6.3 GRAPE DISEASES 6.3.1 Downy Mildew of Grape

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.

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.

Significance

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.

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.

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.

Symptoms

Plasmopara viticola The mycelium diameter varies from 1 to 60 microm

eters 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.

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 bran

ches in a similar manner.

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.

The Pathogen

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.

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.

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.

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.

Development of Disease

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 milde

w have been copper-based products such as the Bordeaux mixture, some broadspectrum protective fungicides, and several systemic fungicides.

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.

Control

• 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

小结

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.

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 ma

nner but sometimes marked by concentric zones. The symptoms resemble those of hitter rot of grape, which

is caused by another anthracnose fungus, Greeneria uvico

la (formerly Melanconium fuligcnum).

Symptoms

The ripe rot-affecred berry becomes more or less densely c

overed with numerous acervuli pustules from which, in humi

d weather, pinkish masses of spores ooze out. Later, the spore masses become darker, almost reddish-bro

wn. The rotted berries become sunken at the point of infecti

on and gradually become more or less shriveled and mumm

ified, while the pustules continue to produce spores. Infected berries often "shell" or drop off before the rot cause

s them to dry up.

Symptoms

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.

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.

Pathogen

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.

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.

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.

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.

Disease development

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.

Control

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

6.3.3 Black Rot on grapevine

• 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.

Significance

• 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.

Symptoms

Early black rot symptoms: Rapidly spreading sunken lesions on young berries. 

Full blown symptoms: Affected berries are completely blighted and develop small black pustules.

Asexual fruiting structures seen as small black dots within a necrotic spot

Leaf showing necrotic (dead), circular, tan spots with red margins.

• Guignardia bidwelii (Ellis) Viala & Ravaz

Pathogen

• 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.

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.

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.

Disease development

• 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.

Control

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

6.4 Citrus diseases

6.4.1 Citrus canker

• 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.

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.

Significance

• 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.

Symptoms

Xanthomonas campestris pv. citri

Pathogen

Disease development

• Citrus canker outbreaks are prevented and managed in a number of ways. In countries that do not have canker, the disease is prevented from entering the country by quarantine measures.

• In countries with new outbreaks, eradication programs that are started soon after the disease has been discovered have been successful; such programs rely on destruction of affected orchards.

Control

• When eradication has been unsuccessful and the disease has become established, management options include replacing susceptible citrus cultivars with resistant cultivars, applying preventive sprays of copper-based bactericides, and destroying infected trees and all surrounding trees within an appropriate radius.

Control

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：细菌病害；生物学特性；生理分化• 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（品种

抗性；树龄大小；寄主感病期）；栽培管理• 综合防治：严格检疫；建立无病苗圃；选用抗病品

种；化学防治

6.4.2 Citrus scab

• Citrus scab, caused by the fungus Elsinoe fawcetti Bitanc. and Jenk., affects the fruit, leaves, and twigs of susceptible varieties of citrus.

• This disease should be controlled primarily on fruit intended for the fresh market.

• Scab is a serious problem on only some varieties. It can be particularly severe on lemons, Temples, and Murcotts and on Minneola tangelos. It is often a problem on grapefruit, but rarely occurs on round oranges.

• Sweet orange is generally only infected if trees are located very close to infected trees of other varieties.

Significance

• Scab symptoms can appear on leaves as early as 4 days after infection.

• The disease starts as small, pale orange, usually somewhat circular, elevated spots.

• As the leaves develop, these infections become well-defined warts or protuberances on one side of the leaf, often with a conical depression on the opposite side .

Symptoms

• The lesions may be single or irregularly grouped. The crests of these wart-like growths usually become covered with a scabby, corky tissue pale in color, but sometimes dark if colonized by other fungi.

• The infected spots often run together and cover large areas with a corky, scab growth. Badly infected leaves become crinkled, distorted, and stunted having very little resemblance to normal foliage.

• The characteristic effects of the scab disease on twigs are the development of small masses of similar corky outgrowths on the surfaces.

Symptoms

• Scab symptoms on fruit can appear 7 days after infection. The disease starts on the fruit by forming irregular scabby spots or caked masses which vary from cream-colored to pale yellow in young fruits to drab or olive-gray with age.

• This change in color is accentuated by saprophytic fungi growing on the surface of the scabby tissue.

Symptoms

• Fruits severely attacked when very young often become misshapen, with predominant warty projections or conical growths extending from the surface, especially on Temples.

• On grapefruit, infected areas tend to flatten out, resembling windscar injury .

• On tangelos, lemons, and sour orange, the scabby areas are at the tip of blister-shaped projections on the rind.

Symptoms

Raised conical lesions of citurs scab on grapefruit leaves

Tangerine leaf severely affected by scab

Raised scab pustules on young grapefruit

Close-up of scab lesions on grapefruit.

Sphaceloma fawcettiiSexual:Elsinoe fawcettii.

Pathogen

• Tissues are susceptible to scab only while young. Leaves become immune to infection in only a few days, whereas fruit remains susceptible for up to 2 months.

• The number of spores available to infect susceptible tissues determines the number of lesions produced.

Disease development

• Water is the most important single factor affecting the severity of the disease, and is involved in spore production, dissemination, and germination.

• Spores are dispersed by rainfall or irrigation and to some extent by wind. Dews can result in heavy spore production.

• The optimum temperatures for spore formation, germination, and infection are 75-82°F (24-28°C).

• However, infection can still occur at temperatures below 75°F (24°C) if wetting periods are long.

