6.3 GRAPE DISEASES 6.3.1 Downy Mildew of Grape

1. 6.3 GRAPE DISEASES 6.3.1 Downy Mildew of Grape

2. Significance Downy mildew is still most destructive in Europe and in the eastern half of the United States, where it may cause severe epidemics year after year and, in some years, in other humid parts of the world. Dry areas are usually free of the disease.

3. Significance • Downy mildew affects the leaves, fruit, and shoots of grapevines. • It causes losses through killing of leaf tissues and defoliation, through production of low-quality, unsightly, or entirely destroyed grapes, and through weakening, dwarfing, and killing of young shoots. • When the weather is favorable, downy mildew can easily destroy 50 to 75% of the crop in one season.

4. Symptoms • At first, small, pale yellow, irregular spots appear on the upper surface of the leaves, and a white downy growth of the sporangiophores of the oomycete appears on the underside of the spots. • Later, the infected leaf areas are killed and turn brown, while the sporangiophores of the oomycete turn gray. The spots often enlarge, coalesce to form large dead areas on the leaf, and frequently result in premature defoliation.

8. Symptoms • All young grapevine tissues are particularly susceptible to infection. • Infected grapes are quickly covered with the downy growth, may become distorted or thickened, and may die.

9. Symptoms • If infection takes place after the berries are half-grown, the oomycete grows mostly internally, the berries become leathery and somewhat wrinkled and develop a reddish marbling to brown coloration. • In late or localized infections of shoots, the shoots usually are not killed but show various degrees of distortion.

10. The Pathogen • Plasmopara viticola • The mycelium diameter varies from 1 to 60 micrometers because the hyphae take the shape of the intercellular spaces of the infected tissues. Globose haustoria grow into the cells. • The mycelium produces sporangiophores on the underside of the leaves and on the stems through stomata and, in young fruit, through lenticels.

11. The Pathogen • Four to six or more sporangiophores arise through a single stoma. • Each produces four to six branches at nearly right angles to its main stem. • Each branch produces two or three secondary branches in a similar manner.

12. The Pathogen • At the tips of the branches, single, lemon-shaped sporangia (conidia) are produced. The oomycete also produces numerous oospores. • It appears, however, that P. viticola is heterothallic, consisting of two mating types. P1 and P2, that must be present for sexual reproduction to occur.

13. Development of Disease The pathogen overwinters as oospores in dead leaf lesions and shoots and, in certain areas, as mycelium in infected, but not killed, twigs. 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.

14. Development of Disease Leaf hairs provide a basic protection barrier against the downy mildew pathogen, but in varieties lacking additional or different defense strategies it is overcome. The mycelium then spreads into the intercellular spaces of the leaf, and when it reaches the substomatal cavity it forms a cushion, of mycelium from which sporangiophores arise and emerge through the stoma.

15. Development of Disease 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. A disease cycle may take from 5 to 18 days depending on temperature, humidity, and varietal susceptibility. In the stems, the affected cells are killed and collapse, producing brown, sunken areas in the stem.

16. Development of Disease In the young berries, infection is also intercellular; chlorophyll breaks down and disappears, and the cells collapse and turn brown. At the end of the growing season the oomycete forms oospores in the infected old leaves and sometimes in the shoots and berries.

18. Control • Several American grape varieties show considerable resistance to downy mildew, but most European (vinifera) varieties are quite susceptible. Even the relatively resistant varieties, however, require protection through chemicals . • The most effective fungicides for the control of downy mildew have been copper-based products such as the Bordeaux mixture, some broadspectrum protective fungicides, and several systemic fungicides.

19. Control • The applications begin before bloom and are continued at 7- to 10-day intervals or, depending on the frequency and duration of rainfall, during the growing season. • Disease prediction systems, based on the duration of leaf wetness, relative humidity, and temperature, are used to identify infection periods and to time fungicide applications. • In recent years, sprays of systemic fungicides in combination with copper or broad- spectrum preventive fungicides have given excellent control of grape downy mildew.

