Viruses, bacteria, nematodes and Fungi as agents of plant diseases.

Transcription

1 DEPARTMENT OF BIOLOGICAL SCIENCES REDEEMER S UNIVERSITY BIO 405 : PLANT PATHOLOGY LECTURE NOTES BY DURUGBO ERNEST UZODIMMA (Ph.D.) COURSE OUTLINE Introduction Infection and host parasite relationships. Viruses, bacteria, nematodes and Fungi as agents of plant diseases. The structure, reproduction/life cycles and classification of plant pathogenic fungi, bacteria and viruses. Methods of isolation of pathogens in pure state and their classification Koch s postulate Diseases of some economic plants (particularly food crops) in Nigeria. Symptoms, etiology, cultural characteristics and control measures 1

2 INTRODUCTION Plant pathology is coined from three Greek words Phyton meaning plant, pathos suffering, and logos knowledge So, literally plant pathology deals with the knowledge or study of plant suffering (diseases). The study of the diseases of plants covers the entire field of biological and scientific activity concerned with the understanding of this complex phenomenon. Phytopathology is the study of nature, development, and control of plant diseases. Disease is an interaction among the host, parasite, and environment. According to the dictionary plant disease means any departure from health, presenting marked symptoms, malady, illness and disorder. Diseased plants are distinguished by changes in their morphological structures or physiological processes, which are brought about by unfavourable environment or by parasitic agencies. Stakman and Harrar (1957) defined plant diseases as a physiological disorder or structural abnormality that is harmful to the plant or to any of its parts or products that reduces the economic value. Agrios (1997) defined diseases in plants as series of invisible and visible responses of plant cells and tissues to a pathogenic micro-organism or environmental factors that results in adverse changes in the form, function, or integrity of the plant and may lead to partial impairment or death of the plant or its parts. WHY STUDY PLANT PATHOLOGY AS A MICROBIOLOGIST i) Man is dependent on plants and their products for food, both for himself and his livestock. Lack of food or a decrease in the quality of the food due to disease would have adverse effects. Therefore, there s need to study these plant diseases. ii) Microbiological research would help in the improvement of food crops and long term storage of their products. iii) Among the different occupations is farming through which people earn income, so when their plants or their products are diseased, there would be economic losses. iv) Plants are sources of such important products as gums, dyes, oils, clothing, wine, alcohol, and drugs. There is therefore need to maintain good health of these plants. v) As a scientist, the quest to investigate what is happening around us also encourages study of plant diseases. vi) Many nations depend on plants and their products for international trade such as timber, cocoa, oils, rice, rubber etc, hence the need to study these resources and if need be improve them for better yields. 2

3 vii) Plants are also used as ornamentals to beautify the environment which if diseased will not be useful in this regard. viii) As a scientist the quest to investigate what is happening around us also encourages studies of plant diseases. CAUSE OF DISEASES Variation in the environment could make a plant liable to infection. Here, the microorganism is not the sole causal factor. After a microorganism has successfully infected a susceptible plant, the subsequent interaction between the plant and microorganism which results in disease is also subject to environmental influences. Most causal organisms are parasites. A parasite is not synonymous with the pathogen. At times, organisms are parasitic without becoming causal factors in the disease. Many parasitic establishments do not become pathogenic if the parasite compensates for the damage done by its presence. For example, the mycorrhizal fungus is parasitic on the roots of trees, but it is not pathogenic. Another example is the root nodule bacterium (Rhizobium leguminosarum). Infection and host-parasite relationship Pathogenesis caused by an infectious agent begins as soon as the pathogen comes in contact with any portion of a susceptible plant. Most of the bacterial and fungal pathogens come into contact with their hosts plants by insects, nematodes, and other vectors. Motile propagules of fungal pathogens are attracted to the root exudates such as the zoospore of many plant pathogens (Hickman & Ho, 1966). Attraction and accumulation of zoospores and nematodes to the roots in the soil behind the root tip has been demonstrated. Recognition between Host and Pathogen It s not still clear how pathogens recognize their hosts and vice versa. Recognition plays a central role in the interactions between plants and their pathogens. These pathogens must be able to recognize the presence of the host plant in the environment and often must recognize specific surface features of their hosts in order to effect successful penetration and infection. Successful pathogens must be able to recognize and overcome plant defense responses. Plants have also evolved sophisticated mechanisms to detect the multitude of potential pathogens in their environment and to activate diverse defense mechanisms. Research has been geared towards identifying and characterizing the molecules involved in recognition between host and pathogen. Numerous host components act as signals for the activation of pathogens. i) They include fatty acids of the plant host cuticle which activates the production of cutinase enzymes which breakdown cutin of the host galacturonan molecules of pectin which stimulate the production of pectin lyase enzymes by the fungal or bacterial pathogen. ii) Others are phenolic compounds which could stimulate the activation of and germination of propagules of some pathogens iii) Phenolics and sugars released from plant wounds that activate a series of genes in certain pathogens leading to infection. 3