Disease development

• Spores that infect young fruit arise from old, though still active, overwintering lesions on the fruit or leaves and/or from any scab infections that have developed on the new spring flush.

• Lesions on new shoot growth can build up inoculum very rapidly if there is sufficient rain or overhead sprinkler irrigation during the critical period of leaf expansion.

Disease development

• The frequency and duration of wetting, and not temperature, have a major influence on inoculum production.

• Spores are dispersed mainly by splashing water. Leaves are most susceptible as they emerge from the bud, and they become immune by the time they have reached about 1/4 of their final width.

• Fruit remains susceptible for up to 8 weeks or until late May during a year of normal bloom.

Disease development

• Spores of E. fawcetti require only short periods of wetting to germinate and cause infection. A minimum period of wetting of only 2 to 3 hours is necessary for infection by freshly released spores.

• Therefore, periods of wetting following cold-front-induced rainfall are often long enough to permit infection.

• The generally higher incidence of scab in groves located in low-lying areas, as compared with more elevated areas, is considered to be due to the heavier dews in such areas which assist spore production.

Disease development

• Scab can be particularly severe on summer growth flushes.

• Summer wet periods associated with rain showers and dew are highly conducive for spore germination and infection.

• Scab occurrence on summer growth flush tissue causes insufficient injury to affect the tree growth, but it has significance in providing overwintering inoculum for the following year.

• It is impractical to try to control scab on the summer growth flushes by fungicide treatments

Disease development

• Sprays intended for scab control are justified primarily for those groves that have a recent history of the disease.

• If the disease has been very severe before, it is usually desirable to prevent a buildup of inoculum on any shoot growth that develops prior to bloom as such infections can rapidly increase the amount of inoculum available to infect fruit.

Control

• Traditionally, two to three fungicide applications have been made for scab control. The first is applied in early spring when the flush has emerged about 2-3 inches.

• This application can be omitted if there were very low levels of scab in the previous year. The second application is made at petal fall.

• A third spray is applied about 3 weeks later to control scab and melanose.

Control

• Scab becomes less of a problem as trees grow older, but routine spraying will often continue to be necessary, particularly on the more susceptible varieties, such as Minneola tangelos, Murcotts, Temples, Page tangelos, and lemons. Fungicide information can be found in the Florida Citrus Pest Management Guide.

Control

• Where feasible, it is advisable to irrigate scab susceptible varieties very thoroughly immediately before growth commences in the spring if overhead irrigation is used. It may then be possible to delay the next irrigation until after the new flush has expanded sufficiently to become resistant to infection.

• In any event, irrigation should be kept to the minimum commensurate with tree requirements.

Control

• If foliage from the previous season is severely affected, light hedging may be advisable to reduce inoculum.

• With grapefruit, it may be helpful to harvest the fruit prior to the spring flush if fruit is severely damaged.

• This practice can also lower inoculum levels for the next season's fruit.

Control

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：分类地位• 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：农业防治；药剂防治

6.4.3 Citrus anthracnose

• Anthracnose lesions associated with bruised or injured rind appear as brown to reddish brown or black spots that may be firm and dry, or if sufficiently advanced, the rind becomes softened.

• Under humid conditions, the masses of spores on the lesion surface appear pink or salmon-colored.

• Anthracnose lesions associated with uninjured rind are initially silvery gray and leathery, and they retain the same degree of firmness and elevation as the adjacent healthy rind.

Symptoms

• As the decay advances, the rind becomes brown to grayish black, and eventually a soft rot occurs.

• Lesions may develop on any area of the fruit surface, and may form tear-drop patterns in association with appressoria formed by water dispersed spores.

• Lesions may also form immediately around the button where the fungus colonized the senescent button before spreading into the adjacent healthy rind.

Symptoms

• Colletotrichum gloeosporioides.

Pathogen

Disease development

• Good cultural practices that produce thrifty trees with minimal amounts of deadwood, or removal of deadwood by pruning can aid in the control of anthracnose.

• Proper ethylene concentrations of 5-10ppm required for optimum degreening should be maintained, because higher levels will not enhance degreening but will significantly increase the incidence of anthracnose.

Control

• Spot picking or delayed harvest for better natural fruit color development will reduce the time of degreening required, and subsequently less decay.

Control

病害发生规律： 病菌主要以菌丝体在病枝梢上越冬，也可以在病叶、病果上越冬，翌年分生孢子通过风雨传播，萌发后直接侵入寄主组织，或通过气孔或伤口侵入，炭疽病菌有潜伏侵染特性，而从伤口侵入时则无潜伏现象，果园初侵染源主要来自枯死枝梢及病果梗。

影响发病的条件： 甜橙、柑、温州蜜柑、早桔及柠檬发病较重； 冬季冻害重，早春气温低和阴雨年份发病重； 夏秋季高温多雨有利于病害发生； 管理粗放，树势衰弱，或偏施氮肥的柑桔园发病较重； 土质粘重、土层浅、有机质含量少、地下水位高、排水不良的果园发病重； 砧木对抗病性有影响。

防治要点：

冬季剪除病枝梢、病果台，清扫地面落叶。 生长期加强水肥管理，增施磷、钾肥，提高抗病力。 春、夏、秋梢嫩梢期及幼果期各喷药一次，易发生枯蒂落果的品种，秋季注意喷药保护果柄，防止病菌侵入。

小 结• 发生概况：分布？ 危害？经济损失• 病害识别：为害部位 ？发病时期？症状特点？• 病原：细菌病害；生物学特性；生理分化• 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；品种和生育期；栽

培管理• 综合防治：加强栽培管理；喷药保护；生物防治