3.1
Pest Information – Diseases
BITTER
ROT (see “Summer
rots”).
PHOMOPSIS CANE AND
LEAF SPOT AND FRUIT ROT
Several important insects and diseases that occur in New York and Pennsylvania vineyards are described below to help growers manage these pests with practices and pesticides appropriate for their production systems.
ANGULAR LEAF SCORCH was first described in 1985. Symptoms of this fungal disease are similar to those of rotbrenner, a disease of grapevines found in the cool grape-growing regions of Europe, which is caused by a very closely related fungus. Angular leaf scorch occurs sporadically, and is most likely to become a problem in years when high rainfall occurs during early shoot growth.
Disease symptoms occur mainly on the leaves and first appear as faint chlorotic spots. As these lesions grow larger, they change from yellow to reddish-brown and the margin often becomes pronounced (depending on the cultivar, the margin may be yellow, red, or absent). Lesions are confined by major veins, becoming "angular" or wedge-shaped. They eventually kill the tissue, often causing infected leaves to fall prematurely.
The fungus survives winter in infected leaves on the vineyard floor. Mature spores are ready for discharge in spring when grape buds begin to grow. During rainfall, spores are released into the air from fruiting structures, and susceptible tissue is infected.
Cultural practices that increase air circulation through the canopy can shorten periods of leaf wetness that favors disease development. Destruction of leaf litter by cultivation, before bud break, can also reduce disease pressure. Where control is needed, fungicides should be applied before rainfall, beginning about the 3-inch stage and continuing through fruit set. Although there are no specific labels for the control of this disease, mancozeb products (e.g., Dithane, Penncozeb) applied according to label directions to control Phomopsis, black rot, and downy mildew also have provided good control of angular leaf scorch. In Europe, the strobilurin fungicides (Abound, Flint, Pristine, Sovran) have provided good control of the closely related disease, rotbrenner. However, experience with angular leaf scorch is lacking, and use of these materials before the prebloom stage is not recommended. See Table 3.1.2 for varietal susceptibility to this disease.
ANTHRACNOSE is a disease that occurs primarily in very wet years, with damage typically being limited to a few highly susceptible cultivars. In NY/PA, most outbreaks have occurred on Vidal blanc and a few minor seedless table grape varieties, especially Reliance. There is also some indication that the new cold-hardy cultivar, La Crescent, may be in this category. Symptoms occur on leaves, green shoots, and clusters. On leaves, numerous small, circular brown spots appear, which later turn gray in the center and develop dark brown to black margins. In severe attacks, lesions may coalesce and cause distortion of the leaf blade and eventually death of the entire leaf. Infected shoots develop dark, noticeably sunken lesions; a slightly raised area may form around the margins of the lesions, whose centers may extend into the pith of the shoots. Severely infected shoots are often stunted. On berries, spots approximately 0.25-in in diameter develop, with whitish-gray centers surrounded by reddish brown to black margins, producing an appearance that superficially resembles a bird’s eye. Sometimes, severely affected berries may shrivel and dry into mummies.
The fungus overwinters primarily on infected canes. In spring, spores are produced from the overwintering fungal structures and are dispersed by splashing raindrops to young, susceptible tissues. Infection can occur across a wide range of temperatures, but temperatures in the mid-70’s to low 80’s (°F; 24-27°C) are optimal. Additional spores, which also are splash-dispersed, are produced upon new infections, and these can rapidly spread the disease through multiple repeating cycles of new infection and additional spore production. Hence, outbreaks occur most frequently in years with multiple rain events throughout the season.
Fungal inoculum to start the disease cycle comes primarily from infected canes. Diseased canes should be pruned during the dormant season and removed from the vineyard or destroyed. If numerous infected berries remain on the vineyard floor, the spores originating from them can be largely negated by covering the berries with soil through cultivation or, if practical, covering them with mulch. Early-season sprays of mancozeb, captan, or ziram targeted against Phomopsis also provide significant control of anthracnose, although this latter disease is not listed as a target on most labels. A “delayed dormant” application of lime sulfur can be useful in vineyards where the disease has become established and/or “organic’ vineyards where traditional fungicides cannot be used (limits production of infectious spores from overwintered cankers).
BITTER ROT (see “Summer rots”).