4 iv) The action of the elicitors is also noteworthy. Elicitor refers to molecules that stimulate any plant defense mechanism, originally the word was used for molecules and other stimuli that induced the synthesis and accumulation of antimicrobial compounds (phytoalexins) in plant cells. Examples of plant defense mechanisms induced by elicitors include: - The synthesis and accumulation of antimicrobial phytoalexins - The production of glycosyl hydrolases capable of attacking surface polymers of pathogens - The synthesis of proteins that inhibit degradative enzymes produced by pathogens - The production of activated oxygen species and the modification of plant cell walls by the deposition of callose hydroxy proline rich glycoproteins and / or lignin HOST PENETRATION AND INFECTION When a pathogen comes in contact with a host plant, a complex of interacting factors including temperature, moisture, susceptibility of plant tissues, effects of other microorganisms present, aggressiveness of pathogen, etc determine whether infection will occur or not. Pathogenity is the property of a micro-organism whereby it becomes a part f the causal complex. Pathogenity is a process or the chain of events by which disease development takes place. These events include: Pre penetration Penetration Post penetration or further colonization of plant by the pathogen. Pre- penetration In fungal pathogens, this includes spore germination and growth of the resulting tube on the surface of the host plant. Fungal invasion is mostly by germ tubes or structures derived from them. There are 2 well known situations involving hyphae of Rhizoctonia solani often aggregate to form an infection cushion on from which multiple penetrations occurs by means of appressoria and penetration pegs. Again the rhizomorphs of Armillaria mellea penetrates intact root periderm. Penetration by bacteria and viruses is achieved in various ways Sporangia of Chytridiomycetes and Oomycetes and parasitic slime moulds produce zoospores in wet condition, and germ tubes in dry conditions. Uredinales (Rusts) and Ustilaginales (Smuts) normally germinate by producing a promycelium on which basidiospores are borne. Budding and fission are characteristics of some yeasts and bacteria, although conidia, ascospores (e.g Taphrina sp.) and basidiospores (Ustilaginales) may bud under certain conditions. 4

5 In order to procure food, plant pathogens have to establish an intimate connection with their hosts; i.e. they have to enter into tissues or send haustoria into surface cells. Entry is mostly through germ tube from spores but can also often be accomplished by mycelial hyphae. The pathogen invades plants through natural openings (stomata, lenticels, nectarthods, hydathodes). - Through wounds (natural or due to insects or nematodes) - Direct penetration In Uredinales (Rusts), the basidiospores penetrate directly while uredospores generally invade through the stomata. Entry through natural openings 5

6 Such fungi as Puccinia graminis tritici (stem rust fungus) enter through the stoma. Urediniospores of the pathogen will germinate in water on a leaf surface in an hour under optimum conditions. The germ tube from the uredospore grows over the leaf and approaches the stoma. The protoplasm of the germ tube accumulates at the tip to leave the older parts empty. Lenticels In many dicots, a protective layer (periderm) develops as the epidermis becomes redundant. It also forms on surfaces exposed by abscission of plant parts or on surfaces produced by the wounding of tissues, or surfaces of such storage tissues as potato tubers. Streptomyces scabies which causes common scab of potato, enters young developing tubers through the stomata or lenticels. Oospora pustulans which causes skin scab of potato tubers enters through lenticels. Nectarthodes Erwinia amylovora enters through nactaries of apple and pear flowers. In pears, nectaries occur in saucer shaped tissues between the point of emergence of the style and stamens. Droplets of nectar are exuded through stoma - like nectarthodes. Hydathodes Hydathodes are structures through which water is discharged from the interior of leaves. Xanthomonas campestris (bacteria) enters only through hydathodes. Trichomes Corynebacterium michigaense, which causes bacterial cankers of tomato plants always enters through trichomes. Direct penetration Sclerotinia sclerotiorum Entry through wounds All bacteria, most fungi, some viruses, and all viriods enter plant through various types of wounds. Penetration of viruses, mollicites, fastidious bacteria, and protozoan through wounds depend on the deposition of these pathogens by their vectors in fresh wounds created at the time of inoculation. Many of the low grade pathogens are not able to penetrate the host surface in the absence of a wound. However, once they are in the plant, they are able to cause severe disease symptoms. Wounds may be natural or caused by activities of man or other agencies, such as insect bites or nematodes. It is also a known fact that many agricultural practices involve accidental or even deliberate wounding. 6

