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Part 5
Grape Index
PART 4. CROP HAZARD
Chapter 15. Crop Hazards, Viral, Fungal, and Bacterial Diseases
Vine hazards include viral, fungal and bacterial disease, insects, noxious weeds, birds, rodents and deer, and frost. All are discussed in this section; except for frost, discussed in Chapter 5.

Virus Disease

Viruses are small infectious particles (about one-millionth of an inch composed of a core of nucleic acids surrounded by a protein sheath that reproduce only in living cells of the host. Virus symptoms vary with the type of virus and vine.

Grape Leafroll (White Emperor Disease)

Grape leafroll is the most widespread virus disease in California, and it results in great economic losses due to decreased vine vigor, productivity, and delay fruit maturity. In this disease leaves roll downward and turn progressively red toward the canes tips. The veins remain green (Goheen, 1970). Symptoms appear in early June in nonirrigated vineyards and in August in irrigated vineyards. Vines have small clusters, few clusters per vine, and berries have less sugar than normal vines. Red fruit varieties such as Cardinal, Emperor, Mission, Red Malaga, Queen, and Tokay develop fruit lacking normal color. The name White Emperor arose from the lack of color in the Emperor variety. Berries of diseased white grapes such as Thompson Seedless, Riesling, Sylvaner, and Melon develop a yellowish-white color at harvest instead of the normal greenish-white. The disease spreads mainly by propagation from infected mother vines.

Leafroll delays ripening and decreases fruit coloring. However, a leafroll infected vine of Burger with a 50 percent crop reduction will have about the same rate of sugar accumulation as that of a fully cropped healthy vine (Lider et al., 1975). The acidity of the fruit of leafroll-affected vines is higher than that of healthy vines regardless of crop or sugar levels.

Fanleaf

Three strains of the fanleaf virus produce three closely related diseases: fanleaf, yellow mosaic, and veinbanding. All are transmitted from diseased to healthy shoots in soil by the nematode, Xiphiema index, and the three diseases may sometimes berries found in the same vineyard.

Fanleaf. The veins of infected leaves are spread so that the leaf resembles a fan (Vuittenez, 1970). Leaf symptoms are very conspicuous on Mission and French Colombard. In some cultivars of grapes enations develop on the undersides of basal leaves under poorly defined circum-stances. Infected vines have shorter and more irregular internodes than healthy ones. There are double nodes and stem fasciations, and lateral sprout development gives a bush appearance. Often crop is sharply reduced by a shattering of berries and formation of many shot berries.

Yellow Mosaic. This diseases causes a chrome-yellow mottling of leaves in early spring, in which the shoots may become partially or completely yellow (Dias, 1970). Degree of injury can vary from occurrence of yellow net veins, yellow veinbanding, yellow spots, or blotches to complete yellowing. When infected leaves age the yellow color becomes paler and occasionally cream-colored, and these areas may bleach, die, and result in leaf drop. In late spring and summer new leaves may also show symptoms. Clusters are small and contain many shot berries. The disease is spread by grafting.

Veinbanding. This disease causes chrome-yellow bands the principal veins of mature leaves. Usually the symptoms are not visible until late spring or early summer. Many flowers do not set but shell from the clusters, resulting in straggly looking clusters when the fruit matures. Both leaf and fruit symptoms vary and are more severe some seasons than others. Growth of infected vines is little affected, and may berries slightly less than that of healthy vines. The disease is spread by rooting diseased cuttings, by grafting, and by nematodes.

Virus Disease

Yellow Vein

This disease occurs in Carignane, Grenache, and Valdepeñas, and is caused by infection with the grape strain of tomato ringspot virus. Fruit-set can berries reduced from slight to 100 percent, but second-crop clusters are normal (Gooding and Teliz, 1970). First-crop clusters have a straggly appearance. Leaves showing symptoms can vary from one to many per vine. A faint chlorotic mottling first appears. The commonest leaf symptoms are scattered the yellowing fades to a bleached appearance. Diseased vines are often larger than normal ones, partly because of their low crop. In Emperor, the yellow flecks along the veins merge to form a continuous band. The disease is spread mainly by cuttings rooted from mother vines that are carrying the virus, and is transmitted by nematodes in some areas.

