Plant Diseases. Dr.Ahed A.Hadi Matloob Assistance Professor in Biological Control Department

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1 Republic of Iraq Dr.Ahed A.Hadi Ministry of higher education & scientific researches Al Furat Al Awsat tech. University Al-Musaib Tech. College-Biological control Dep. Dr.Ahed A.Hadi Matloob Assistance Professor in Biological Control Department

2 Dr.Ahed A.Hadi lec.1 Plant pathology: is a science that studies plant diseases and attempts to improve the chances for survival of plants when they are faced with unfavorable environmental conditions and parasitic micro-organisms that cause disease. by their presence, prevent the cultivation and growth of food plants in some areas; or food plants may be cultivated and grown but plant diseases may attack them, destroy parts or all of the plants, and reduce much of their produce before they can be harvested or consumed. plant pathology is joined by the sciences of entomology and weed science. Importance of the Globally, enormous losses of the crops are caused by the plant diseases. Important historical evidences of plant disease epidemics are Irish Famine due to late blight of potato (Ireland, 1845), Bengal famine due to brown spot of rice (India, 1942) and Coffee rust (Sri Lanka, 1967). Such epidemics had left their effect on the economy of the affected countries. The losses are usually lower in the more developed countries and higher in the developing countries, i.e., countries that need food the most. It has been estimated that of the 36.5% average of total losses, 14.1% are caused by diseases, 10.2% by insects, and 12.2% by weeds. Considering that 14.1% of the crops are lost to plant diseases alone, the total annual worldwide crop loss 2

3 from plant diseases is about $220 billion. To these should be added 6 12% losses of crops after harvest, which are particularly high in developing tropical countries where training and resources such as refrigeration are generally lacking. Also, these losses do not include losses caused by environmental factors such as freezes, droughts, air pollutants, nutrient deficiencies, and toxicities. The Concept of Disease in Plants Because it is not known whether plants feel pain or discomfort and because, in any case, plants do not speak or otherwise communicate with us, it is difficult to pinpoint exactly when a plant is diseased. It is accepted that a plant is healthy, or normal, when it can carry out its physiological functions to the best of its genetic potential. When the ability of the cells of a plant or plant part to carry out one or more of these essential functions is interfered with by either a pathogenic organism or an adverse environmental factor, the activities of the cells are disrupted, altered, or inhibited, the cells malfunction or die, and the plant becomes diseased. Disease in plants, can be defined as the series of invisible and visible responses of plant cells and tissues to a pathogenic organism or environmental factor that result in adverse changes 3

4 in the form, function, or integrity of the plant and may lead to partial impairment or death of plant parts or of the entire plant. HISTORY OF PLANT PATHOLOGY In many of the early references, plant diseases were considered to be a curse and a punishment of the people by God for wrongs and sins they had committed. Greek philosopher Theophrastus (about 286 BC) recorded some plant diseases about 2400 years ago. yet Theophrastus and his contemporaries, being unable to explain plant diseases, believed that God controlled the weather that brought about the disease. therefore, that avoidance or control of the disease depended on people doing things that would please that same superpower. In the fourth century b.c.; the Romans suffered so much from hunger caused by the repeated destruction of cereal crops by rusts and other diseases that they created a separate god, whom they named Robigus. To please Robigus, the Romans offered prayers and sacrifices in the belief that he would protect them from the dreaded rusts. The Romans even established a special holiday for Robigus, the Robigalia, in an attempt to please and pacify Robigus so he would not send the rusts to destroy their crops. some ancient writers, e.g., Homer (c b.c.), mention the therapeutic properties of sulfur on plant diseases, and Democritus (c. 470 b.c.) recommended controlling plant blights by sprinkling plants with the olive grounds left after extraction of the olive oil. This branch of science could maintain a proper record on the plant 4

5 disease and their causal organisms only after development of compound microscope by the Dutch worker Antony von Leeuwenhoek in He first visualized bacteria in 1683 under his microscope. Some of the most famous phytopathologists that contributed to development the plant diseases science were: 1) ANTON De BARY (Germany): He was the father and founder of modern Mycology. He was the founder of modern experimental plant pathologyin 1863, he studied the epidemics of late blight and renamed the casual organism as Phytophthora infestans. He discovered heteroecious nature of rust fungi (1865). He gave detailed account on life cycles of downy mildew genera. He studied about vegetable rotting fungi and damping off fungi. He wrote a book named Morphology and Physiology of fungi, lichens and Myxomycetes (1866). He reported the role of enzymes and toxins in tissue disintegration caused by Sclerotinia sclerotiorum 2) T. J. BURRUILL (USA)(1878): He proved for the first time that fire blight of apple and pear was caused by a bacterium (now known as Erwinia amylovora) 3) Needham (1743): made The first report of nematodes associated with a plant disease which was Anguina tritici caused Wheat seed galls. 4) DOI (JAPANESE)(1967) 5

6 he found that mycoplasma like organisms (MLO) could be responsible for the disease of the yellows type. Doi observed that MLO's are constantly present in phloem. 5)BEIJERINCK (Dutch)(1898) Founder of virology He proved that the virus inciting tobacco mosaic is not a living microorganism. He believed it to be contagium vivum fluidum (infectious living fluid) then named it virus 6)DIENER (1971) discovered the potato spindle tuber was caused by small naked RNA which he called viroid D C B A FIGURE 1 A. Theophrastus, the father of botany. B. Antonius van Leeuwenhoek C. Anton DeBary D. Diener. Causes of Plant diseases are caused by pathogens. Hence a pathogen is always associated with a disease. In other way, disease is a symptom caused by the invasion of a pathogen that is able to survive, perpetuate and spread. Further, the word pathogen can be broadly defined as any agent or factor that incites pathos 6

7 or disease in an organism or host. In strict sense, the causes of plant diseases are grouped under following categories: 1. Animate or biotic causes: Pathogens of living nature are categorized into the following groups. (i) Fungi (vi) plant parasite (ii) Bacteria (vii) Protozoa (iii) Phytoplasma (viii) Nematodes (iv) Rickettsia-like organisms (ix) Viruses (v) Algae (X) Viroids 2. Inanimate or abiotic causes: In true sense these factors cause damages (any reduction in the quality or quantity of yield or loss of revenue) to the plants rather than causing disease. The causes are: (i) Deficiencies or excess of nutrients. (ii) Light (iii) Moisture (iv) Temperature (v) Air pollutants (e.g. black tip of mango) (vi) Lack of oxygen (e.g. hollow and black heart of potato) (vii) Toxicity of pesticides (viii) Improper cultural practices (ix) Abnormality in soil conditions (acidity, alkalinity). ******************** 7

8 components of disease Specific conditions must be present for biotic disease to develop. There must be a susceptible host plant, the pathogen (fungi, bacteria, viruses, etc.) or when it is affected by an abiotic agent, and environmental conditions conducive to disease development; this called Disease Triangle, these must come together in a given point in time. These conditions make up what is called the Plant Disease Pyramid (Epidemic-lec. 4). Biotic disease cannot occur if one of these pieces is missing. [Figure 2] We can see that human putting in the top of Plant Disease Pyramid because his role in disease development and dissemination of pathogens by some wrong processes. Human A B Figure 2, A. Disease Triangle B. Plant Disease Pyramid 8

9 SOME TERMS AND CONCEPTS USED IN PLANT PATHOLOGY - Infection: The initiation and establishment of a parasite within a host plant. - Pathogen: is any agent that causes Disease - Parasite: Organisms which derive the materials they need for growth from living plants - Pathogenicity is the ability of the pathogen to cause disease. - Symptom: The external or internal reactions or alterations of a plant as a result of a disease. such as wilt, yellow, spot, rot, etc. - Sign: The pathogen or its parts or products seen on a host plant. Such as hyphae, spores etc. Inoculum potential: It is the inoculum needed for successful infection. It is a function of inoculum density and their capacity. 9

