Germination of Spores and Seeds Almost all pathogens in their vegetative state are capable of initiating infection immediately. Fungal spores and seeds of parasitic higher plants, however, must first germinate. Spores germinate by producing a typical mycelium that infects and grows into host plants or they produce a short germ tube that produces a specialized infectious structure, the haustorium. In order to germinate, spores require a favorable temperature and also moisture in the form of rain, dew, or a film of water on the plant surface or at least high relative humidity. Hatching of Nematode Eggs Nematode eggs also require conditions of favorable temperature and moisture to become activated and hatch. In most nematodes, the egg contains the first juvenile stage before or soon after the egg is laid. This juvenile immediately undergoes a molt and gives rise to the second juvenile stage, which may remain dormant in the egg for various periods of time. Thus, when the egg finally hatches, it is the second-stage juvenile that emerges, and it either finds and penetrates a host plant or undergoes additional molts that produce further juvenile stages and adults.
Penetration Pathogens penetrate plant surfaces by direct penetration of cell walls, through natural openings, or through wounds. Some fungi penetrate tissues in only one of these ways, others in more than one. Bacteria enter plants mostly through wounds, less frequently through natural openings, and never directly through unbroken cell walls. Viruses, viroids, mollicutes, fastidious bacteria, and protozoa enter through wounds made by vectors, although some viruses and viroids may also enter through wounds. Parasitic higher plants enter their hosts by direct penetration. Nematodes enter plants by direct penetration and, sometimes, through natural openings. Penetration does not always lead to infection (penetration in plants that are not susceptible) Direct Penetration through Intact Plant Surfaces Direct penetration through intact plant surfaces is probably the most common type of penetration by fungi, oomycetes, and nematodes and the only type of penetration by parasitic higher plants. None of the other pathogens can enter plants by direct penetration. Penetration through Natural Openings Many fungi and bacteria enter plants through stomata, and some enter through hydathodes, nectarthodes, and Lenticels. Stomata are most numerous on the lower side of leaves. Bacteria present in a film of water over a stoma and, if water soaking occurs, can swim through the stoma easily and into the substomatal cavity where they can multiply and start infection. Fungal spores generally germinate on the plant surface, and the germ tube may then grow through the stoma. Hydathodes are more or less permanently open pores at the margins and tips of leaves; they are connected to the veins and secrete droplets of liquid, called guttation drops, containing various nutrients. Some bacteria use these pores as a means of entry into leaves, but few fungi seem to enter plants through hydathodes. Some bacteria also enter blossoms through the nectarthodes or nectaries. Lenticels are openings on fruits, stems, and tubers that are filled with loosely connected cells that allow the passage of air. During the growing season, lenticels are open, but even so, relatively few fungi and bacteria penetrate tissues through them.
Infection Infection is the process by which pathogens establish contact with susceptible cells or tissues of the host and procure nutrients from them. Following infection, pathogens grow, multiply, or both within the plant tissues and invade and colonize the plant to a lesser or greater extent. Successful infections result in the appearance of symptoms. Some infections, however, remain latent, i.e., they do not produce symptoms right away but at a later time when the environmental conditions or the stage of maturity of the plant become more favorable. Symptoms may appear as soon as 2 to 4 days after inoculation, as happens in some localized viral diseases of herbaceous plants, or as late as 2 to 3 years after inoculation, as in the case of some viral, mollicute, and other diseases of trees. In most plant diseases, however, symptoms appear from a few days to a few weeks after inoculation. The time interval between inoculation and the appearance of disease symptoms is called the incubation period. The length of the incubation period of various Diseases varies with the particular pathogen host combination, with the stage of development of the host, and with the temperature in the environment of the infected plant. Infection requirements: 1- The plant variety must be susceptible to the particular pathogen and at a susceptible stage. 2- The pathogen must be in a pathogenic stage that can infect immediately without requiring a resting (dormancy) period first, or infective juvenile stages or adults of nematodes. 3- The temperature and moisture conditions in the environment of the plant must favor the growth and multiplication of the pathogen.