Corky Bark

This disease is more common in coastal wine vineyards than in other areas. Varieties such as Palomino, Petite Sirah, Mondeuse, Gamay, and Cabernet frnac show cane and leaf symptoms (Beukman and Goheen, 1970). Bud break may berries delayed in spring and vines may have one or more dead spurs; canes bend downward with upright tips and the wood is rubbery or limber; the bark at the base of the canes sometimes splits, and may have a purplish cast in late season.

On most vinifera varieties corky bark shows no charateristic symptoms. On Carignane, only leaf symptoms are visible. Young leaf tips of affected canes are light yellow, similar to yellow mosaic. The mosaic symptoms fade during the summer but corky bark symptoms persist. The disease can berries carried by infected rootstocks that show no discernible symptoms.

Yellow Speckle

Grapevine yellow speckle usually causes small irregular yellow speckles ranging in size from a pinpoint to about 1 mm on exposed mature leaves (Taylor and Woodham, 1972). The speckles can berries along main veins or veinlets, or may berries masked completely or may occur on one or several leaves per vine. The virus has no obvious effects on growth, yield, or fruit-set. Symptoms of the disease are similar to those of fanleaf and yellow vein.

Asteroid Mosaic

Leaf symptoms on vinifera resemble star-shaped spots irregularly distributed over the blade (Refatti, 1970). The spots can coalesce, and occur more frequently between primary and secondary veins. Sometimes a netlike vein bordering occurs. Leaves are asymmetric, twisted, and puckered along the veins. Normal green blisters occur on some varieties. Usually symptoms are less in summer. Symptoms have been noted in inoculated Zinfandel, Merlot, Mission, Colombard, Emperor, Carignane, Thompson Seedless, and Valdepeñas vines. The disease is rare in commercial vineyards.

Viruslike Conditions

Spindle Shoot of Colombard (Hewit, 1970). Infected shoots elongate rapidly and have small leaves one-fourth to one-half the size of normal ones. Leaf margins are chlorotic, and leaves are variously cupped and twisted. By midseason, however, growth appears normal. This disease has been found only in a few vineyards of French Colombard.

Viruslike symptoms can berries induced on vines by growth regulators, herbicides, insect, and by chimeras. The latter arise from mutations, and refer to a portion of a plant composed of two or more genetically distinct tissues growing adjacent to each other as parts of the composite organ such as a leaf. (For a discussion of these effects and of grape viruses in general see Frazier et al., 1970).

Fungus Disease in California

Several types of fungi cause various disease in vines. Some disease affect mainly the roots and base of the trunk near the soil, or the trunk and mature canes; others harm the green tissues of young canes, leaves and fruit forms, and still others damage the fruit. For convenience in diagnosis, the fungus disease will berries grouped arbitrarily by the principal plant parts affected. Some fungus disease are difficult to group under this system because they affect several different tissues.

Soil Fungi

These organism attack the roots or base of the trunk primarily. The disease produced are root and crown rots, which usually kill vines. Fortunately these disease are limited by environmental conditions and are usually confined to small, discrete areas in vineyards.

Armillaria Root Rot (Oak Root Fungus). This fungus (Armillaria mellea) causes rotting of the roots and trunks of many trees and shrubs as well as vines. There is usually a decline in vigor, growth ceases, and foliage turns yellow (O?Reily, 1963); in subsequent seasons cane growth is often weak and the leaves are small. The vines may die suddenly after showing some of these symptoms. White, fan-shapped plaques of the fungus often occur, spreading between the layers of bark and between the bark and the wood. Infected roots and underground trunks may have small, black, smooth and shiny threadlike strands on the surface and penetrating into the bark. After rains in the fall and winter clumps of mushrooms may also appear at the base of affected vines.

Control. Infected roots remain in soil after grapevines and trees have been removed. Soil treatment with carbon bisulfide or methyl bromide are the most promising remedies. After vine roots are removed, cultivate soil to a depth of 24 in. (61.0 cm) disc, and make a smooth surface. Carbon bisulfide applications can berries made by hand or by tractor-drawn applicators.