10 Dr.Ahed A.Hadi lec.2 HOST RANGE OF PATHOGENS Pathogens differ with respect to the kinds of plants that they can attack, organs and tissues that they can infect, and with respect to the age of the organ or tissue of the plant. - Some pathogens are restricted to a single species, others to one genus of plants, and still others have a wide range of hosts. - Some pathogens grow especially on roots, others on stems, leaves or on fleshy fruits or vegetables. - Some pathogens, e.g., vascular parasites, attack specifically certain kinds of tissues, such as phloem or xylem. - Some pathogens attack seedlings or the young parts of plants, whereas others attack only mature tissues. STAGES IN THE DEVELOPMENT OF DISEASE: THE DISEASE CYCLE In every infectious disease a series of events occurs in succession and leads to the development of the disease and the pathogen. This chain of events is called a disease cycle. The events in a disease cycle are inoculation, penetration, establishment of infection, colonization (invasion), growth and reproduction of the pathogen, dissemination of the pathogen, and survival of the pathogen in the absence of the host, i.e., overwintering or oversummering (overseasoning) of the pathogen. Usually there is one cycle of disease which called Monocyclic disease, In some diseases there may be several 0

11 infection cycles within one disease cycle, called polycyclic disease (figure 3) figure 3. Mono and Polycyclic disease. Steps of Disease Development The steps of the disease cycle are inoculation, penetration, infection, invasion, reproduction, and dissemination. Then overseasoning. 1. Inoculation is the placement of the pathogen s infectious unit or propagule on or in close proximity to the host cell wall. -The inoculum is any part of the pathogen that can initiate infection. Types of Inoculum An inoculum that survives dormant in the winter or summer and causes the original infections in the spring or in the autumn is called a primary inoculum, and the infections it causes are called primary infections. An inoculum produced from primary infections is called a secondary inoculum and causes secondary infections. 1

12 2. Penetration The propagule then penetrates the cell wall of the host. In fungi, the propagule may germinate and the germ tube may penetrate the wall directly or indirectly through a wound or natural opening. Bacteria enter plants mostly through wounds and less frequently through natural openings. Viruses, viroids, mollicutes, fastidious bacteria enter through wounds made by vectors. Fungi, nematodes and parasitic higher plants enter through direct penetration and less frequently through natural openings and wounds. 3. INFECTION pathogens grow and multiply within the plant tissues. Invasion of plant tissues by the pathogen, and growth and reproduction of the pathogen (colonization) are two concurrent stages of disease development. 4- dissemination After infection takes place, the pathogen may grow and invade other parts of the host. The pathogen will continue to reproduce and the new propagules will be dispersed or disseminated by a variety of means such as wind, rain, within or on vectors, seed, or on contaminated debris or equipment. 5- overseasoning in the end of season pathogen survive in plant debris, in the soil, or formed survival bodies. Incubation period: The period between penetration of a host by a pathogen and the first appearance of symptoms on the host. It varies with pathogens, hosts and environmental conditions. 2

13 Levels of Parasitism Pathogens can be classified into several groups. - obligate parasites. (Biotrophs) are pathogens that require living host tissue to complete their life cycle. e. g. fungi, such as powdery mildews, rusts and some members of the Oomycota, such as downy mildews and white rusts,viruses and viroids; phytoparasitic nematodes. and they are called obligate parasites. - facultative saprophyte : is a pathogen that often behaves as a parasite but under certain conditions behaves as a saprophyte on organic matter, - facultative parasite: A pathogen that often behaves as a saprophyte but under some conditions becomes a parasite. Necrotrophs: Non biotrophic organisms kill before feeding on the cells or cellular contents. These organisms that live on dead tissues. 3

14 Dr.Ahed A.Hadi HOW PATHOGENS ATTACK PLANTS lec.3 Healthy plant is a community of cells built in a fortress-like fashion. Therefore, for a pathogen to infect a plant it must be able to make its way into and through the plant, obtain nutrients from the plant, and neutralize the defense reactions of the plant. Pathogens accomplish these activities mostly through secretions of chemical substances that affect certain components or metabolic mechanisms of their hosts. Penetration and invasion, however, some cases be entirely the result of the mechanical force exerted by certain pathogens on the cell walls of the plant. MECHANICAL FORCES EXERTED BY PATHOGENS ON HOST TISSUES Only some fungi, parasitic higher plants, and nematodes appear to apply mechanical pressure to the plant surface they are about to penetrate. cutinase and cellulase enzymes released from the spore surface help the spore adhere to the plant surface. After contact is established, the diameter of the tip of the hypha or radicle in contact with the host increases and forms the flattened, bulb-like structure called the appressorium (Figs. 4). This increases the area of adherence between the two organisms and securely fastens the pathogen to the plant. From the appressorium, a fine growing point, called the penetration peg arises and advances into and through the cuticle and cell wall. 4

15 After penetration of the cuticle, the hyphal tube diameter often increases considerably. The penetration tube attains the diameter normal for the hyphae of the particular fungus only after it has passed through the cell wall. Nematodes penetrate plant surfaces by means of the stylet, which is thrust back and forth and exerts mechanical pressure on the cell wall. figure 4 germination and penetration of fungal spore and invasion CHEMICAL WEAPONS OF PATHOGENS The main groups of substances secreted by pathogens in plants enzymes, toxins, growth regulators, and polysaccharides. and their relative importance may be different from one disease to another. Thus, in some diseases, such as soft rots ( enzymes ), crown gall ( growth regulators), the Bipolaris blight of Victoria oats ( toxin ) and some of wilt types (Polysaccharides)). 5

16 1-Enzymes: Most pathogens derive energy principally from enzymatic break down of food materials from host tissue.the epidermis of plants is covered by cuticle, whose major chemical substance is cutin in addition to cuticular wax. - Cuticular wax: Plant waxes are found as granular or rod like projections or as a continuous layer outside / within the cuticle.. Most of the fungi and parasitic higher plants penetrate wax layers by means of mechanical force alone (no enzyme). - Cutin: Cutin is admixed with waxes on upper side and with pectin and cellulose on the lower side. Cutinases break cutin molecules, Cutinases reaches its highest concentration at penetrating point of the germ tube and at infection peg of appressorium forming fungi. - Pectic substances: These are major components of middle lamella (intercellular cement that holds in place the cells of plant tissues). They also make up a large portion of primary cell wall in which they form an amorphous gel filling the spaces between cellulose microfibrils, The enzymes that degrade pectic substances are known as pectinases. - Cellulose: Cellulose occurs in all higher plants as the skeletal substance of cell walls in the form of microfibrils. These microfibrils are like bundles of iron bars in a reinforced concrete building. Cellulose is degraded by cellulases - Lignin: Lignin is found in the middle lamella, as well as in the secondary cell wall of xylem vessels and the fibres that 6

17 strengthen plants. White rot fungi (Basidiomycetes) secrete one or more ligninases which enable them to utilize lignin. - proteins: proteins are degraded by enzymes, proteases or proteinases. - Lipids: Various types of lipids occur in all plant cells. The most important ones are phospholipids and glycolipids. These lipids contain fatty acids. Lipolytic enzymes, called lipases (phospholipases, glycolipases) hydrolyze lipids and release fatty acids. - Starch: It is a glucose polymer and exists in two forms: amylose, a linear molecule, and amylopectin, a highly branched molecule. Starch is degraded by enzyme, amylases. 2- ROLE OF TOXINS IN PLANT PATHOGENESIS Toxin can be defined as a microbial metabolite excreted or released by cells which in very low concentration is directly toxic to the cells of the host. - Classification based on specificity of toxins 1. Host specific / Host selective toxins: These are the metabolic products of the pathogens which are selectively toxic only to the susceptible host of the pathogen Ex: Victorin, T-toxin and Phyto-alternarin. 2. Non-specific / Non-selective toxins These are the metabolic products of the pathogen, but do not have host specificity and affect the protoplasm of many 7