7 Other natural wounds occur due to emergence of lateral roots from the main roots. This is a means of entry for the tobacco root rot fungus Thielaviopsis basicola and the bean root fungus Fusarium solani. Other fungi, nematodes could cause wounds through which pathogens enter. They could also enter through root hairs and buds. PLANT PROTECTION 1. The principal method of control of plant diseases are avoidance, exclusion, eradication, protection and immunization 2. In order to eliminate all the impediments to maximum production of food, it is crucial to control the spread of plant pathogens by cultural methods, chemical methods and by breeding disease resistant varieties. Cultural methods Stevens (1960) discussed the cultural methods of disease control. According to him, these measures involve agricultural cropping, harvesting and storage, tillage, crop rotation, soil management, growing of resistant varieties, planning of land use and other related practices. 1. Avoidance of pathogen Many plant diseases can be prevented by a proper selection of the land or field, choice of time of sowing, selection of varieties, seed and planting stock, and by modification of cultural practices. The aim of these measures is to enable the host to avoid contact with the pathogen or to ensure that the susceptible stage of the plant and favorable conditions for the pathogen should not coincide. i. Proper selection of Geographical area: Many fungal and bacterial diseases are more severe in wet areas than in dry areas. Tolyposporium penicillariae the pathogen which causes smut disease of bajra pearl millet (Pennisetum glaucum) and the ergot of bajra caused Claviceps fusiformis are more severe in wet areas, in regions where rain occurs for long durations during the flowering stage of the crop. 7

8 ii. Selection of field The selection of a suitable area for cultivation is very important for better yields and protection of the crops. In the case of many soil borne pathogens, if the same field is chosen for cultivating a specific crop, there is every probability that the disease will appear in a more severe way due to build up of the inoculum potential of the pathogen. The same crop should not be grown in the same field where in the previous year, it was infected with a pathogen capable of surviving in the soil. Colletotrichum falcatum (sugarcane red rot) can persist in the soil for a couple of months. Even this short period is enough for development of the disease in sugarcane. For soil borne diseases, prevention is achieved by not growing the same crop in the same field year after year i.e. late blight of potato, powdery mildew of wheat, ergot of bajra, smut of bajra, root knot of vegetable crops, and bacterial wilt of solanaceous plants. Drainage is also important, low lying, waterlogged field favours red rot of sugar cane. iii. Choice of time of sowing Pathogens are able to infect susceptible plants only under certain environmental conditions. Downy mildews require high moisture for infection. If the crop is grown such as not to be attacked by pathogen at susceptible stage, disease incidence will be reduced. Rhizoctonia root rot of grain is severe if grain is sown immediately after the rains, because the pathogen develops rapidly under high temperature and moisture conditions. Sowing should be delayed slightly. iv. Disease escaping varieties Those that escape damage because of their growth characteristics and inherent characters i.e. maturing varieties of wheat or pea escape damage due to Puccinia graminis tritici and Erysiphe polygoni v. Selection of seed and planting stock Seeds need to be collected from disease free areas where the disease has not occurred due to certain environmental factors. The planting of stocks from disease free fields is an important control measure for red rot of sugar cane. Abroad, tuber and stem indexing are used to obtain disease free stems and seeds. vi. Modification of cultural practices i.e. the distance between plants, time and amount of irrigation, quality and quantity of fertilizers or organic manure time and method of planting, mixed cropping, depth of sowing etc if adjusted can reduce diseases. vii. Eradication of alternate or collateral hosts Many plant pathogens complete their life cycle on two hosts, one of them generally a wild plant. The destruction of such wild host plant will break the chain of diseases. i.e. Pyricularia grisea (P. oryzae) can infect or survive on several wild species of rice or grasses such as Panicum repens, Digitaria marginata and Setaria intermedia. All these must be cleared to eradicate the disease. viii. Crop rotation Most of the time cultivation of the same crop or related crop leads to the proliferation of pathogenic soil borne fungus and the gradual increase in the disease. If the crops are planted over and over, the soil will get sick due to overload of the pathogen and, so planting non-susceptible crops in-between the cultivation periods will be helpful. Especially for red rot of sugarcane and smut of bajra. Plant pathogenic members of the Ascomycota and Basidiomycota Ascomycota i) Dutch elm disease.. Ophiostoma ulmi 8