Invasion Various pathogens invade hosts in different ways and to different extents. Fungi 1. Subcuticular colonization: Some fungi produce mycelium that grows only in the area between the cuticle and the epidermis (apple scab and black spot of rose) 2. Superficials colonization: produce mycelium only on the surface of the plant but send haustoria into the epidermal (powdery mildews) Most fungi spread into all the tissues of the plant organs (leaves, stems, and roots) 3. Intracellular mycelium: produce mycelium growing directly through the cells 4. Intercellular mycelium: produce mycelium growing between the cells 5. Vascular wilt Fungi: produce mycelium invade the xylem vessels of plants and cause wilts Bacteria invade tissues intercellulary, when parts of the cell walls dissolve, bacteria also grow intracellularly. Bacteria causing vascular wilts, invade the xylem vessels. Nematodes invade tissues intercellularly, but some can invade intracellularly as well. Many nematodes do not invade cells or tissues at all but feed by piercing epidermal cells with their stylets. Viruses, viroids, mollicutes, fastidious bacteria, and protozoa invade tissues by moving from cell to cell intracellularly. Viruses and viroids invade all types of living plant cells, mollicutes and protozoa invade phloem sieve tubes and perhaps a few adjacent phloem parenchyma cells, most fastidious bacteria invade xylem vessels and a few invade only phloem sieve tubes. tissues throughout the plant.
Invasion Local Many infections caused by fungi, bacteria, nematodes, viruses, and parasitic higher plants are local, they involve a single cell, a few cells, or a small area of the plant. These infections may remain localized throughout the growing season or they may enlarge slightly or very slowly. Systemic Other infections enlarge more or less rapidly and may involve an entire plant organ (flower, fruit, leaf), a large part of the plant (a branch), or the entire plant. Infections caused by fastidious xylem- or phloem inhabiting bacteria, mollicutes, and protozoa and natural infections caused by viruses and viroids are systemic, the pathogen, from one initial point in a plant, spreads and invades most or all susceptible cells and tissues throughout the plant.
Growth and Reproduction of the Pathogen (Colonization) Individual fungi and parasitic higher plants generally invade and infect tissues by growing on or into them from one initial point of inoculation. Most of these pathogens, whether inducing a small lesion, a large infected area, or a general necrosis of the plant, continue to grow and branch out within the infected host indefinitely so that the same pathogen individual spreads into more and more plant tissues until the spread of the infection is stopped or the plant is dead. fungi causing vascular wilts often invade plants by producing and releasing spores within the vessels, and as the spores are carried in the sap stream they invade vessels far away from the mycelium, germinate there, and produce a mycelium, which invades more vessels. The bacteria, mollicutes, viruses, viroids, nematodes, and protozoa, do not increase much, if at all, in size with time, These pathogens invade and infect new tissues within the plant by reproducing at a rapid rate and increasing their numbers tremendously in the infected tissues. Plant pathogens reproduce in a variety of ways. Fungi reproduce by means of spores, which may be either asexual (mitospores, i.e., products of mitosis, roughly equivalent to the buds on a twig or the tubers of a potato plant), or sexual (meiospores, i.e. products of meiosis, roughly equivalent to the seeds of plants). Parasitic higher plants reproduce just like all plants, i.e., by seeds. Bacteria and mollicutes reproduce by fission in which one mature individual splits into two equal, smaller individuals. Viruses and viroids are replicated by the cell, just as a page placed on a photocopying machine. The rate of reproduction varies considerably among the various kinds of pathogens
A) on the underside of a grape leaf infected with the grape downy mildew pathogen Plasmopara viticola and (B) on the root of a lettuce plant infected with Plasmopara lactucae-radicis.