18 unrelated plant species that are normally not infected by the pathogen Ex: Tab-toxin, Fusaric acid, Piricularin, Lycomarasmin. Effect of toxins on host tissues A) Changes in cell permeability: Toxins kill plant cells by altering the permeability of plasma membrane B) Disruption of normal metabolic processes, Increase in respiration due to disturbed salt balance. Malfunctioning of enzyme system. C) Interfere with the growth regulatory system of host plant. 3- ROLE OF GROWTH REGULATORS IN PLANT PATHOGENESIS Growth regulators Growth regulators are of two types: 1. Growth promoting substances and 2. Growth inhibiting substances Auxins, gibberellins and cytokinins are growth promoting substances, whereas, dormin, ethylene and abscissic acid are growth inhibiting substances. The imbalance in growth promoting and growth inhibiting substances causes hypertrophy (excessive increase in cell size) and atrophy (decrease in cell size). Symptoms may appear as tumors, galls, knots, witches broom, stunting, excessive root branching, defoliation and suppression of bud growth. 8

19 4- ROLE OF POLYSACCHARIDES IN PATHOGENESIS Polysaccharides: Fungi, bacteria and nematodes release varying amounts of mucilaginous substances that coat their bodies and provide interface between the outer surface of the micro-organism and its environment. In the vascular wilts, large polysaccharide molecules released by the pathogen in the xylem causes mechanical blockage of vascular bundles and initiate wilting. Ex: Ralstonia solanacearum (Bacterial wilt of Solanaceous plants) 9

20 Dr.Ahed A.Hadi - Epidemic diseases lec.4 An epidemic has been defined as any increase of disease in a population. The study of epidemics and of the factors that influence them is called epidemiology. Epidemiology is concerned simultaneously with populations of pathogens and host plants as they occur in an evolving environment THE ELEMENTS OF AN EPIDEMIC Plant disease epidemics develop as a result of the timely combination of the same elements that result in plant disease: susceptible host plants, a virulent pathogen, and favorable environmental conditions over a relatively long period of time. Humans may unwittingly help initiate and develop epidemics through some of their activities, (the disease triangle, tetrahedron, or pyramid, (discussed in Chapter 1) (Fig.-2). HOST FACTORS THAT AFFECT THE DEVELOPMENT OF EPIDEMICS Several internal and external factors of particular host plants play an important role in the development of epidemics involving those hosts. * Levels of Genetic Resistance or Susceptibility of the Host. * Degree of Genetic Uniformity of Host Plants a new pathogen race will appear that can attack their genome and result in an epidemic. 0

21 * Type of Crop- In diseases of annual crops, such as corn, vegetables, rice, and cotton, epidemics generally develop much more rapidly (usually in a few weeks) than they do in diseases of branches and stems of perennial woody crops such as fruit and forest trees. Age of Host Plants the age of the host plant at the time of arrival of the pathogen may affect considerably the development of infection and of an epidemic. PATHOGEN FACTORS THAT AFFECT DEVELOPMENT OF EPIDEMICS *Levels of Virulence Virulent pathogens capable of infecting the host rapidly ensure a faster production of larger amounts of inoculum, and, thereby, disease, than pathogens of lesser virulence. *Quantity of Inoculum near Hosts The greater the number of pathogen propagules (bacteria, fungal spores and sclerotia, nematode eggs, virus infected plants, etc.) within or near fields of host plants, the more inoculum reaches the hosts and at an earlier time, thereby increasing the chances of an epidemic greatly. *Type of Reproduction of the Pathogen All pathogens produce many offspring, Some plant pathogenic fungi, bacteria, and viruses have short reproduction cycles and therefore are polycyclic, i.e., they can produce many generations in a single growing season. Polycyclic pathogens include fungi 1

22 that cause rusts, mildews, and leaf spots and are responsible for most of the sudden, catastrophic plant disease epidemics in the world. *Ecology of the Pathogen Some pathogens, such as most fungi and all parasitic higher plants, produce their inoculum (spores and seeds, respectively) on the surface of the aerial parts of the host. From there, spores and seeds can be dispersed with ease over a range of distances and can cause widespread epidemics. Other pathogens, such as vascular fungi and bacteria, mollicutes, viruses, and protozoa, reproduce inside the plant. In this case, spread of the pathogen is rare or impossible without the help of vectors. *Mode of Spread of the Pathogen The spores of many plant pathogenic fungi, such as those causing rusts, mildews, and leaf spots, are released into the air and can be dispersed by air breezes or strong winds over distances varying from a few centimeters up to several kilometers. These kinds of fungi are responsible for the most frequent and most widespread epidemics. ENVIRONMENTAL FACTORS THAT AFFECT DEVELOPMENT OF EPIDEMICS The environment may affect the availability, growth stage, succulence, and genetic susceptibility of the host plants. It may also affect the survival, vigor, rate of multiplication, sporulation, and ease, direction, and distance of dispersal of the pathogen, as 2

23 well as the rate of spore germination and penetration. In addition, the environment may affect the number and activity of the vectors of the pathogen. The most important environmental factors that affect the development of plant disease epidemics are moisture, temperature, and the activities of humans in terms of cultural practices and control measures. EFFECT OF HUMAN CULTURAL PRACTICES AND CONTROL MEASURES Many activities of humans have a direct or indirect effect on plant disease epidemics, some of them favoring and some reducing the frequency and the rate of epidemics. *Site Selection and Preparation *Selection of Propagative Material The use of seed, nursery stock, and other propagative material that carries various pathogens increases the amount of initial inoculum within the crop and favors the development of epidemics greatly. The use of pathogen-free or treated propagative material can reduce the chance of epidemics greatly. *Cultural Practices Continuous monoculture, large acreages planted to the same variety of crop, high levels of nitrogen fertilization, no-till culture, dense plantings, overhead irrigation, injury by herbicide application, and poor sanitation all increase the possibility and severity of epidemics. *Disease Control Measures 3

24 *Introduction of New Pathogens The ease and frequency of worldwide travel have also increased the movement of seeds, tubers, nursery stock, and other agricultural goods. These events increase the possibility of introducing pathogens into areas where the hosts have not had a chance to evolve resistance to these pathogens. ************************************** Sources of Inoculum - In some fungal and bacterial diseases of perennial plants, such as shrubs and trees, the inoculum is produced on the branches, trunks, or roots of the plants. - The inoculum sometimes is present in the plant debris or soil; - Other times it comes into the field with the seed, transplants, tubers, or other propagative organs. - the inoculum survives in perennial weeds or alternate hosts. - Fungi, bacteria, parasitic higher plants, and nematodes either produce their inoculum on the surface of infected plants. - Viruses, viroids, mollicutes, fastidious bacteria, and protozoa produce their inoculum within the plants transmitted from one plant to another by some kind of vector, such as an insect. 4