9 ii) The ergot of rye Claviceps purpurea iii) Ergot of maize pearl millet bajra Claviceps fusiformis iv) Late blight of potato.. Phytophthora infectans v) Red ear rot of maize Gibberella zeae vi) Destroys coffee plantations in Africa.. Gibberella xylarioides vii) Foolish disease of rice in Asia, also source of Gibberellic acid Gibberella fujikuroi viii) Early blight of potato..alternaria solani ix) Early blight of tomato..alternaria solani x) Leaves curl of peaches and almonds Taphrina spp. xi) Plum pockets Taphrina deformans xii) Leaves spot of turmeric Taphrina maculans xiii) Witches broom of cherries Taphrina cerasi xiv) Gall of coriander Protomyces macrosporus xv) Root rot of sweet potato Ceratocystic fimbriata xvi) Butt rot of pineapple.. Ceratocystic paradoxa xvii) Oak wilt in North America.Ceratocystic fagacearum xviii) Serious canker disease of large and hardwood trees such as apples and pears Nectria galligena xix) Butt rot of apples.. Glomerella cingulata xx) Red rot of sugar cane.glomerella tucumanensis xxi) Fruit rot of egg plant.. Diaporthe vexans xxii) Melanose of citrus plants.. Diaporte citri xxiii) Rice blast.. Magnaporthe grisea xxiv) Chest nut blight Cryphonectria parasitica xxv) Root rot of hard wood and tree cankers Xylaria digitata xxvi) Root diseases of fruit trees and vines.. Rosellinia necatrix xxvii) Scab of mangoes Elsinoe magniferae xxviii) Scab of sour orange Elsinoe fawcetti xxix) Anthracnose of grapes Elsinoe ampelina xxx) Tar spot of maple seeds. Rhystina acerinum xxxi) Rot and wilt of many vegetable crops Sclerotinium sclerotiorum xxxii) Leaf spot of goose berry and currant. Mycosphaerella ribis xxxiii) Leaf spot of strawberry.. Mycosphaerella fragariae xxxiv) Leaf spot of groundnuts.. Mycosphaerella arachidicola xxxv) Stem rot of paddy rice. Sclererotium oryzae xxxvi) Root and stem rots.. Rhizoctonia solani xxxvii) Singatoka disease of banana.. Cercospora musae xxxviii) Tikae disease of groundnut Cercospora musae Basidiomycota i) Fomes sp attacks heart wood of various trees ii) Heterobasidion annosum causes serious problem in freshly stumps, causes rots and.. of trees iii) Ganoderma sp causes basal and stem rot of various trees iv) Polyporus sp causes wood rot 9

10 v) Armuillaria mellea causes root rot of forest trees vi) Graphiola phoenicis causes spot on date palm vii) Tilletia ayresii causes the smut of Guinea grass viii) Tolyposporium penicillariae causes smut of pearl millet (bajra) ix) Exobasidium vexans causes blister blight of tea in Sri Lanka and India x) Puccinia graminis tritici causes black rust of wheat xi) Puccinia recondita causes leaf/ brown rust of wheat xii) Puccinia striiformis causes yellow rust of wheat and barley xiii) Puccinia arachidis causes rust on the leaves of groundnut xiv) Uromyce fabae causes rust of broad beans xv) Hemileia vastartix causes rust of coffee xvi) Gymnosporangium juniperus causes rust of juniper xvii) G. virginianae causes rust of juniper xviii) Melamspora lini causes rust of flax xix) Phragmidium spp causes rust of rosaceous plants xx) Ustilago maydis causes smut of maize xxi) U. segetum var tritici causes loose smut of wheat xxii) Sphacelotheca sorghi causes grain smut of Jowar (sorghum) xxiii) Sphacelotheca cruenta causes loose smut of Jowar xxiv) Sprosporium reliana causes head smut of Jowar Bacteria as agents of plant diseases i) Bacterial leaf blight of rice Xanthomonas campestris pv. oryza ii) Potato bacterial ring rot caused by Clavibacter michiganense subsp. sepedonicus iii) Pseudomonas syringae pv. tabaci causes wild fire of tobacco iv) Pseudomonas syringae pv. syringae causes citrus blast, pear blast v) Xanthomonas campestris pv. citri causes citrus vi) Pseudomonas syringae pv. syringae causes leaf spot of beans vii) Pseudomonas syringae pv. phaseolicola causes halo blight of beans viii) Pseudomonas syringae pv. tomato causes bacterial speck of tomato and canker ix) Pseudomonas syringae pv. cacry causes angular leaf spot of cucumber x) Xanthomonas campestris pv. phaseoli common blight of beans xi) Xanthomonas pv. phaseoli common blight of beans xii) Xanthomonas campestris pv. malvacearum causes angular leaf spot of cotton xiii) Xanthomonas campestris pv. vesicatoria causes bacterial spot of tomato and pepper xiv) Xanthomonas campestris pv. manihotis causes bacterial blight of cassava xv) Xanthomonas campestris pv. campestris causes black rot/ vein of crucifers xvi)clavibacter michigaense subsp. michigaense causes bacterial cankers and wilt of tomato xvii) Erwinia stewartii causes stewart wilt of corn xviii) Erwinia amylovora causes fire blast of pome fruits 10