BLACK ROT
is one of the most serious diseases of grapes in the eastern United States.
Although relatively easy to manage with the aid of conventional fungicides, it
has the potential to cause substantial crop loss in wet
years. Fruit rot is the most damaging phase of the disease, but all green
tissues of the vine are susceptible to infection.
Infected leaves develop relatively small, brown circular lesions surrounded by distinct dark margins; black, pimple-like fruiting bodies (“pycnidia”) are scattered within these spot-like lesions. Black, elongated lesions on petioles (leaf stems) may cause affected leaves to wilt and drop. Large, black, elliptical lesions on infected shoots may contribute to breakage by wind. The disease is most common and damaging on berries. These are chocolate brown when first infected, but soon become dark brown with numerous black, pimple-like pycnidia on the surface. They eventually shrivel into hard, black raisin-like mummies, most of which remain firmly attached to the berry stem.
The black rot fungus overwinters primarily in mummified fruit on the vineyard floor or in mummies retained within the vine. It can also overwinter within cane lesions when these develop. Rain triggers the release of infective spores from all sources, and infection occurs if susceptible tissues remain wet for a sufficient length of time, which depends on temperature (Table 3.1.1). Spores within cane lesions are available for infection starting at bud break; however, the majority of overwintering spores in most vineyards (those within mummified fruit on the ground) first become available about 2–3 weeks after bud break, reach peak levels about 1–2 weeks before bloom, and are usually depleted within one to several weeks after the start of bloom, depending on the season. However, in years with dry spring weather when only a few rains occur, the fungus does not discharge all of its spores as early as usual, and significant spore discharge may extend several weeks beyond bloom if this is when rains finally develop.
CAUTION: Most mummified fruit remain firmly attached to the berry stems and are not removed by machine harvesting. When these are not dropped to the ground during dormant pruning operations, they will produce large numbers of spores within the canopy throughout the period of berry development the following year. Research has shown that this prolonged period of high spore production, combined with the closeness of the spores to newly-developing berries, significantly increases the pressure for berry rot. Therefore, removal of mummies from the canopy is a critical component of black rot management programs.
Pycnidia develop within the lesions caused by current season infections and begin releasing a new generation of spores about 2 weeks after infection first occurs. These spores then cause new infections under favorable rainy conditions. Such secondary rounds of new spore release and re-infection are responsible for disease spread, and are the cause of most economic loss when it occurs. Fruit are highly susceptible to infection for the first 2–3 weeks after cap fall. They become progressively less susceptible as they continue to develop, finally becoming highly resistant about 5–8 weeks after bloom, depending on the variety and year. In general, “Concord” fruit appear to become resistant about 1–2 weeks earlier than those of V. vinifera varieties. Thus, the most critical time to control berry infections is during the first few weeks after the start of bloom.
Removal of mummified clusters from the canopy during pruning significantly reduces disease pressure for the coming season; burying mummies on the ground before or soon after budbreak, by cultivation or covering them with mulch, also can contribute to a reduction of inoculum if disease was severe the previous season. All spray products currently approved for “organic” production are weak against black rot, although copper has moderate efficacy if applied very regularly. Therefore, growers of organic grapes should pay strict attention to the above sanitation procedures, because they are the most important defenses against this disease, which can be the “Achilles heel” of organic grape production in eastern viticulture. Cultural practices that open the canopy also are beneficial because they promote drying and improve spray coverage. See Table 3.1.2 for varietal susceptibility to this disease.
Spray Timing: Fungicide applications once were recommended to begin at about 3- to 5-inch shoot growth and continue at regular intervals of 10–14 days until veraison. However, most disease control is provided by the immediate prebloom and first postbloom fungicide sprays, and virtually complete control has been provided in the vast majority of vineyards when these sprays have been followed by a second postbloom application. This second post-bloom application is potentially critical if black rot symptoms are visible on either leaves or fruit at that time, and should be considered prudent in most vineyards if this portion of the season is wet. If severe black rot developed during the previous year, or if the vineyard has a history of consistent black rot development, an additional spray two weeks before bloom may be beneficial, particularly under wet conditions. Because damaging levels of black rot usually result from spread within the cluster, sprays should continue through the end of July if more than a trace level of berry rot is present and weather conditions are suitable for infection (see Table 3.1.1).