25 Dr.Ahed A.Hadi lec.5 ENVIRONMENTAL EFFECTS ON THE DEVELOPMENT OF INFECTIOUS PLANT DISEASE 1- EFFECT OF TEMPERATURE Pathogens differ in their preference for higher or lower temperatures. some species of the fungi Typhula and Fusarium, which cause snow mold of cereals and turf grasses, thrive only in cold regions. Also, the late blight pathogen Phytophthora infestans is most serious in the winter. Many diseases, such as the brown rot of stone fruits caused by Monilinia fructicola, are favored by relatively high temperatures. Several diseases, such as the fusarial wilts, many anthracnoses caused by Colletotrichum, and the bacterial wilts of solanaceous plants caused by Ralstonia solanacearum, are favored by high temperatures and are limited to hot areas. 2- EFFECT OF MOISTURE Moisture, may exist as rain or irrigation water on the plant surface or around the roots, as relative humidity in the air, and as dew. - Moisture is indispensable for the germination of fungal spores and penetration of the host by the germ tube. - It is also indispensable for the activation of bacterial, fungal, and nematode pathogens before they can infect the plant. 5

26 - Moisture, in such forms as splashing rain and running water, also plays an important role in the distribution and spread of many of pathogens on the same plant and on their spread from one plant to another. - Finally, moisture increases the succulence of host plants and thus their susceptibility to certain pathogens, which affects the extent and severity of disease. late blight of potato, apple scab, downy mildew of grapes, and fire blight are found or are severe only in areas with high rainfall or high relative humidity during the growing season. In many diseases affecting underground parts of plants, such as roots, tubers, and young seedlings, e.g., in the Pythium damping off of seedlings and seed decays, the severity of the disease is proportional to the amount of soil moisture and is greatest near the saturation point. Several other fungi, e.g., Fusarium solani, which is the cause of dry root rot of beans, and Macrophomina phaseoli, the cause of charcoal rot of sorghum and of root rot of cotton, grow fairly well in rather dry environments. EFFECT OF WIND Wind influences infectious plant diseases by: - increasing the spread of plant pathogens. - increasing the number of wounds on host plants. 6

27 - accelerating the drying of wet surfaces of plants. EFFECT OF LIGHT Several diseases are depended on the intensity and the duration of light which may increase or decrease the susceptibility of plants to infection and also the severity of the disease. ex. Reduced light intensity generally increases the susceptibility of plants to virus infections. EFFECT OF SOIL PH AND SOIL STRUCTURE The ph of the soil is important in the occurrence and severity of plant diseases caused by certain soilborne pathogens. For example, the clubroot of crucifers caused by Plasmodiophora brassicae is most prevalent and severe at about ph 5.7, the common scab of potato caused by Striptomyces scabies can be severe from ph 5.2 to 8.0 or above. EFFECT OF HOST PLANT NUTRITION Nutrition affects the rate of growth and the state of readiness of plants to defend themselves against pathogenic attack. 7

28 Dr.Ahed A.Hadi lec.6 PLANT DISEASES CAUSED BY FUNGI Fungi are small, generally microscopic, eukaryotic, usually filamentous, branched, that lack chlorophyll. More than 10,000 species of fungi can cause disease in plants. CHARACTERISTICS OF PLANT PATHOGENIC FUNGI Morphology Most fungi have a filamentous vegetative body called a mycelium. The individual branches of the mycelium are called hyphae. may or may not be partitioned by cross walls (septa). Reproduction Fungi reproduce chiefly by means of spores. Spores are reproductive bodies consisting of one or a few cells. Spores may be formed asexually, like buds produced on a twig, or as the result of sexual fertilization. In some fungi, asexual spores are produced inside a sac called a sporangium. Some of these spores can swim by means of flagella and are called zoospores. Other fungi produce asexual spores called conidia by the cutting off of terminal or lateral cells from special hyphae called conidiophores. In some fungi, terminal cells of a hypha enlarge, round up, form a thick wall, and separate to form chlamydospores. 8

29 Sexual reproduction occurs in most groups of fungi. In Zygomycetes, produce a zygospore. In Ascomycetes, ascospores. In Basidiomycetes, sexual spores are produced called the basidiospores. In the Oomycetes, oospores. Some fungi (formerly known as fungi imperfect or deuteromycetes), no sexual reproduction is known or it has not yet been discovered. Diseases Caused by class: Plasmodiophoromycetes Their body is a plasmodium, an amoeboid mass of protoplasm that has many nuclei and no definite cell wall. Three Plasmodiophoromycetes cause the following common diseases of plants which were The pathogens are obligate parasites: Plasmodiophora, causing clubroot of crucifers Polymyxa, causing a root disease of cereals and grasses Spongospora, causing the powdery scab of potato 1-CLUBROOT OF CRUCIFERS The clubroot disease of cruciferous plants, such as cabbage and cauliflower, is widely distributed all over the world. Clubroot can cause serious losses to susceptible varieties. 9

30 Symptoms Infected plants at first have pale green to yellowish leaves. Later, infected plants show wilting in the middle of hot, sunny days, recovering during the night. plants stunted and fail to produce marketable heads. The most characteristic symptoms of the disease appear on the roots (Fig. 6) as spindle-like, spherical, knobby, or club-shaped swellings. The Pathogen: Plasmodiophora brassicae Its body is a plasmodium. The plasmodium gives rise to zoosporangia or to resting spores which on germination, produce zoospores. Development of Disease Wintering\ as resting spores in the soil and debris. Inoculation\ The single zoospore produced from resting spores Penetration and infection\ zoospore penetrates (directly or from the wounds) root hairs, and there develops into a plasmodium. After a few days, the plasmodium cleaves into multinucleate portions and each develops into a zoosporangium containing four to eight secondary zoospores. Dissemination\ The zoospores are discharged outside the host through pores dissolved in the host cell wall (FIG 5). 0

31 Zoospores formed in Zoosporangia still in host Zoospores are released Through pores Fungus-filled cells of cabbage root Multinucleate plasmodium Resting spore releasing resting spores FIGURE 5. Life cycle of Plasmodiophora brassicae Figure 5. club-root on crucifers 1

32 (The plasmodium-infected clubs not only utilize much of the food required for the normal growth of the plant, they also interfere with the absorption and translocation of mineral nutrients and water through the root system. This results stunting and wilting of the aboveground parts of the plant). Control 1- Addition hydrated lime to raise the soil to ph 7.2. At that ph, spores of the clubroot organism germinate poorly or not at all. 2- treating Seed bed areas with appropriate soil fumigants approximately two weeks before planting. can be kept free of clubroot. 3- using the clean, clubroot-free seedlings, and watered with a solution of an effective fungicide. 4- Some varieties of cruciferous hosts are resistant to certain races of the clubroot organism and can be grown in areas infested with these races. 2

33 Dr.Ahed A.Hadi Diseases Caused by Oomycetes lec.7 Oomycetes produce oospores (sexual spore) and zoospores (asexual spores). The most important plant pathogenic pathogen belong the order called Peronosporales. This order includes several of the most important genera of plant pathogens known such as: -Pythium sp., one of the most common and most important causes of seed rot, seedling damping-off, and root rot of all types of plants, and also of soft rots of fleshy fruits in contact with the soil (Figure 7). -Phytophthora sp., late blight of potato and several others causing root rots, fruit rots, and blights of many other annual and perennial plants, and root and stem rots, cankers and diebacks of trees. -Downy Mildews The Pathogen: There are many genera in the family Peronosporaceae The genera are distinguished by the highly differentiated branching patterns of sporangiophores that emerge through stomata on lower leaf surfaces, Some of the most common or most serious downy mildew oomycetes and the diseases they cause are listed below. Bremia, sporangiophore with binary branch and terminal swellings like hand, causing downy mildew of lettuce(b. lactucae). 3