11 xix)erwinia trachiphila causes bacterial wilt of cucurbits xx) Corynebacterium fluccum faciens causes bacterial wilt of cucurbits xxi)pseudomonas solanacearum causes mock disease of banana xxii) Agrobacterium spp. eg A. rumefaciens causes bacterial spot of tomato and pepper xxiii) Xylella spp. xxiv) Xylophilus spp. xxv) Acidovora spp. xxvi) Gram negative Clavibacter spp xxvii) Curtobacterium spp. causes bacterial spot of tomato and pepper xxviii) Streptococcus spp. xxix) Streptomyces spp. eg S. scabies causes bacterial spot of tomato and pepper Modes and effects of nematode attack on plants Nematodes parasitic on plants obtain their food by sucking juices from them. Feeding is accomplished through a hollow needle like mouth part called a spear or stylet. The nematode pushes the stylet into plant cells and injects a liquid containing enzymes, which digests the plant cell contents. The liquid contents are then sucked back into the nematode digestive tract through the stylet. Effects of nematode attack i) Feeding by nematodes on plants lowers their natural resistance. ii) Reduces vigor and yield of plants iii) Affords easy entrance of wilt and soft rot producing fungi or bacteria and other nematodes. iv) Die back of twigs and shoots v) Stunting, loss of green color and yellowing of leaves vi) Wilting of leaves, stems etc of the plant vii) Wilting on hot bright days viii) Lack of response to water and fertilizers Five nematode diseases and their causative agents Root lesion nematode.. Pratylenchus spp Golden nematode of potato Heterodera rostochiensis Citrus nematode. Tylenchulus semipenetrans Bulb and stem nematode Ditylenchus dipsaci Burrowing nematode. Radopholus similis Leaf and foliar nematodes Aphelenchoides spp Stinging nematode.. Belonoliamus spp Spiral nematodes of banana roots... Helicotylenchus multicinctus/dihyste Northern root knot nematode Meloidogyne hapla Control of nematode diseases Rotation with non host plants Growing of resistant varieties and species 11

12 Using certified nematode-free nursery stock Use of soil fumigants Use of steam or dry heat on soil which kills eggs or nematodes Biological method of eradicating plant pathogens Garrett (1965) defined biological control of plant diseases as any condition under which or practice whereby survival or activity of pathogen is reduced through the agency of any living organism (except man himself) with the result that there is a reduction in the incidence of the disease caused by the pathogen. (Chattopadhyay et al., 2007) Types of biocontrol 1. Classical biocontrol Control of pests introduced from another region through importing specialized natural enemies of the pest from its native range. This biocontrol has been most successful with perennial crops (Fruit plantation and forest). It is aimed at establishing a sustained population of the natural enemies. For example, the successful import and release of the predatory ladybird Rodolia cardinalis for control of the accidentally introduced citrus pest Icerya purchasi in Mediterranean Europe around 1900 (Greathead, 1976). 2. Augmented biocontrol Augmentation is the direct manipulation of natural enemies to increase their effectiveness. Augmentation biocontrol is not permanent, and requires reapplication of the agent. Types of Augmentation biocontrol 12

13 Inundation: involves the mass production and release of large numbers of the control agent, such as the Trichogramma egg parasitoids of various lepidopteran pests including the cotton bollworm Heliothis virescens ( Van et al., 2003). Inoculative - The organisms are intentionally released with the expectation that they will then multiply and control the pest species for an extended period, but not permanently 3. Conservative biocontrol In conservation biological control aspects of the habitat or cultural practices are altered to increase the number or efficiency of predators, parasites, pathogens or herbivores. Conservation is the identification and modification of any number of factors to increase the effectiveness of natural enemies. for example Changing the timing of various aspects of cultivation. 13