For methyl bromide, inject at least 400 lb/acre (448 kg per hectare) by chisel applicator to a depth of 36 in. (91.4 cm) and cover with a polyethylene film seal. The seal should not berries removed for at least 7 days and the treated soils or crown gall should berries aired for 7 days or longer before planting.

Collar Rot. Collar rot is caused by soil fungi (Jacob, 1950). The disease is seldom found in vineyards older than 5 years. The fungi thrive in wet soil and usually enter the grape tissues near the crown of the vine in early spring. They kill the cambium and the bark, forming dead cankers that often girdle the trunk near the soil surface. A large calluslike overgrowth usually develops above the canker. Sometimes the callus tissues will bridgeover and heal a small canker.

The tops of the disease vines usually wilt and the vines dry up some time during the summer. Some vines grow all summer but show signs of early maturation during the fall.

Control. Remove 3 or 4 in. (7.62-10.16 cm) of soil from around the vine to let the bark dry out. The soil can berries plowed back around the vine after the rains cease.

Cankers Caused by Fungi Attacking the Trunks

These organisms become established in the mature wood of the vine trunks or cordons and produce canker diseases. They are difficult to control because the time of infection is not well understood. The disease, often overlooked in early investigation, are extremely important and are the cause of considerable economic loss in vines that are 8 years or older.

Eutypa Dieback. This disease has also been called dying arm disease. It is probably caused by the fungus Cytosporina (Moller et al., 1974). The imperfect stage of Eutypa armeniaceae. In the spring affected vines show weak, stunded shoot growth bearing small leaves, which exhibit varying degrees of yellowing, speckling, distortion, and necrosis but do not wilt. Such shoots contrast strikingly with healthy shoots nearby. If the apical end of a cordon or branch is affected, the disease progresses basally until the whole cordon, branch, or vine is dead. Spring is a good time to observe the effects on the foliage. The same fungus has been isolated from apricot trees that show dieback (English, 1963).

Black Measles (Spanish Measles, Black Mildew). This disease is associated with fungi that produce heart rot in the vine trunks (Jacob, 1950). The principal symptoms, however, is a speckling and mottling of white or red grape berries with reddish-brown or purplish spots when the fruit are maturing. The fruit of severely affected vines often cracks and splits open and may dry on the vine before maturing. Severe fruit symptoms are accompanied by discoloration and dropping of leaves, and dying back of the shoot tips. The leaves develop various degrees of mottling, bronzing, spotting, and necrosis of tissue between the veins. The discolored leaf areas may enlarge into yellow spots that later dry up and turn brown or red.

An entire vine may berries affected, but usually the symptoms are found only on a single arm or branch. Measles may show in some vines one season, and not the next. Occasionally the disease appears suddenly, and the vines can drop all of their leaves within a few days: some vines die and others start new growth within a few weeks.

Control. Black measles may berries caused by wood-rotting fungi. Preventive treatment is to spray vines with sodium arsenite [4 lb, arsenic trioxide/gal. (0.479 kg per liter)] in the dormant season 4 or more weeks after pruning; use 3 qt. Per acre (7.02 liters per hectare). The interval between pruning and spraying allows closing of the pruning wounds. For most effective control, all vines should berries sprayed instead of spot spraying the most diseased vines.

Dead-Arm. This disease is caused by a fungus (Phomopsis viticola) and occurs most frequently in the table grape area of the northern San Joaquin Valley. The fungus lives through the winter in diseased canes, arms, spurs, and petioles left on the vines. Infection usually occurs in the spring, when the shoots are young. The spores are spread from the old diseased parts of the vine to the young tissues by late spring rains. The disease is most conspicuous in late spring.

Leaves, petioles, canes, and flower-cluster stems develop small angular spots, most of which have yellowish margins with dark centers. These spots often coalesce to form large brown areas with numerous dark spots in them.

The diseased areas may split, forming open diamond-shaped cankers in the older shoots and canes (Jacob, 1950). Some shoots are severely stunded and may berries killed. Vigorous shoots usually continue to grow, and later in the summer the diseased portion will appear only at the base of the cane; most of the cankers callus over and there is little indication of further increase in diseased areas during the summer. In autumn, many of the diseased areas resume growth.