34 Peronospora, sporangiophore binary branches curved with ends like claws, causing downy mildew of onion (P. destructor). Plasmopara, sporangiophore branches 2-8 with right angles causing downy mildew of grape (P. viticola). Pseudoperonospora, sporangiophore like Plasmopara and Peronospora right angles wit claw ends, causing downy mildew of cucurbits (P. cubensis). Sclerospora, sporangiophore compound binary branch, causing downy mildew of grasses and millets (S. graminicola).(fig 8) Basidiophore, sporangiophore without branches and has basidium shape. Development of Disease (life cycle). - overwinters as oospores in dead leaf lesions and shoots. - During rainy periods in the spring the oospores germinate to produce a sporangium. - The sporangium or its zoospores are transported by wind or water to the wet leaves near the ground, which they infect through stomata of the lower surface, The mycelium then spreads into the intercellular spaces of the leaf - sporangiophores arise and emerge through the stoma. - The sporangia may be carried by wind or rain to nearby healthy plants, germinate quickly, and produce many zoospores that cause secondary infections and thus spread the disease rapidly. 4

35 - At the end of the growing season fungus form Oospore (Fig 9) A B C FIGURE 7. (A) Pythium seed rot., One healthy bean (B) Pythium, barley seedlings (C) Soft rots of squash (A) and potato (B) caused by Pythium 5

36 Figure 8. Some genera of fungi causing Downy mildew 6

37 Fig 9. life cycle of Downy mildew pathogen 7

38 Dr.Ahed A.Hadi Diseases Caused by Zygomycetes lec.8 Zygomycetes have well-developed mycelia without cross walls and produce non-motile spores in sporangia; their sexual spore is a thick-walled Zygospore. Three genera of Zygomycetes are known to cause disease in plants or plant products: (1) Choanephora, which attacks the withering floral parts of many plants after fertilization and from there invades the fruit and causes a soft rot of primarily summer squash but also of pumpkin, pepper, and okra. (2) Rhizopus and (3) Mucor, both common bread mold fungi, which in addition cause soft rot of many fleshy fruits, vegetables, flowers. Other genera are fungi that become associated with roots of plants (Mycorrhizae) e.g., Endogone, Glomus, that are beneficial to plants. Rhizopus SOFT ROT OF FRUITS AND VEGETABLES Rhizopus soft rot of fruits and vegetables occurs throughout the world on harvested fleshy organs of vegetable, fruit, and flower crops during storage, transit, and marketing of these products. The Pathogen: Rhizopus spp. The mycelium of the fungus produces long, aerial sporangiophores at the tips of which black spherical sporangia develop. the sexually produced spore is called a zygospore (zygospore produced by the union of two morphologically 8

39 similar gametes) and is the overwintering of the fungus. When it germinates it produces a sporangiophore bearing a sporangium full of sporangiospores.(figure 10). Figure 10. life cycle of Rhizopus Development of Disease -Throughout the year, sporangiospores float about and if they land on wounds of fleshy fruits, roots, corms, or bulbs. - sporangiospores germinate. The resulting hyphae secrete pectinolytic and cellulolytic enzymes, which break down and dissolve the pectic substances of the middle lamella soft rot.. - The fungus continues to grow inside the tissues. When the epidermis breaks, the fungus emerges through the wounds and produces aerial sporangiophores, sporangia, the mycelium can penetrate even healthy fruit. When the food supply in the infected tissues begins to diminish and compatible strains are 9

40 present together, zygospores are produced. Zygospores help the fungus survive (Figure 11). A Figure 11. A. Zygospore B. Soft rot on squash B 0

41 Dr.Ahed A.Hadi lec.9 DISEASES CAUSED BY ASCOMYCOTA The Ascomycota are true fungi with a well-developed, septate mycelium The sexual structure of the ascomycetes is an ascus, which is shaped like a sac and contains the products called ascospores The Powdery Mildews Powdery mildews are obligate parasites. have specialized feeding cells called haustoria that absorb nutrients from their hosts. Pathogen: The powdery mildew diseases of the various crop or other plants are caused by many species of fungi of the family Erysiphaceae grouped onto several main genera. These genera are distinguished from one another by the number (one versus several) of asci per cleistothecium and by the morphology of hyphal appendages growing out of the wall of the cleistothecium. The main genera are illustrated in, and the most important diseases they cause are listed here. Blumeria, B. graminis causing powdery mildew on cereals and grasses. 1

42 Erysiphe, E. cichoracearum causing powdery mildew of begonia, chrysanthemum, cucurbits; E. polygoni of legumes, beets, crucifers, and cucurbits; E. betae of beets; and E. orontii of tomato Leveillula, L. taurica causing powdery mildew of eggplant and pepper. Microsphaera, M. alni causing powdery mildew of many shade trees and woody ornamentals. Phyllactinia spp., causing powdery mildew of shade and forest trees. Podosphaera, P. leucotricha causing powdery mildew of apple, pear, and quince; P. oxyacanthae, of apricot, cherry, peach, and plum; and P. xanthii, of cucurbits. Sphaerotheca,, S. pannosa of peach and rose. Uncinula necator, causing powdery mildew of grape. Development of Disease. - The fungus overwinters mostly as mycelium in the buds. Cleistothecia form occasionally toward the end of the season. (Fig. 13). - Ascospores or conidia are carried by wind to young green tissues. - the spores germinate and infect these tissues. The germ tube grows directly into the epidermal cells and forms a haustorium by which the fungus obtains its nutrients. 2

43 - The germ tube, however, continues to grow and branch on the surface of the plant tissue, producing a network of mycelium that sends haustoria into the epidermal cells. The absorption of nutrients from the cells depletes their food supply, weakens them, and may sometimes lead to their death. Photosynthesis in the affected areas is reduced greatly. - The aerial mycelium produces numerous conidia, which cause new infections on the expanding leaves and shoots. In the end season sexual spore (ascospore) be formed in cleistothecia to surviving. FIGURE 13 Life cycle of a powdery mildew (a) Conidium. (b) Condium germinating and forming an appressorium on the leaf surface. (c) Conideophores and conidia. (d) Sexual reproduction between compatible mating types. (e) Ascocarps, ascus, and ascospores. Note germinating 3

44 Dr.Ahed A.Hadi DISEASES CAUSED BY BASIDIOMYCETES lec.10 Basidiomycetes are fungi that produce their sexual spores, called Basidiospores, on a club-shaped spore producing structure called a Basidium. Most Basidiomycetes are fleshy fungi, such as the common mushrooms, the puffballs, also include two very common and very destructive groups of plant pathogenic fungi that cause the rust and the smut diseases of plants. SMUTS Plant smuts, caused by Basidiomycetes of the order Ustilaginales, occur throughout the world. smuts attack the grain kernels themselves and replace the kernel contents with the black, dusty spore masses that resemble soot or smut. Most smut fungi attack the ovaries of grains and grasses and develop in them and in the fruit, the kernels of grain crops, which they destroy completely. Most smut fungi produce only two kinds of spores: Teliospores and Basidiospores. The most common smut fungi and the diseases they cause are the following: Ustilago, causing corn smut [U. zeae (maydis)], loose smut of cereals ( U. nuda, and U. tritici), Tilletia, causing covered smut or bunt of wheat [T. caries and T. foetida)] Sphacelotheca, causing the sorghum smuts (S. sorghi, and S. reiliana) 4