Dissemination of the Pathogen A few pathogens, such as nematodes, oomycetes, zoosporic fungi, and bacteria, can move short distances on their own power and thus can move from one host to another one very close to it. Fungal hyphae can grow between tissues in contact and sometimes through the soil toward nearby roots for a few to many centimeters. Both of these means of dissemination, are quite limited, especially in the case of zoospores and bacteria. The spores of some fungi are expelled forcibly from the sporophore or sporocarp by a squirting or puffing action that results in the successive or simultaneous discharge of spores up to a centimeter or so above the sporophore. The seeds of some parasitic plants are also expelled forcibly and may arch over distances of several meters. Almost all dissemination of pathogens is carried out passively by such agents as air and insects.to a lesser extent, water, certain other animals, and humans may be involved. Dissemination by Air While airborne, some of the spores may touch wet surfaces and get trapped; when air movement stops or when it rains, the rest of the spores land or are washed out from the air and are brought down by the raindrops. The spores of other fungi, particularly those of the cereal rusts, are very hardy and occur commonly at all levels and at high altitudes (several thousand meters) above infected fields. Spores of these fungi are often carried over distances of several kilometers, even hundreds of kilometers. Dissemination by Water, Insects, Mites, Nematodes, and Other Vectors Dissemination by Pollen, Seed, Transplants, Budwood, and Nursery Stock Dissemination by Humans some plant pathogens, e.g., the zoospores of some fungi and certain parasitic plants, can transmit viruses as they move from one plant to another (zoospores) or as they grow and form a bridge between two plants (dodder).
Overwintering and/or Oversummering of Pathogens perennial plants: Pathogens can survive in plants but the tops of some perennial plants die, Annual plants: die at the end of the growing season, Their pathogens must be able to survive such periods in the absence of their hosts. Thus, They have evolved mechanisms by which they can survive the cold winters or dry summers; Fungi fungi overwinter -in plants as mycelium, resting spores, sclerotia (hard masses of mycelium) -On perennial plants, in diseased tissues, cankers, buds, on fallen, infected leaves or fruits -In annual plants survive the winter or summer as mycelium in infected plant debris, -in the soil, as mycelium, spores, or sclerotia; Some plant pathogens are soil inhabitants and they are able to survive indefinitely as saprophytes. Soil inhabitants are generally unspecialized parasites that have a wide host range. Other fungi are soil transients, i.e., they are rather specialized parasites that generally live in close association with their host but may survive in the soil for relatively short periods of time -in or on seeds and other propagative organs, such as tubers. -In some areas, fungi survive by continuous infection of host plants grown throughout the year, such as cabbage, or of plants grown in the greenhouse -on winter crops grown in warmer climates and move from them; -Some rust fungi infect alternately an annual and a perennial host, and the fungus goes from the one to the other host and overwinters in the perennial host.
Bacteria overwinter and oversummer as bacteria in essentially the same ways as described for fungi: in infected plants, seeds, and tubers, in infected plant debris or in the soil. Bacteria survive poorly when present in small numbers and free in the soil but survive well when masses of them are embedded in the hardened, slimy polysaccharides that usually surround them. Some bacteria also overwinter within the bodies of their insect vectors. Viruses, viroids, mollicutes, fastidious bacteria, and protozoa survive only in living plant tissues such as the roots of perennial plants, vegetative propagating organs, and the seeds of some hosts. A few viruses survive within their insect vectors, and some viruses and viroids may survive on contaminated tools and in infected plant debris. Nematodes usually overwinter or oversummer as eggs in the soil and as eggs or nematodes in plant roots or in plant debris. Some nematodes produce juvenile stages or adults that can remain dormant in seeds or on bulbs for many months or years. Parasitic higher plants survive either as seeds, usually in the soil, or as their infective vegetative form on their host.