Rally (^Nova) and ΔElite provide excellent control of black rot, and are labeled for up to 72 hours of postinfection activity. Recent research indicates that their effective period of postinfection activity is one to several days longer than this, although delaying postinfection applications beyond 72 hr should be employed on an emergency basis, not as a deliberate strategy. The strobilurin fungicides (Abound, Flint, Pristine, Sovran) are excellent protectants but provide only very limited postinfection activity. Mancozeb and ziram are good protectants but are more subject to wash-off than the strobilurins, so must be reapplied more frequently during high-rainfall conditions. Other fungicides should not be relied upon for black rot management. Copper fungicides are the most effective organically-approved materials, but must be applied frequently (e.g., 1-wk intervals) under high disease pressure conditions.
BOTRYTIS is a fungus that causes a bunch rot of berries and also may blight blossoms, leaves, and shoots. The bunch rot phase of the disease can cause severe economic losses, particularly on tight-clustered French hybrid and Vitis vinifera cultivars. Ripe berries are susceptible to direct attack and are particularly susceptible to infection through wounds such as those caused by insects, hail, or rain cracking. Once established, infections can spread rapidly throughout the cluster during the preharvest period (especially if berries are tightly compressed), causing extensive loss in yield and quality. This disease can be distinguished from other causes of bunch rot by the characteristic masses of gray “fuzzy” spores produced by the Botrytis fungus on infected plant parts, especially during humid weather.
The fungus overwinters in debris on the vineyard floor or on the vine. Old cluster stems are a particularly important source of carry-over between seasons. Spores are produced throughout the growing season, although their numbers appear to be greatest near bloom and after veraison. Production of spores and subsequent infection are greatly favored by prolonged periods of wetness or very high humidity, particularly at moderate temperatures (60–75°F).
The Botrytis fungus is most capable of attacking injured or senescing tissues; hence, infections usually occur as blossoms wither, as fruit ripen, or through wounds. Wounds caused by the grape berry moth are particularly common sites of infection. Under wet conditions, blossom parts can become infected between cap fall and bunch closing; such infections can lead to latent (dormant) infections of the young berries, which then become active as the berries begin to ripen. Although direct losses from these early infections appear to be modest, they often provide a starting point for sudden and significant disease spread within the clusters if wet weather occurs before harvest. Berries infected by powdery mildew between fruit set and bunch closing also can serve as starting points for a Botrytis epidemic; hence, good control of powdery mildew during this period is an important component of a good Botrytis management program.
Botrytis management is best accomplished through a combination of cultural and chemical methods. In fact, perhaps more than for any other common disease, consistent control of Botrytis requires the conscientious use of cultural management practices even when the best fungicides are applied. Any practice that improves air circulation and thereby reduces humidity within the canopy is of significant benefit. Such practices include site selection to avoid fog pockets and heavily wooded areas; management of canopy densities through pruning, shoot positioning, and selectively removing leaves in the cluster zone immediately after fruit have set; and avoiding excessive nitrogen fertilization. Loose clusters also significantly reduce Botrytis development, and the use of clones (e.g., the ‘Mariafeld’ clones of cv. Pinot noir) or viticultural techniques that provide loose clusters can aid in its control. Well-timed sprays of an effective fungicide also are important in many years. Research conducted during the early 1980s in the Finger Lakes region of New York indicated that veraison and preharvest were the most important application times, and this became the recommendation thereafter. However, over the past decade, research in this region and on Long Island has consistently shown that applications at late bloom and bunch closure can be just as important or even more so than those at veraison and preharvest, particularly if the weather is wet during the earlier period. Such early sprays appear to be particularly important on extremely tight-clustered varieties such as Vignoles. Thus, growers who have had trouble controlling Botrytis when they have delayed applications until veraison (or later) should consider appropriate protection against the disease if the weather is wet during the bloom and/or pre-bunch closure period.
Eight fungicides are now labeled for control of Botrytis: Adament, Elevate, Endura, Flint, Pristine, Rovral, Scala, and Vangard. These materials are expensive, and the Botrytis fungus can develop resistance to them. Therefore, they should not be used unnecessarily, nor should a grower rely on a single compound or class of compounds for control over time. Note that for rotational purposes, Vangard and Scala are in the same class; Adament, Flint and Pristine should be considered in the same class; and Pristine and Endura should be considered in the same class. Note also that for Adament, Endura, Flint, and Pristine, the labeled rate for Botrytis control is higher than that for control of other diseases against which these materials are active.