14 Use of fungus 1. Trichoderma viride is a saprophytic fungus which can parasitize the mycelia of other fungi due to the secretion of an antibiotic substance gliotoxin. 2. Pseudomonas fluorescens (Dagger G) works against Pythuima Rhizoctonia species 3. Some mycorrhizal fungi used to control fungal root pathogen 4. Soil amendments potato scab controlled by ploughing organic materials into the soil. Rhizoctonia disease controlled by nitrogen rich amendments such as tops of bean plants (legumes) 5. Entomophthora spp. and entomogenous spp are used to kill insect pests. For example Entomophthora muscae is used t kill house flies. 14

15 6. Clonostachys rosea is a potential biological control agent against Fusarium culmorum which causes diseases of wheat and barley. It has also proved to be efficient in the control of diseases caused by Fusarium spp, Pythium spp., Alternaria spp, Botrytis spp., Bipolaris spp, and Tilletia spp. 7. Trichoderma harzianum and Trichoderma virens are both used to control Fusarium, Pythium, Rhizoctonia, Botrytis, Sclerotinia, Phytophthora spp. powdery mildews, downy mildew. 8. Ampelomyces quisqualis is used to control powdery mildew. 9. The use of the fungus Peniophora gigantea is to inoculate tree stumps to prevent the infection of adjacent trees by the wood decay fungus Heterobasidion annosum. Use of bacteria i) Pseudomonas fluorescens (Dagger G) works against Pythium and Rhizoctonia species. ii) Bacillus subtilis is used to control diseases caused by Fusarium spp, Pythium spp., Phytophthora spp Rhizoctonia spp., Colletortichum spp., Cercospora spp, Venturia spp., and Diplocarpon spp. iii) Agrobacterium radiobacter (gallinol A (Strain 84) is used to target or reduce pathogens of fruit trees before planting to prevent infection that causes crown gall brought about by Agrobacterium tumefacians. iv) Genes from the bacterium Bacillus thurigiensis have been inserted into plants to protect against attack by insects, such plants with inserted genes are known as GMO s (genetically modified organisms) Chemical control Many groups of chemicals for plant disease control such as heterocyclic nitrogen compounds, quinines, phenols and antibiotics. Isolating plant pathogens from aerial parts Isolation of fungal pathogen s from the aerial part of plants Each member of the Oomycetes are suspected after thorough washing, thin slices are cut from the advancing edge of leaf which is usually discoloured. The slices are floated in shallow water, they are removed and plated out. Pieces of infected host tissues are used to separate cultures eg cultures with motile spores can be separated from the walls without motile spores by picking of zoospores. Two zoospore producing species can be separated if they can produce/can be induced to produce sporangia and zoospores at different times by growing them at different temperatures. Basidiomycetes may be picked from sporophores (fresh/dried.. plant herbarium material resuscitated by exposure to humidity water overnight. By allowing the basidiospores to germinate on 5% malt agar at 25 C. Sporophores may be produced from mycelium isolated directly from basidiospores by. Moist sterile saw dust containing 5% maize meal and 2% bone meal as inoculums from wood and paper pulp 15

16 Disease symptoms Plant disease symptoms caused by fungal parasitism include the following. Spots: localized lesions on host leaves. Wilts: fungal colonization of root or stem vascular tissue and subsequent inhibition of translocation leading to wilting. Blight: browning of leaves, branches, twigs or floral organs. Rots: disintegration of roots, stems, fruits, tubers, fleshy leaves, corms or bulbs. Cankers: localized, often sunken, wounds on woody stems. Die back: necrosis of twigs initiated at the tip and advancing to the twig base. Abnormal growth: enlarged gall-like or wartlike swelling of plant stems, roots, tubers, leaves or twigs, root and shoot proliferation, etc. Damping off: rapid death and collapse of young seedlings. Decline: loss of plant vigour; retarded development. Anthracnose: necrotic (often sunken) ulcer-like blemishes on stems, fruits, leaves or flowers. Scab: localized lesions of scabby appearance on host fruit, leaves, tubers. Rusts: many small, often rust-coloured lesions on leaves or stems. Mildews: chlorotic or necrotic leaves, stems and fruits covered with mycelium and fruiting bodies of the fungus, often giving a white woolly appearance. 16