DISEASE CYCLES and EPIDEMICS Monocyclic pathogens: Some pathogens complete only one disease cycle in 1 year monocyclic or single-cycle pathogens: smuts, rusts, which require two alternate hosts, many soil-borne diseases, root rots and vascular wilts. In monocyclic pathogens the primary inoculum is the only inoculum available for the entire season, and there is no secondary inoculum and no secondary infection. Polycyclic pathogens: In most diseases, the pathogen has more than one generation per growth season, polycyclic, or multicyclic pathogens. 2 to 30 disease cycles per year. with each cycle the amount of inoculum is multiplied. Polycyclic pathogens are disseminated primarily by air or airborne vectors (insects) and are responsible for epidemics on most crops, downy mildews, late blight of potato, powdery mildews, leaf spots and blights, grain rusts, and insectborne viruses. In polycyclic fungal pathogens, the primary inoculum often consists of the sexual (perfect) spore or sclerotia. once primary infection takes place, large numbers of asexual spores (secondary inoculum) are produced at each infection site and these spores can themselves cause new (secondary) infections that produce more asexual spores for more infections. polyetic (multiyear) pathogens: In some diseases of trees, fungal vascular wilts, phytoplasmal declines, and viral infections, the infecting pathogen may not complete a disease cycle, it may not produce inoculum that can be disseminated and initiate new infections, until at least the following year and some may take longer. Such diseases are basically monocyclic, but if they take more than a year to complete the cycle, they are called polyetic (multiyear). several rusts of trees and the mistletoes (Viscum album,(دارواش that take several years to go through all the stages of their life cycle and to initiate new infections. Dutch elm disease, cedar apple rust, white pine blister rust, and citrus tristeza.
EFFECTS of PATHOGENS on PLANT PHYSIOLOGICAL FUNCTIONS While pathogens infect plants in the course of their obtaining food for themselves, depending on the kind of pathogen and on the plant organ and tissue they infect, pathogens interfere with the different physiological functions of the plant and lead to the development of different symptoms. 1. Effect of pathogens on photosynthesis Chlorosis, necrosis on green plant parts, reduce growth and amounts of fruits In some fungal and bacterial diseases, photosynthesis is reduced because of toxins, such as tentoxin and tabtoxin. 2. Effect of pathogens on translocation of water and nutrients in the host plant When a pathogen interferes with the upward movement of inorganic nutrients and water or with the downward movement of organic substances, diseased conditions result in the parts of the plant denied these materials. The diseased parts, in turn, will be unable to carry out their own functions and will deny the rest of the plant their services or their products, thus causing disease of the entire plant. 3. Effect of pathogens on host plant respiration Many pathogens, such as damping-off fungi, root-rotting fungi and bacteria, most nematodes, and some viruses, cause an extensive destruction of the roots before any symptoms appear on the aboveground parts of the plant. Some bacteria and nematodes cause root galls or root knots which interfere with the normal absorption of water and nutrients by the roots. Root injury affects the amount of functioning roots directly and decreases proportionately the amount of water absorbed by the roots. Some vascular parasites, along with their other effects, seem to inhibit root hair production, which reduces water absorption. 4. Effect of pathogens on permeability of cell membranes Changes in cell membrane permeability are often the first detectable responses of cells to infection by pathogens, to most host-specific and several nonspecific toxins, to certain pathogen enzymes, and to certain toxic chemicals, such as air pollutants. The most commonly observed effect of changes in cell membrane permeability is the loss of electrolytes, i.e., of small water-soluble ions and molecules from the cell. P105 chapter3