20. 小结 发生概况：分布？ 危害？经济损失 轻病害识别：为害部位？发病时期？症状特点？ 病原：形态特点；生理特性 病害发生发展规律：越冬、入侵方式、传播、 发病及其影响因素：气象因素；品种抗病性；栽培管理 综合防治：清除越冬菌源；加强栽培管理；化学防治；选用抗病品种

21. 6.3.2 Ripe Rot of Grape Introduction The disease occurs worldwide but is most serious in areas with warm, humid weather during the ripening of the fruit.

22. Symptoms • Ripe rot appears when the fruit is nearly mature and may continue its destruction or fruit after it has been picked and during shipment and marketing. • Symptoms begin as small spots that soon spread to over half the berry. • Eventually the whole berry rots, usually in a continuous manner but sometimes marked by concentric zones. • The symptoms resemble those of hitter rot of grape, which is caused by another anthracnose fungus, Greeneria uvicola (formerly Melanconium fuligcnum).

23. Symptoms • The ripe rot-affecred berry becomes more or less densely covered with numerous acervuli pustules from which, in humid weather, pinkish masses of spores ooze out. • Later, the spore masses become darker, almost reddish-brown. The rotted berries become sunken at the point of infection and gradually become more or less shriveled and mummified, while the pustules continue to produce spores. • Infected berries often "shell" or drop off before the rot causes them to dry up.

25. Pathogen • Colletotrichum gloeosporioides (perfect stage, Glomerella cingulata) • The anamorphs of fungus produce colorless, one-celled, ovoid, cylindrical, and sometimes curved or dumbbell-shaped conidia in acervuli. Masses of conidia appear pink or salmon colored.

26. Pathogen • Acervuli are subepidermal and break our through the surface of the plant tissue. • Glomerella produces ascospores in asci in perithecia. • Much more frequently, however, the fungus produces conidia-hearing acervuli of its anamorphs Colletotrichum or Gloeosporium spp.

27. Disease development • The fungus overseasons in diseased stems, leaves, and fruit as mycelium or spores, in the seed of most affected annual hosts, and in cankers of perennial hosts. • Ascospores or conidia produced by the surviving mycelium in the spring cause primary infections. • Conidia cause all secondary infections during the entire season as long as temperature and humidity are favorable.

28. Disease development • Germ tubes penetrate uninjured tissue directly. The mycelium grows intercellularly and may remain latent for some time before the cells begin to collapse and rot. • The mycelium then produces acervuli and conidia just below the cuticle, which rupture the cuticle and release conidia that cause more infections. • Infections of young fruit generally remain latent until the fruit is past a certain stage of development and maturity, at which point the infections develop fully.

29. Disease development • The fungus is favored by high temperatures and humid or moist weather. • Conidia are released and spread only when the acervuli are wet and are generally spread by splashing and blowing rain or by coming in contact with insects, other animals, tools, and so on. • Conidia germinate only in the presence of water and penetrate the host tissues directly. • In the beginning the hyphae grow rapidly, intercellularly and intracellularly, but cause little or no risible discoloration or other symptoms.

30. Disease development • Then, more or less suddenly, especially when fruit begins to ripen, the fungus becomes more aggressive and symptoms appear. • In many hosts the fungus reaches the seed and is either carried on the seeds or, in some, may even invade a small number of Seeds without causing any apparent injury to them. • There is considerable variability in the kinds of host plants . • There may be several races with varying pathogenicity within each species of the fungus.

31. Control • The control of Glomerella/Colletotrichum diseases depends on the use of disease-free seed grown in arid areas or use of treated seed. • Use of resistant varieties when available; • Removal and burning of dead twigs, branches, and fruit infected with the fungus in woody plants; • Finally, spraying with appropriate fungicides.

32. 小结 • 发生概况：分布？ 危害？ • 病害识别：为害部位？发病时期？症状特点？ • 病原：形态特点；生理特性 • 病害发生发展规律：越冬、入侵方式、传播、 • 发病及其影响因素：气象因素；品种抗病性；栽培管理 • 综合防治：清除越冬菌源；加强栽培管理；化学防治；选用抗病品种

33. 6.3.3 Black Rot on grapevine

34. Significance • Black Rot on grapevine is a dangerous disease in viticulture. • The ascomycete fungus Guignardia bidwellii is able to cause considerable economic losses under favourable climatic conditions in insufficient protected vineyards. • The fungus has been protracted from North America to Europe just as the downy and the powdery mildew of grapevine with rootstock material in the 19th century.