45 Urocystis, causing onion smut (U. cepulae) 1-COVERED SMUT, OR BUNT, OF WHEAT Covered smut, or bunt, or stinking smut of wheat occurs in all wheat-growing areas of the world. Bunt destroys the contents (except cover) of infected kernels and replaces them with the spores of the fungus. Pathogen. Tilletia caries and T. foetida Development of Disease -The pathogens of common bunt overwinter as teliospores on contaminated wheat kernels and less frequently in the soil. - Teliospore on the kernel or near the seedling germinates through the production of the basidium, the mycelium they produce infects the young seedling and penetrate the wheat seedlings after seedling emergence. -After penetration, the mycelium grows intercellularly and invades the developing leaves and the meristematic tissue at the growing point of the plant. When the plant forms the head of the grain, the mycelium invades all parts of it and consume the contents of the kernel cells. kernels are usually break and release their spores on harvest or threshing. The liberated spores contaminate the healthy kernels and are also blown away by air currents, thus contaminating the soil. 5

46 2-LOOSE SMUT OF CEREALS In an infected plant, usually all the heads and all the spike lets and kernels of each head are smutted The Pathogens: Ustilago nuda and Ustilago tritici 3- CORN SMUT (common smut) Minute galls form on the leaves and stems, Infected areas are permeated by the fungus mycelium, which stimulates the host cells to divide and enlarge, thus forming galls. Galls are first covered with a greenish white membrane. Later, as the galls mature, they reach a size from 1 to 15 centimeters in diameter, which contained the millions of sooty teliospores, which are released into the air. The Pathogen: Ustilago zeae, The fungus overwinters as teliospores in crop debris and in the soil, where they can survive for several years. 6

47 A B C Figure 15. A. Loose smut on Barley, B. common smut on corn, C. covered smut on wheat 7

48 Dr.Ahed A.Hadi RUSTS lec.11 Plant rusts, caused by Basidiomycetes of the order Uredinales, are among the most destructive plant diseases. The most important rust fungi and the diseases they cause are listed here - Puccinia, causing severe and often catastrophic diseases on numerous hosts such as the stem rust of wheat and all other small grains (P. graminis); yellow or stripe rust of wheat, barley, and rye (P. striiformis); leaf or brown rust of wheat and leaf rust of barley (P. hordei); corn rust (P. sorghi); - Gymnosporangium, causing cedar-apple rust (G. juniperivirginianae) - Uromyces, causing the rusts of legumes (U. appendiculatus) WHEAT RUSTS Wheat rusts are caused by three related fungi: - Stripe rust ( yellow rust ) is caused by Puccinia striiformis f. sp. tritici. - Leaf rust (orange rust) is caused by Puccinia triticina. - Stem rust is caused by Puccinia graminis f. sp. tritici. STEM RUST ON WHEAT The symptoms on wheat appear as elliptical blisters or pustules, known as uredia, that develop parallel with the long axis of the 8

49 stem, leaf, or leaf sheath powdery mass of brick red-colored uredospores. Later in the season, as the plant approaches maturity, the pustules turn black as the fungus produces teliospores instead of uredospores and uredia are transformed into black telia. Development of Disease. The fungus overwinters as teliospores on infected wheat debris. Teliospores germinate in the spring and produce a basidium on which form four basidiospores. The basidiospores are carried by air currents for a few hundred meters. Basidiospores landing on young barberry leaves germinate and penetrate the epidermal cells. After that, the mycelium grows mostly intercellularly. Within 3 or 4 days the mycelium develops into a spermagonium which ruptures the epidermis, and its opening emerges on the surface of the plant tissue. This called spermatial stage then form aecial stage. mycelium grows intercellularly toward the lower side of the leaf, where it forms thick mycelial mats that develop into aecia. On the lower surface of the leaf The aecia form in groups and protrude considerably beyond the surface of the barberry plant. Aeciospores are released in late spring and are carried by wind to nearby wheat plants on which they germinate and infect wheat stems, leaves, or sheaths through stomata. After the mycelium grows intercellularly for a while, uredospores are produced that exert pressure on the epidermis, which is pushed outward and forms a uredial pustule. Finally, the epidermis is 9

50 broken irregularly, revealing several hundred thousand rustcolored uredospores, which give a powdery appearance to the uredium The uredospores are easily blown away by air currents, Uredospores cause new infections on wheat plants up to the time the plant reaches maturity. the uredia produce teliospores instead of uredospores or new telia may develop from recent uredospore infections. Teliospores do not germinate immediately and do not infect wheat; rather, they are the overwintering stage of the fungus. 0

51 Basidiospores infect barberry leaf directly Receptive hypha Spermatia Spermatia fertilize compatible receptive hypha Fertilized receptive hypha Barberry stem and leaves Spermagonia on barberry leaf Dikaryotic mycelium Basidiospore Telia and uredia on wheat stem or leaf Aecium Germinating teliospore teliospore Clusters of aecia on under side of barberry leaf Overwintering teliospore Aeciospores Karyogamy Telia on wheat at the end of season Uredospore infects wheat through stomata Uredium on wheat Aeciospore infects wheat stem or leaf through stomata FIGURE 16. Disease cycle of stem rust of wheat caused by Puccinia graminis tritici. Note\ this life cycle has two hosts with 5 stages thus called long life cycle diseases, Some pathogens complete only one, or even part of one, disease cycle in tow stage telial and basidail stages on one host and these are called short life cycle. 1

52 Dr.Ahed A.Hadi lec.12 PLANT DISEASES CAUSED BY BACTERIA Bacteria and mollicutes are prokaryotes. These are generally single-celled microorganisms whose genetic material (DNA) is not bound by a membrane and therefore is not organized into a nucleus. Characteristics of Plant Pathogenic Bacteria Most of the DNA in bacteria is present as a single circular chromosome. Additional DNA is found in many bacteria as independently reproducing plasmids composed of smaller amounts of DNA. Most plant pathogenic bacteria are Gram negative, with the exception of Clavibacter (Corynebacterium).Phytopathological bacteria are either rod (0.6 to 3.5 micrometers in diameter). or filamentous shaped, may or may not be flagellate, The cell walls of bacteria of most species are enveloped by a viscous, gummy material, which, if thin and diffuse, is called a slime layer, but if thick, forming a definitive mass around the cell, is called a capsule. and reproduce by binary fission. Traditionally, bacteria were classified based on Gram stain, cell shape, cultural Pathogenic bacteria are known as wound pathogens because they usually do not penetrate the host directly. They may also enter through natural plant openings such as nectaries, hydathodes, and stoma. 2

53 They are disseminated by air currents, water, insects, plant materials, and contaminated equipment. Symptoms Caused by Bacteria Plant pathogenic bacteria induce as many kinds of symptoms on the plants they infect as do fungi. They cause leaf spots and blights, soft rots of fruits, roots, and storage organs, wilts, overgrowths, scabs, and cankers. GENERA OF PLANT PATHOGENIC BACTERIA Agrobacterium Rods motile by 1 to 6 peritrichous flagella. Agrobacterium species cause proliferations on many plants: smooth and rough galls (A. tumefaciens), Tumorigenic Agrobacterium species carry parasitic plasmids vectoring tumor DNA (T-DNA). Pseudomonas Rods motile by one or more polar flagella. Many plant pathogens produce water-soluble pigments that fluoresce light blue to greenish-yellow with ultraviolet light. Pseudomonas marginalis is a soft-rotting pathogen of many plants. Xanthomonas Rods motile by one polar flagellum. Colonies are yellow due to xanthomonadin pigments. Xanthomonas campestris has over 140 pathovars, including important pathogens of rice, field beans and cabbage. Erwinia- Rods motile by peritrichous flagella. E. amylovora, which causes fire blight of pear and apple, and the soft-rotting. 3