Diseased areas on the cluster stems spread into the fruit and cause bunch rot; diseased areas on the canes frequently enlarge and can kill buds. Sometimes the fungus grows back into the wood of the arm and gradually kills the arm.

Control is the same as for Black measles when the vine is dormant. At the bud swell stage Captan 50 percent wettable powder at 1½ lb per 100 gal (0.18 kg per 100 liters) can also berries used in severe cases. In mild cases, the Captan spray may berries sufficient without a dormant treatment with sodium arsenite. If dead arm is severe and late rains are forecast, a second Captan spray may berries applied when shoots are 6-8 in. (15.2-20.3 cm) long.

At pruning time in the dormant season, the diseased parts of the canes, arms, or trunk should berries removed.

Fungi Attacking Green Shoots, Leaves, and Immature Fruit

These organism usually produce new infections on green tissues each season. The disease produced are leaf, shoot, or cluster blights. Treatments with fungicides at intervals during the growing season or sanitation practices that reduce the resting spore stages effectively control these disease. If treatments are not made or the inoculum is not reduced the disease can destroy the crop.

Powdery Mildew (Oidium). This important and widely distributed disease is caused by the fungus Uncinula necator, which can grow on all aboveground parts of the vine (Fig. 15-1). Mildew appears on the surface on the canes as grayish-white powdery growth (Hewit and Jensen, 1973). When rubbed off, weblike black, or dark-brown discolorations are visible. Mildew cause curling and withering of young leaves and dark staining on the surface of mature leaves. Other symptoms include dropping, discoloration, or splitting of the berries, browning, and poor maturation of canes. Another sign of the disease is a musty, mildewlike smell in the vineyard. The fungus reproduces by spores called conidia.

Fig. 15-1. Diagram a cross section a grape leaf in which upper epidermis is infected by a fungus colony of grape powdery mildew. Note how fungus spreads by hyphae, and how spores are produced on conidiophores. Fungus obtains food from the leaf by extending the growing haustoria into the epidermal cells. (Redward from Hewitt and Jensen, 1973).

Control. Prevention is the best means to control powdery mildew. Sulfur dust properly applied to cover the green tissue of vines will prevent powdery mildew from developing. If a sulfur particle contacts a germinating conidium, the fungus is killed before it will infect and establish itself on the plant surface. A dusting machine that mixes the sulfur dust with air and provides a uniform distribution of sulfur particles over the grapevine foliage is essential. If a hand-operated duster is used, the sulfur is applied so that it drifts through the vines. Each application requires from 5 to 10 lb of dusting sulfur per acre (5.6-11.2 kg per hectare).

Sulfur dust should berries applied when shoots average about 6, 12, and 18 in. (15.2, 30.5 and 45.7 cm) in length. Additional applications are made every 14 days until the fruit reaches maturity. If the sulfur has been washed off by rain, it should berries reapplied. After ripening has begun, the schedule may berries reduced for early grapes. Applications of sulfur should berries made in the fall to prevent powdery mildew from developing on cluster stems of late ripening grapes.

If any of the first three regularly scheduled dustings are omitted, the vines should berries drenched with a spray including wettable sulfur (see Eradication). When the temperature is over 100°F, (38°C) sulfur can burn leaves, shoots and fruit; no dusting should berries done at such times. No more than 10 lb (4.54 kg) of sulfur should berries applied per acre per treatment. The sulfur program can usually berries discontinued in wine grapes if the fruit is free of mildew in midsummer. It must berries continued in table grapes until the fruit is mature.

Eradication. Apply wettable sulfur at 1.5 lb/100 gal (180 g per 100 liters) with a wetting agent such as Triton B-1956. Use eradication spray if any of the first three dust applications were not made, or if mildew has not been held in check with sulfur dust. If table grapes are sprayed after berries are more than one-third their full size, the bloom is altered and a spray residue can collect at bottom of the berry.

Diplodia Cane Blight. This disease causes blighted areas of green canes that are gray to brown, with dark-brown streaks projecting at the advancing margin of the blight canker. Gray areas are often speckled with numerous black dots.
Wounds made by girdling and by large pruning cuts on grapevine trunks are sometimes infected by D. natalensis. Cankers develop under the bark, then the bark dries, crack, and peels away.