35. Symptoms • Berries: sunken brown spots develop and rapidly spread through the entire berry, small black pustules develop in the center of the spots, the berry desiccates and becomes mummified in a matter of days • Leaves: tan spots develop with small black pustules in the center, the leaf blade remains flat.

36. Early black rot symptoms: Rapidly spreading sunken lesions on young berries. Full blown symptoms: Affected berries are completely blighted and develop small black pustules.

37. Asexual fruiting structures seen as small black dots within a necrotic spot Leaf showing necrotic (dead), circular, tan spots with red margins.

38. Pathogen • Guignardia bidwelii (Ellis) Viala & Ravaz

39. Disease development • Small, circular reddish-brown spots 1/8 to 1/4 inch in diameter appear, usually on the lower leaves, in mid- to late-June. • The lesions are the result of spring ascospore infections originating from old black rot fungus infected berries and canes. • Only a few hundredths of an inch of rain are necessary to promote ascospore release and subsequent leaf infection. • The lesions develop a black border and small black, pimple-like pycnidia are found in the reddish-brown part of the lesion.

40. Disease development • Ascospores and conidiospores (secondary spores) infect the blossoms during each rain. However, the results of early blossom and fruit infection are not visible until mid-July or early August, when infected grape berries begin to shrivel in the clusters. • Leaf, cane and tendril infection can occur only when the tissue is young, but berries can be infected until almost fully grown if an active fungicide residue is not present.

41. Disease development • Mid-season infection of green fruit is evidenced by a whitish circular spot 1/8 inch in diameter on the fruit at the point of infection. • About two weeks later, after the fungus has grown throughout the berry, it begins to shrivel and look like a hard bluish raisin. • These eventually fall to the ground and are an inoculum source for the rest of the season. • American varieties such as 'Concord' and 'Niagara' are quite susceptible to the pathogen; 'Delaware' and certain French-American hybrids are moderately resistant. 'Aurore' is a particularly susceptible variety.

42. Control • Black rot can be effectively controlled by a combination of approaches including; use of resistant cultivars, vineyard sanitation and chemical control practices. • Vineyard hygiene is critical for successful black rot control. The destruction of infected plant parts and mummies each year will reduce the amount of inoculum present. Weed control and pruning allows for free circulation of air and fast drying of plants. This makes conditions less suitable for growth and spread of the fungus, and provides increased fungicide spray coverage.

43. 小结 • 发生概况：分布？ 危害？经济损失 • 病害识别：为害？发病时期？症状特点？ • 病原：形态特点；生理特性 • 病害发生发展规律：越冬、入侵方式、传播、 • 发病及其影响因素：气象因素；品种抗病性；栽培管理 • 综合防治：清除越冬菌源；加强栽培管理；化学防治；选用抗病品种

44. 6.4 Citrus diseases 6.4.1 Citrus canker

45. Significance • Citrus canker is a disease affecting citrus species that is caused by the bacterium Xanthomonas axonopodis. • Infection causes lesions on the leaves, stems, and fruit of citrus trees, including lime, oranges, and grapefruit. • While not harmful to humans, canker significantly affects the vitality of citrus trees, causing leaves and fruit to drop prematurely; a fruit infected with canker is safe to eat but too unsightly to be sold.

46. Significance • The disease, which is believed to have originated in South East Asia, is extremely persistent when it becomes established in an area. • Citrus orchards have been destroyed in attempts to eradicate the disease. • Australia, Brazil and the United States are currently suffering from canker outbreaks.

47. Symptoms • The bacterium enters the plant tissue and causes the development of blister-like formations (lesions). • Lesions are usually raised, coloured tan to brown, surrounded by an oily, water-soaked margin and a yellow ring or halo. Large or older lesions may have a crater-like appearance. • Leaf, fruit and stem tissue may be infected. Leaf tissue offers more opportunity for infection and as such typically displays the most numerous lesions over time. It is unusual to see multiple lesions on fruit or stems if lesions are not present on leaves.

49. Pathogen Xanthomonas campestris pv. citri

50. Disease development