CROWN GALL is a bacterial disease whose characteristic symptom is fleshy galls produced mostly on the lower trunk, but which may form anywhere on the trunks where injuries occur. Large galls may develop rapidly and completely girdle young vines in one season. Galled vines frequently produce inferior shoot growth, and portions of the vine above the galls may die. Current-season galls are first apparent in early summer as white, fleshy, callus growth. Galls turn brown by late summer, and in the fall, become dry and corky. The crown gall bacterium is systemically present in the vast majority of grape vines, but seldom causes disease unless the vine is injured. Budding and grafting cause injuries that occasionally elicit disease development at those wounding sites, as does “tractor blight”, but cold injury is by far the most important factor in the Northeast. Therefore, management practices that minimize the risk of cold injury are currently the only practical technique for managing the disease. These include careful site selection for cold-sensitive cultivars and cultural practices that promote winter hardiness. Hilling above the union of grafted vines protects buds from freezing and ensures the development of new scion shoots that may be needed for trunk renewal. The use of multiple-trunk vines and regular replacement of diseased or dead trunks with renewals helps to manage the disease at a tolerable level.
DOWNY MILDEW is caused by a fungus that can infect berries, leaves, and young shoots. Leaf lesions appear as yellow or reddish-brown areas on the upper surface, with corresponding white, downy, or cottony fungal growth directly opposite on the lower surface (note that downy mildew growth appears only on the lower leaf surface and looks cottony, whereas powdery mildew growth can occur on both sides of the leaf and looks more like baby powder). Leaf lesions become brown and dead with age, and severely infected leaves fall prematurely. Young, infected shoots and cluster stems may curl and are characteristically covered with the white, ”downy” growth of the fungus on mornings following rain or dew the night before. Berries on infected cluster stems may fail to set or can turn brown and eventually shrivel, depending on the time of infection. Berries that are directly infected while very young may become entirely covered with a fuzzy white fungal growth when wet from evening rain or early morning dew. Cluster infections that occur later in the season cause berries to remain hard, with a mottled light green to brown or red appearance. Frequent rainfall and high humidity are the most important environmental factors promoting downy mildew epidemics. The downy mildew organism overwinters as dormant spores within infected leaves on the vineyard floor or (more commonly) within the upper soil layer, and first becomes active in the spring about 2–3 weeks before bloom. Infective spores are then produced during rainy periods if temperatures are above 52°F, and are splashed from the soil onto susceptible tissues to cause the season’s first (primary) infections. (Note that inoculum for such early-season infections come strictly from within the vineyard.) Epidemic disease development can then result from repeated cycles of new infections, which are caused by new spores produced within the white fungal growth on diseased tissues. These spores are produced only at night when the relative humidity is extremely high (>95%). They can be blown relatively long distances and cause infection when they land on susceptible tissues that remain wet for just a few hours. (Note that such disease spread can also originate from nearby vines outside the vineyard.) The generation period for the fungus (time from spore germination and infection to the production of a new “crop” of secondary spores) is only 4 to 5 days at optimum temperatures in the mid- to upper-70s (°F), allowing explosive disease development during extended periods of warm, humid weather with periodic rain showers. On some cultivars, including all V. vinifera varieties, this can be particularly destructive during the several week period before and after bloom, when fruit clusters are highly susceptible to infection. Young leaves remain highly susceptible to infection so long as they continue to be produced, although even older leaves can become diseased under high-pressure conditions. Uncontrolled infections can cause extensive defoliation in wet years, limiting both fruit ripening and vine winter hardiness. Winter kill of buds or even entire vines is not uncommon when spraying stops too early on susceptible varieties in a bad downy mildew season. Disease can develop at a wide range of temperatures, from the low 50s to the mid-80s, although the rate of spread is slower while at these edges of the range.