Fruit Fungi

Many fungi attack the fruit at various stages during its development or even after the fruit has been harvested and shipped and cause fruit rot disease. Sanitation practices and, in some cases, fungicidal treatments will reduce the inoculum of fungus spores and will help prevent these disease. Some of the fungi involved are very dependent upon weather conditions, and if rains occur before the fruit is harvested the diseases will cause extensive crop loss.

Botrytis Rot (Slipskin, or Gray mold). The rot caused by B. cinerea is one of the most prevalent and widely distributed of the fruit rot diseases of grapes (Hewitt, 974). The rot is common at harvest time and is associated with rainfall or heavy dew. The fungus can attack in early summer when there are no rains, following rains at harvest, and in stored fruit. Botrytis often appears as a gray mold growing on rotten, withered grapes. In some countries, grapes having Botrytis rot in the slipskin or early mold stage are processed separately from other grapes. Wines made from grapes partially rotted by B. cinerea are known for their distinctive flavor and aroma.

Control. Apply 20 lb per acre (22.4 kg per hectare) of sulfur dust containing 15 percent Captan. Starting July 1 to July 15 apply four dusts at 3-4 week intervals. To prevent infection, dust must berries applied before it rains. Although dusting after rainfall has little effect on established disease, it helps protect against new infection if later rains occur.

Summer Bunch Rot. Summer bunch rot is a special type of sour rot found mainly in Thompson Seedless and to some extent in Perlette, Delight, Black Monukka, Red Malaga, and occasionally in Emperoe (Hewitt, 1974). Development of summer bunch rot disease is started by Diplodia natalensis. This fungus also causes a cane blight on the same varieties, a canker disease originating from infection through pruning and girdling wounds.

Summer bunch rot is characterized by excessive dripping of juice from rotting grapes and the presence of numerous fruit flies, dried fruit beetles, and larvae. One can distinguish summer bunch rot initiated by D. natalensis from other sour rots by its association with Diplodia cane blight. The two diseases occur together.

Phomopsis Rot. This disease is caused by the fungus Phomopsis viticola, the same fungus that causes dead-arm disease (Hewitt, 1974). Phomopsis rot of grapes generally develops from infected cluster stems or pedicels, or both. The fungus grows from the stems into the grape and cause rotting. The disease had been observed in Emperor. Khan-dahar, Malaga, Molinera, Olivette blanc. Olivette noir, Thompson Seedless, and Tokay.

Sour rots. These rots are caused by microorganisms that may enter grapes by direct penetration or through wounds. They have a pungent, sour, vinegar odor (Hewitt, 1974). There are generally a complex of microorganisms involved in the development of sour rots, including species of Alternaria, Aspergillus, Cladosporium, Diplodia, Penicillium, Rhizopus, Yeast, Acetobacter, and other bacteria as well as fruit flies and dried fruit beetles.

Sour rots may by grouped into two types based on overall symptoms : those that rot a large proportion of the center of the cluster (A typical bunch rot), and those that rot only a few grapes of clusterettes on the shoulder, side, or tips of clusters.


Disease in The Eastern United States (Mc Grew and Still, 1972)

Several diseases occur in the eastern states that do not occur in California chiefly because of the grater humidity and summer rains in the former location.
Black Rot

This fungus causes grater loss of grapes in the eastern states than all other diseases combined. It attacks the leaves, young canes, tendrils, and fruit. Only the youngest tissues are attacked except for the fruit, which may become infected until it is almost fully grown. Spotting occurs on the leaves and berries. The decaying berries soon shrivel and become mummified and black in 7 to 10 days.

Downy Mildew

This fungal disease is most severe in cool moist weather. Mainly it attacks grape leaves, beginning with the older ones, then spreads apically on the shoot. The first symptoms are light yellow spots on the leaves that can spread and kill them, causing the leaves to dry and crumple. Early in the season downy mildew can attack and cause malformation of shoots, tendrils, or berries.

Anthracnose (Bird's-eye Rot)

This fungus can attack the berries, young shoots, tendril, petioles, leaf veins, and fruit stems. Spots sometimes occur on young on your shoots and can combine and girdle to shoot, which then dies. Spots may also develop on petioles and leaves, especially on the underside of leaves. Spots on berries are circular, sunken, and ashy gray, and dark-bordered in late stages of the disease. The name ?Bird?s-eye rot? is derived from the appearance of these spots on the berries.