54 Bacillus Large rods motile by peritrichous flagella with oval central endospores. Strongly Gram-positive. Bacillus species may cause rots of tobacco leaves, tomato seedlings, and soybean and white stripe of wheat. Streptomyces Vegetative, extensively branched hyphae. Aerial mycelium matures to form three or more spores in chains. Streptomyces scabies and S. acidiscabies cause potato scab. Some PROKARIOTE caused plant diseases are Phytoplasma Polymprphic. The aster yellows phytoplasma causes proliferation or yellows in over 120 host plants. Sprioplasma Spiral shape supported around a central protein rod embedded in the membrane. Plant pathogenic spiroplasmas cause stubborn disease of Citrus species (S. citri) and corn stunt (S. kunkelii). 1- FIRE BLIGHT OF PEAR AND APPLE.Fire blight causes damage to pear and apple orchards in many parts of the world. Symptoms: Infected flowers become water soaked, then shrivel, turn brownish black, and fall or remain hanging in the tree. Soon leaves on the same spur or on nearby twigs develop brown-black blotches along the midrib and main veins or along the margins and between the veins. The tip of the twig is hooked, and the leaves turn black and cling to the twig. From fruit spurs and 4

55 twigs the symptoms progress down to the branches, where cankers are formed. The Pathogen Erwinia amylovora. Development of Disease Bacteria overwinter at the margins of cankers and possibly in buds and apparently healthy wood tissue. In the spring, bacteria in the cankers become active again, multiply, and spread into the adjoining healthy bark. During humid or wet weather, bacterial masses exude through lenticels and cracks. The bacterial ooze appears at about the time when the pear blossoms are opening. Various insects, such as bees, flies, and ants, are attracted to the sweet, sticky, bacteria-filled exudate, become smeared with it, and spread it to the flowers they visit afterward. In some cases, 5

56 bacteria are also spread from oozing cankers to flowers by splashing rain. When the ooze dries, it often forms aerial strands that can be spread by wind and serve as inoculum. Bacteria multiply rapidly in the nectar and, through the nectar thodes, enter the tissues of the flower. Bees visiting an infected flower carry bacteria from its nectar to all the succeeding blossoms that they visit. Once inside the flower, bacteria multiply quickly and cause death and collapse of nearby cells. Bacteria move quickly through the intercellular spaces and also through the macerated middle lamella and flower cells. In some cases, fairly large cavities form that are filled with bacteria. From the flower, bacteria move down thepedicel into the fruit spur. Infection of the spur results in the death of all flowers, leaves, and fruit on it Penetration and invasion of leaves are similar to those of flowers. Bacteria may enter through stomata and hydathodes, but usually they enter through wounds made by insects, hail storms, bacteria move rapidly from the vessels to other tissues, killing cells, and causing blight and canker symptoms in the process. Invasion of large twigs and branches is restricted primarily to the cortex. Infection of succulent tissues is rapid under warm, humid conditions. bacteria may progress from spurs or shoots into the second-year, third-year, and older growth, killing the bark all along the way. 6

57 Dr.Ahed A.Hadi lec.13 PLANT DISEASES CAUSED BY VIRUSES Virus is a nucleoprotein that multiplies only in living cells and has the ability to cause disease. All viruses parasitize cells and cause a multitude of diseases in all forms of living organisms. A plant may sometimes be infected by more than one kind of virus at the same time. Virus particles (known as virions) consist of two or three parts: i) the genetic material made from either DNA or RNA, long molecules that carry genetic information; ii) a protein coat that protects these genes; and in some cases iii) an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simple helical and icosahedral forms to more complex structures. CHARACTERISTICS OF PLANT VIRUSES In general, there are three main morphological virus types: 1- Helical These viruses are composed of a single type of capsomer stacked around a central axis( nucliec acid) to form a helical structure, This arrangement results in rod-shaped or filamentous virions. 7

58 2- Icosahedral icosahedral or near-spherical. A regular icosahedron is the optimum way of forming a closed shell from identical sub-units. Pentose (5) and hexons(6) 3-Complex These viruses possess a capsid that is neither purely helical nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall. Some bacteriophages. Structure of tobacco mosaic virus: RNA coiled in a helix of repeating protein sub-units Electron micrograph of icosahedral adenovirus Herpes viruses have a lipid envelope 8

59 FIGURE Electron micrographs of the various shapes of plant viruses. (A) Rod-shaped virus (tobacco mosaic virus) (36,000 ). (B) Flexuous thread virus (sugarcane mosaic virus) (80,000 ). (C) Isometric virus (cowpea chlorotic mottle virus) (100,000 ). TRANSMISSION OF PLANT VIRUSES Viruses can not moved there are tow type of Plant viruses transmission 1-Transmission between plant cells Viral infections often develop into systemic infections as a means of transmission. The virus often infects many tissues, if not the whole plant, where it can continue to replicate. There are a variety of methods the virus can use to spread throughout the organism but the most common route utilise the vascular 9

60 system, otherwise known as the xylem and phloem, and the plasmodesmata, which interconnect adjacent cells. 2- transmission between plant hosts viruses are transmitted from plant to plant in a number of ways. Modes of transmission include: vegetative propagation, mechanically through sap, through seed, pollen, dodder, and by specific insects, mites, nematodes, and fungi.( Vectors that feed on plant sap). By budding By grafting By cutting By bulbs By corms By stolons By rhizomes By tubers Through natural root grafts Through dodder FIGURE: Transmission of viruses, mollicutes, and other pathogens through vegetative propagation, natural root grafts, and dodder. 0

61 Symptoms around the site of virus inoculation are denoted local symptoms. When virus spreads from the site of inoculation and to other parts of the plant, this is referred to as systemic symptoms. Some symptoms leaf color. In this case leaves can show more severe symptoms such as leaf color and shape, Mottle(abnormal coloration), Mosaic( green, yellow parts on leaf) leaf distortion (e.g. curling) and/or other growth distortions (e.g. stunting of the whole plant, abnormalities in flower or fruit formation). Viruses cause many important plant diseases and are responsible for huge losses in crop production and quality in all parts of the world. Such as - Tobacco Mosaic Virus (TMV) - Cucumber Mosaic Virus (CMV) - Grapevine fan leaf Virus (GFLV) - Citrus tristeza virus (CTV) - Potato leaf curly Virus Yellow mosaic symptoms on lettuce caused by Lettuce mosaic virus. 1

62 Yellow vein-banding symptoms on grapevine caused by Grapevine fanleaf virus. Fruit distortion on eggplant fruit caused by Tomato bushy stunt virus. A healthy fruit is shown on the left. Infection does not always result in visible symptoms (as witnessed by names such as Carnation latent virus. virus infection can result in symptoms of ornamental value, such as 'breaking' of tulips. - Control 2

63 Plant viruses cannot be directly controlled by chemical application. The major means of control (depending on the disease) include: Chemical or biological control of the vector (the organism transmitting the disease, often an insect). Growing resistant crop varieties Use of virus-free planting material: in vegetatively propagated crops (e.g. potatoes, many fruit crops) and where viruses are transmitted through seed major efforts are made through breeding, certification schemes etc., to ensure that the planting material is virus-free. Exclusion: the prevention of disease establishment in areas where it does not yet occur. This is a major objective of plant quarantine procedures throughout the world as well as more local schemes. 3

64 shapes of viruses structure 4

65 Dr.Ahed A.Hadi lec.14 PLANT DISEASES CAUSED BY NEMATODA Nematodes belong to the kingdom Animalia. Nematodes are wormlike in appearance. Life Cycles The life histories of most plant parasitic nematodes are, in general, quite similar. Eggs hatch into juveniles, whose appearance and structure are usually similar to those of the adult nematodes. Juveniles grow in size, and each juvenile stage is terminated by a molt. All nematodes have four juvenile stages, with the first molt usually occurring in the egg. After the final molt the nematodes differentiate into males and females. The female can then produce fertile eggs either after mating with a male or, in the absence of males, parthenogenetically. A life cycle from egg to egg may be completed within 2 to 4 weeks under optimum environmental, especially temperature, conditions but will take longer in cooler HOW NEMATODES AFFECT PLANTS The direct mechanical injury inflicted by nematodes while feeding causes only slight damage to plants. Most of the 5