Any practice that improves air circulation and speeds drying within vine canopies will help to control downy mildew. However, properly timed fungicides are still necessary for reliable disease management. Mancozeb, captan, and copper fungicides provide good protection when applied at 7–14 day intervals (use the shorter end of this range when very rainy, due to wash-off, or during periods of rapid shoot growth). Ridomil is a systemic fungicide that provides outstanding control of downy mildew when applied at 14-day intervals, but it is not effective against any other disease of grapes. (The mancozeb or copper that is prepackaged with commercial Ridomil formulations may provide some benefit against other diseases, although the rate of these mixing components is often low; for instance, the 2.5-pound rate of Ridomil MZ 58 provides the same amount of mancozeb as 2 pounds of a standard mancozeb product such as Dithane or Penncozeb). Ridomil also has significant post-infection activity, but it is highly prone to resistance development and should be applied only a limited number of times in any particular season, in rotation with other DM products. Abound and Pristine have traditionally provided excellent control of downy mildew when applied at 14-day intervals. These are primarily protectant fungicides, with very limited post-infection activity. However, they also are at high risk for resistance development, and resistance has been confirmed in a number of vineyards in the mid-Atlantic, southeastern, and southern regions of the U.S. Similar product failures occurred in NY during 2009; laboratory tests to confirm resistance are still underway. Due to the risk of resistance, these materials should be used only on a limited scale in rotation with unrelated products. Although in the same chemical family, Sovran has only moderate activity against downy mildew and Flint has very little, hence they should not be relied upon under moderate or greater disease pressure. In recent years, several different products containing phosphorous acid (also called “phosphite” or “phosphonate”) have come onto the market; some (e.g., ProPhyt, Phostrol) are labeled for control of downy mildew whereas others are not. (Be sure to note that this is a different chemical than phosphoric acid or phosphate [note the “-ic” or "-ate" suffix], the form of P found in most commercial fertilizers.) The phosphite products have been used widely throughout eastern viticulture regions since 2003, with generally good results. Research in NY has shown that these materials provide both protective and postinfection activity, although the period of residual (protective) activity is limited, particularly in older leaves. Post-infection sprays have been very effective at controlling disease when applied before symptoms occur; this activity is most pronounced at higher rates, especially when applied in “back to back” applications a week or 10 days apart. Sprays applied to active leaf lesions will not eradicate them but will greatly reduce their further production of spores, thereby limiting additional disease spread.
Downy mildew management programs should focus on (a) preventing early disease establishment and destructive cluster infections during the prebloom and early postbloom periods, and (b) limiting secondary spread on the foliage during the summer and early fall. The timing and intensity of these sprays should be determined by varietal susceptibility, weather conditions, and the availability of fungal inoculum. Because primary infections can first occur 2–3 weeks before bloom, protection may need to start at this time on vinifera cultivars and on highly susceptible hybrid and native American cultivars (e.g., Chancellor, Catawba, Niagara) if the weather is wet. This is particularly true if significant disease occurred the previous year, providing high levels of overwintering inoculum within the vineyard. Clusters should be protected on all but the most highly resistant cultivars from the immediate prebloom period through the first or second postbloom spray, depending on the weather. Continued protection against disease spread during the summer should be based on cultivar susceptibility, the extent of favorable weather conditions, and the amount of disease already in the vineyard (secondary inoculum). Downy mildew has the potential for “explosive” spread if the disease is active and weather conditions favor its continued development. However, in many years, hot, drier weather causes the downy mildew fungus to become inactive during mid-summer. Thus, it is worthwhile to scout vineyards during this time for the presence of active disease and to determine the need for protective sprays based on such findings. Also, recognize that fruit lose their susceptibility to infection by midsummer (repeated trials have shown that fungicides applied later than 4 weeks after the start of bloom provide no additional control of cluster infections on highly susceptible varieties), although protection against leaf infections and consequent defoliation may need to continue throughout the summer, depending on weather conditions. See Table 3.1.2 for varietal susceptibility to this disease.
EUTYPA DIEBACK is a fungal disease appearing as cankers on trunks and arms of infected grapevines. New shoots above cankers often appear stunted, with shortened internodes and small, cupped, greenish-yellow leaves in the spring. (Such symptoms on new shoots superficially resemble those caused by Roundup and similar herbicides.) Healthy shoots usually overgrow and obscure affected shoots by early- to midsummer. Shoot and leaf symptoms become progressively worse each season until, eventually, the entire portion of the trunk or cordon above the canker dies.