5. Effect of pathogens on transcription and translation Transcription of cellular DNA into messenger RNA and translation of messenger RNA to produce proteins are two of the most basic, general, and precisely controlled processes in the biology of any normal cell. They vary with the stage of development and the requirements of each cell. Nevertheless, disturbance of any one of these processes, by pathogens or environmental factors, may cause drastic, unfavorable changes in the structure and function of the affected cells by its effect on the expression of genes. Several pathogens, particularly viruses and fungal obligate parasites, such as rusts and powdery mildews, affect the transcription process in infected cells. In some cases, pathogens affect transcription by changing the composition, structure, or function of the chromatin associated with the cell DNA. In some diseases, especially those caused by viruses, the pathogen, through its own enzyme or by modifying the host enzyme (RNA polymerase) that makes RNA, utilizes the host cell nucleotides and machinery to make its own (rather than host) RNA. 6. Effect of pathogens on plant growth Generaly reduced growth, some times increased growth and some abnormal growths such as crown gall, stem gall, leafy gall, hairy root, knot root. 7. Effect of pathogens on plant reproduction such plants remain smaller in size, may produce fewer flowers, and may set fewer fruit and seeds; the latter may be of inferior vigor and vitality and, therefore, if planted, they may produce fewer and weaker new plants. many pathogens have a direct adverse effect on plant reproduction because they attack and kill the flowers, fruit, or seed directly, or interfere and inhibit their production,
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HOW PATHOGENS ATTACK PLANTS 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 to penetrate. In some cases an adhesion pad forms from the spore when it comes in contact with a moist surface, and cutinase and cellulase enzymes released from the spore surface help the spore adhere to the plant surface. Spores of some fungi carry adhesive substances at their tips that, on hydration, allow spores to become attached to various surfaces. Appressorium fastens the pathogen to the plant. Penetration of plant barriers by fungi and parasitic higher plants is almost always assisted by the presence of enzymes secreted by the pathogen at the penetration site, resulting in the softening or dissolution of the barrier. Nematodes penetrate plant surfaces by means of the stylet. Considerable mechanical force is also exerted on host tissues from the inside out by some pathogenic fungi on formation of their fructifications in the tissues beneath the plant surface. CHEMICAL WEAPONS OF PATHOGENS Although some pathogens may use mechanical force to penetrate plant tissues, the activities of pathogens in plants are largely chemical in nature. Therefore, the effects caused by pathogens on plants are almost entirely the result of biochemical reactions taking place between substances secreted by the pathogen and those present in, or produced by, the plant. The main groups of substances secreted by pathogens in plants that seem to be involved in production of disease, either directly or indirectly, are enzymes, toxins, growth regulators, and polysaccharides. In general, plant pathogenic enzymes disintegrate the structural components of host cells, break down food substances in the cell, or affect components of its membranes and the protoplast directly. Toxins seem to act directly on protoplast components and interfere with the permeability of its membranes and with its function. Growth regulators exert a hormonal effect on the cells and either increase or decrease their ability to divide and enlarge. Polysaccharides seem to play a role only in the vascular diseases, in which they interfere passively with the translocation of water in the plants.
HOW PATHOGENS ATTACK PLANTS Enzymes in Plant Disease enzyme, being a protein, is coded for by a specific gene. Some enzymes are present in cells at all times (constitutive). Many are produced only when they are needed by the cell in response to internal or external gene activators (induced). Enzymatic Degradation of Cell Wall Substances Aerial plant part surfaces consist primarily of cuticle and/or cellulose, whereas root cell wall surfaces consist only of cellulose. Cuticle consists primarily of cutin, more or less impregnated with wax and frequently covered with a layer of wax. The lower part of cutin is intermingled with pectin and cellulose lamellae and lower yet there is a layer consisting predominantly of pectic substances; below that there is a layer of cellulose. Polysaccharides of various types are often found in cell walls. Proteins of many different types, both structural, e.g., elastin, which helps loosen the cell wall, and extensin, which helps add rigidity to the cell wall, some enzymes, and some signal molecules that help receive or transmit signals inward or outward, are normal constituents of cell walls. Finally, epidermal cell walls may also contain suberin and lignin. The penetration of pathogens into parenchymatous tissues is facilitated by the breakdown of the internal cell walls, which consist of cellulose, pectins, hemicelluloses, and structural proteins, and of the middle lamella, which consists primarily of pectins. In addition, complete plant tissue disintegration involves the breakdown of lignin. The degradation of each of these substances is brought about by the action of one or more sets of enzymes secreted by the pathogen.