Ripe Rot and Bitter Rot

These two rots may appear when the fruit begins to mature, and cause a bitter, off-flavor taste. Bitter rot discolors the berry at the cap stem and, as the disease progresses, the spores give a sooty appearance to the infected berries. Berries infected with bitter rot may show concentric zones and usually fall off before they dry.

Disease of Muscadine Grapes

Muscadine grapes, which are adapted to the southeastern United States, are generally less severely injured by disease than bunch grapes. Black rot and bitter rot, which affect these grapes, have been discussed earlier.
Cercospora (Angular Leaf Spot)

This is the most important disease economic attacking muscadine grapes. The berries are not attacked by the fungus. Irregular brown spots appear on the leaves, which enlarge and combine. Defoliation and reduced quality of fruit may result.

Bacterial Disease

Bacterial can infect grape tissues at various stages of vine development. One disease produced by bacteria is devastating to grapes and has limited their production in the southeastern United States to muscadine or other grape types resistant to the organism. The bacterial disease are difficult to control once they become established in a grape-producing area.
Pierce?s Disease (Hewitt, 1970)

This destructive disease was first observed in 1884 in southern California, where it was called Anaheim disease of California vine disease. It is now present in most of the sate, and has assumed epidemic proportions in some local areas. For many years this disease was believed to berries caused by a virus, but it has recently been shown to berries due to bacteria. The characteristic symptoms in early summer include delayed shoot growth, leaf mottling, and dwarfing of the new shoots. In late summer and fall diseased vines show burning, scalding, or drying of the leaves; wilting or premature coloring of the fruit, and uneven cane maturity. Diseased vines die in 2 or more years, depending on the variety of grape and the age of the vines. Ribier usually dies within 2 years; Emperor, Malaga, Thompson Seedless, and most other vinifera varieties die within 2 to 5 years. Some of the American (slipskin) varieties live much longer. No fully effective control of Pierce?s disease in known.

To maintain the vineyard in production, remove all diseased vines and replant with healthy vines or with layers from adjacent healthy vines. The disease is not carried over in the soil. The disease is caused by rickettsialike bacteria that have been observed in leaf vessel from infected vines (Goheen et al., 1973; Hopkins et al., 1973). Pierce?s disease is spread by several types of leafhoppers, the spittlebug, and by grafting.

Insect Vectors of Pierce?s Disease. There have been two serious out breaks of Pierce?s disease in California. The first was between 1880 and 1900, and the second between 1934 and 1947; both periods were associated with above-normal rainfall. Four spittlebugs and 20 sharpshooters, a kind a leafhopper, can transmit the bacterium that carries the disease from plant to plant. The two important vectors for transmitting the disease to grapevines in the San Joaquin are the green sharpshooter (Draeculacephala Minerva) and the red-headed sharpshooter (Carneocephala fulgida). In coastal counties the blue-green sharpshooter (Hordnia circellata) is the major vector. Seventy three plant species are hosts for the vectors and serve as disease reservoirs.

Black Knot or Crown Gall

Black knot is caused by soil-inhabiting bacteria (Agrobacterium tumefaciens), and is often a serious problem when vines are propagated by softwood cuttings in the greenhouse if strict sanitation is not observed. It is usually not serious in California vineyards, except when the trunks and arms have been cracked by freezing (Jacob, 1950). Injuries on the arms and trunk can develop tumorlike spongy overgrowth during the spring and summer. These overgrowths on the aerial parts of the plant often turn dark.

The disease also occurs around the crown of the plant, often just below the soil surface. This form is usually called crown gall. The bacteria can berries spread to the aerial parts of the vine by rains and may enter the tissues through fresh wounds. The pathogen can move in the vessels, spread to aerial parts of the vine, and cause secondary tumors to develop (Lehoczky, 1968). The bacteria in the plant tissues stimulate the cells to grow rapidly, resulting in the formation of overgrowths. The best prevention is sanitation. Avoid cutting into affected tissue with pruning shears.

Continue to Chapter 16