In winter or early spring, during rainfall or snowmelt, fungal spores are released from fruiting structures on the dead, infected wood of the cankers. Spores are dispersed by the wind and infection occurs when they enter fresh pruning wounds. Cankers and foliage symptoms are not evident until 2 to 4 years after infection; then, vine deterioration continues until the trunk or arm is finally killed.
Infected cordons or trunks should be removed in late spring when foliar symptoms are noticeable and the resultant wounds remain susceptible for a more limited period of time than if made earlier. Pruning should be at least 6 inches below any dead or discolored wood associated with the canker. Any infected wood or stumps should at the very least be removed from the vineyard, and burned or buried if practical. Topsin-M fungicide has a NY State (SLN) registration for application as a concentrated paste to protect fresh pruning wounds from this disease. Although impractical for routine pruning cuts, this treatment may be worth the time and expense on a limited number of major pruning wounds that could each cause the loss of an entire cordon or trunk (hence, vine) if they became infected. See Table 3.1.2 for varietal susceptibility to this disease.
PHOMOPSIS CANE AND LEAF SPOT AND FRUIT ROT are most likely to become problems when the Phomopsis fungus is allowed to build up on dead canes or pruning stubs in the vines and/or when early-season sprays for this disease are omitted. Thus, hedged vineyards are particularly at risk of incurring economic losses from Phomopsis, although serious problems also occur regularly in hand-pruned vineyards when conditions are favorable and sprays are not properly timed. Economic losses have been especially severe on Niagara, and to a lesser extent, Concord, although many other native, hybrid and V. vinifera cultivars are susceptible as well (see Table 3.1.2). Infected rachises and shoots develop black lesions that may split the green tissue (shoots) or appear sunken (rachises). Numerous lesions give the shoot surface a blackened, scabby appearance, and may coalesce to girdle the rachises. Severe infection weakens the tissues at these spots and can cause infected shoots to break off during high winds, or infected clusters to break before and during harvest, causing fruit to fall to the ground. Small, pinprick-sized lesions, with brown or black centers surrounded by a small and somewhat yellow margin, can be numerous on the leaves early in the season. These infections cause little harm themselves, but provide a good indication that the fungus is present and capable of causing more serious losses on other organs, should they be left unprotected during favorable weather periods. Infected berries remain symptomless until late summer or preharvest, when they turn brown, often beginning at the point of attachment to the pedicel (berry stem), and become covered with black, pimple-like fruiting bodies. Such berries eventually shrivel up into raisin-like "mummies", at which time they look very similar to berries infected with black rot. On fruit, the two diseases are best distinguished by the initial location, timing, and development of symptoms, in addition to the strength of the attachment of individual fruit to the berry stem. Phomopsis lesions typically (but not always) start where the berry is attached to its stem, whereas black rot lesions start at random locations on the fruit. Also, Phomopsis lesions do not appear until late summer or early fall, often just before harvest; in contrast, most black rot symptoms appear by late July or early August, and all diseased berries should be evident by veraison. Finally, berries infected with Phomopsis are usually quite easy to detach from their stem by lightly touching them or giving a gentle pull, whereas those with black rot typically remain attached firmly to the berry stem.
Black fruiting bodies of the Phomopsis fungus overwinter in infected wood (diseased canes or pruning stubs) and rachises. During wet periods, spores ooze from the fruiting bodies and are distributed by raindrops onto nearby susceptible tissues. For this reason, young shoots and clusters directly beneath old canes and pruning stubs are at greater risk than those that are trained to grow above these sources. Extended periods of wet weather are particularly favorable for disease development. Shoot and leaf infections can occur anytime between bud break and early summer, although they are most common during the first few weeks of growth. Shoot and leaf lesions appear within 3 to 4 weeks after infection, but they do not serve as a source of disease spread during the current season. Rachises can be infected anytime after the young clusters first emerge until fungal spores are depleted in early summer, although infections that occur soon after cluster emergence in the early growing season are the most damaging. Infections that occur on the pedicels (berry stems) during this period can also move into the fruit, causing them to rot before harvest. Fruit appear to be most at risk for direct infection from bloom until pea-sized, after which few spores are available to cause new infections. Fruit infection occurs sporadically, since it requires extended periods of rain and wetness; however, serious losses can result if the growing season is especially wet and protection is not maintained with an effective Phomopsis fungicide from the early shoot growth period through fruit set. The trend to omit early season Phomopsis sprays in many juice grape blocks has been accompanied by significant fruit and rachis infections in several wet years.