66 Juvenile and adult ectoparasitic ring nematodes feeding on root. damage seems to be caused by a secretion of saliva injected into the plants while the nematodes are feeding. as the females of species that become established in or on roots permanently, The feeding process causes the affected plant cells to react, resulting in dead or devitalized root tips and buds, lesion formation and tissue breakdown, swellings and galls of various kinds, and crinkled and distorted stems and foliage. Some of these manifestations are caused by the dissolution of infected 6

67 tissues by nematode enzymes, which, with or without the help of toxic metabolites, cause tissue disintegration and the death. NEMATODA help other pathogen ( fungi, bacteria,viruses) to penetrate an infect plants - PLANT NEMATODA have Stylet use in penetration and feeding. ROOT-KNOT NEMATODES: MELOIDOGYNE SPP. Symptoms Aboveground symptoms are reduced growth and fewer, small, pale green, or yellowish leaves that tend to wilt in warm weather. Blossoms and fruits are few and of poor quality or dead. Characteristic symptoms of the disease appear on the underground parts of the plants. Infected roots develop the typical root-knot galls that are two to several times as large in diameter as the healthy root. Several infections along the root give the root a rough, clubbed appearance. Roots infected by certain species of the nematode also develop a bushy root. The Pathogen: Meloidogyne spp. The males are wormlike and about 1.2 to 1.5 millimeters long by 30 to 36 micrometers in diameter. The females are pear shaped and about 0.40 to 1.30 millimeters long by 0.27 to 0.75 millimeters wide. 7

68 Each female lays approximately 500 eggs in a gelatinous substance Development of Disease Second-stage juveniles enter roots behind the root tip Cells near the path of the juveniles begin to enlarge. Two or 3 days after the juvenile has become established, some of the cells around its head begin to enlarge. Their nuclei divide, but no cell walls are laid down. The existing walls between some of the cells break down and disappear, giving rise to giant cells, which are due to substances contained in the saliva secreted by the nematode in the giant cells during feeding. and produce their egg sacs, they push outward, split the cortex, and may become exposed on the surface of the root or remain completely covered, depending on the position of the nematode in relation to the root surface. In addition to the disturbance caused to plants by the nematode galls themselves, damage to infected plants is frequently increased by certain parasitic fungi, which can easily attack the weakened root tissues and the hypertrophied, undifferentiated cells of the galls. Moreover, some fungi, e.g., Fusarium, Rhizoctonia, and the oomycete Pythium, grow and reproduce much faster in the galls than in other areas of the root, thus inducing an earlier breakdown of the root tissues. 8

69 II Stage juveniles invade rootlet and cause formation of giant cells Adult nematodes. Male leaves root II Stage juvenile Attack rootlets II Stage juvenile free in soil II Stage juvenile I Stage juvenile egg Female lays eggs into egg sac Disease cycle of root knot caused by nematodes of the genus Meloidogyne. Control - Root knot can be controlled effectively in the greenhouse with steam sterilization of the soil or soil fumigation - In the field the best control of root knot is obtained by fumigating the soil with approved chemical nematicides. - varieties resistant to root-knot nematodes are also available. - Several cultural practices, such as crop rotation, fallow soil, soil solarization, and certain soil amendments, are also helpful in reducing root-knot losses. 9

70 - Biological control of root knot has been obtained experimentally by treating nematode infested soil with endospores of the bacterium Pasteuria penetrans, which is an obligate parasite of some plant parasitic nematodes, or with preparations of the fungus Trichoderma harzianum; by treating transplants or infested soils with spores of the fungus Dactylella oviparasitica, which parasitizes the eggs of Meloidogyne nematodes; and in some experiments by treating transplants or infested soils with spores of the vesicular arbuscular mycorrhizal fungi Gigaspora and Glomus. Fairly good experimental control of root knot - also been obtained by mixing essential oils from plant spices into nematode-infested soil before planting and through an increase in plants of their local and systemic-induced resistance to root knot nematodes by mixing in the soil or spraying the plants with amino-butyric acid and other amino acids. 0

71 Dr.Ahed A.Hadi lec.15 CONTROL OF PLANT DISEASES The various control methods can be classified as regulatory, cultural, biological, physical, and chemical, depending on the nature of the agents employed. 1- Quarantines and Inspections When plant pathogens are introduced into an area in which host plants have been growing in the absence of the pathogen, such introduced pathogens may cause much more catastrophic epidemics than the existing endemic pathogens. Similar quarantine regulations govern the interstate, and even intrastate, sale of nursery stock, tubers, bulbs, seeds, and other propagative organs, especially of certain crops such as potatoes and fruit trees. The movement and sale of such materials within and between states are controlled by the regulatory agencies of each state. 2- Use of Pathogen-Free Propagating Material When a pathogen is excluded from the propagating material (seed, tubers, bulbs, nursery stock) of a host. Here we can used clean, uninfected or treated propagating material to prevent the diseases appearance. 3- Cultural Methods That Eradicate or Reduce the Inoculum - Crop Rotation 1

72 Soilborne pathogens that infect plants of one or a few species or even families of plants can sometimes be reduced in the soil by planting, for 3 or 4 years, crops belonging to species or families not attacked by the particular pathogen. - Sanitation Sanitation consists of all activities aimed at eliminating or reducing the amount of inoculum present in a plant, a field, or a warehouse and at preventing the spread of the pathogen to other healthy plants and plant products. Thus, plowing under infected plants after harvest, such as leftover infected fruit, stems, tubers, or leaves, helps cover the inoculum with soil and speeds up its disintegration (rotting) and concurrent destruction of most pathogens carried in or on them. Similarly, removing infected leaves of house or garden plants and Pruning infected plants helps remove or reduce the inoculum. - Creating Conditions Unfavorable to the Pathogen Stored products should be aerated properly to hasten the drying of their surfaces and inhibit germination and infection by any fungal or bacterial pathogens present on them. Similarly, spacing plants properly in the field or greenhouse prevents the creation of high-humidity conditions on plant surfaces and inhibits infection by certain pathogens, such as Botrytis and Peronospora tabacina. 2

73 4- Biological Methods That Eradicate or Reduce the Inoculum Biological control of pathogens: the total or partial destruction of pathogen populations by other organisms, occurs routinely in nature. The mycelium and resting spores (oospores) or sclerotia of several phytopathogenic soil oomycetes and fungi such as Pythium, Phytophthora, Rhizoctonia, Sclerotinia, and Sclerotium are invaded and parasitized (mycoparasitism) or are lysed (mycolysis) by several fungi, which as a rule are not pathogenic to plants. Several nonplant pathogenic oomycetes and fungi, including some chytridiomycetes and hyphomycetes, and some pseudomonad and actinomycetous bacteria infect the resting spores of several plant pathogenic fungi. Among the most common mycoparasitic fungi are Trichoderma sp., mainly T. harzianum. 3

74 D C Attachment of the yeast biocontrol agent Pichia guilliermondii on hyphae of the plant pathogenic fungi Botrytis cinerea (A) and Penicillium expansum (B). ( C)Hypha of a nonpathogenic species of Pythium (P. nunn) penetrating (arrow) a hypha of the pathogenic fungus Phytophthora (D) fungi parasite on pathogenic nematoda 4