Diseased canes should be removed during pruning to reduce inoculum. Recent research has shown that dead canes and pruning stubs can produce extremely high levels of Phomopsis spores, and these sources should be specifically targeted for removal as part of a Phomopsis management program. The need for fungicidal protection programs on susceptible cultivars is dependent on the level of inoculum within the vineyard and the frequency and duration of wetness periods. Recent research from Ohio suggests that when inoculum is present, moderately-severe infection can develop after about 26 hr. of wetness at an average temperature of 48°F, 16 hr. at 54°F, and 12 hr. at 60–68°F (shorter and longer periods of wetness at any given temperature should reduce and increase disease severity, respectively). Captan, mancozeb, and ziram have been the most effective fungicides against Phomopsis in local trials. Early-season applications of the strobilurin fungicides have provided only fair control of shoot infections in research trials. However, these materials have provided good protection of rachises and fruit when used from pre-bloom onwards, following at least one earlier application of a traditional protectant fungicide. Copper and sulfur are only weakly effective; thus, organic growers or others seeking to minimize the use of conventional fungicides should pay strict attention to the removal of infected wood from within the canopy. Protective sprays of mancozeb, captan, or ziram may need to begin at as early as 1-inch shoot growth in vineyards with heavy inoculum pressure, and should generally not be delayed beyond the 3- to 5-inch shoot stage. Clusters may benefit from protection from the time of their emergence through the early fruit set period, although the early part of this period (cluster emergence through prebloom) appears to be the critical time to protect against rachis and fruit infections if conditions are favorable for the disease. The first couple of weeks after bloom can also be an important time to maintain protection against fruit infection under wet conditions, particularly if the block has a history of Phomopsis. See Table 3.1.2 for varietal susceptibility to this disease.
POWDERY MILDEW is a fungal disease that affects all green tissues. Diseased tissues appear to be covered with a white to grayish-white powder. Severe leaf infection can result in cupping, drying, and premature drop. Infected berries may fail to ripen properly; remain covered with a dusty mass of the fungus; turn dark brown; and/or shrivel and split, depending on the time and severity of infection. Fruit infection may promote growth of spoilage microorganisms and reduce wine quality on grapes intended for that use, even when symptoms are relatively mild. The powdery mildew fungus overwinters on the bark of the vine as tiny black fruiting bodies (“cleistothecia”). Spores (“ascospores”) contained in the cleistothecia are released during rains of approximately 0.1-inch or more, from bud break until shortly after bloom. They are wind-dispersed to young leaves and clusters, and can infect wet or dry tissue at temperatures of 50°F or higher. These first mildew colonies that result from ascospore infection then produce masses of white, powdery secondary spores (“conidia”). Conidia are wind-dispersed throughout the vineyard and do not require rain for release or infection, although humid conditions particularly favor disease development. New colonies that result from these secondary infections produce additional conidia, which can continue to spread the disease. This repeating cycle of infection, spore production, spore dispersal, and re-infection continues throughout the season if susceptible tissue is present, at a rate that is driven by temperature (Table 3.1.3). Thus, at optimum temperatures in the mid-60s to mid-80s (°F), a new generation of the fungus can multiply every 5–7 days, resulting in an epidemic of powdery mildew unless it is managed efficiently.
In addition to moderately warm temperatures, disease development is strongly favored by high humidity and cloudy weather. Therefore, management programs may need to be intensified (e.g., shorter spray intervals, higher fungicide rates, more efficacious materials) during periods when such conditions occur. Conversely, the harmful impact of sunlight on the powdery mildew fungus can be exploited by pruning and training practices that promote good light exposure throughout the canopy, thereby utilizing this natural “fungicide” to help manage the disease.
Berries are highly susceptible to infection from the immediate prebloom stage until about two to three weeks after fruit set. Severe fruit damage observed later in the season almost always is the result of infections that occurred during this peak period of susceptibility. Berries of Concord become almost completely resistant to infection once they reach about 0.25 inch in diameter, although the rachises remain susceptible until harvest (the economic importance of mid- or late-summer rachis infections on processing fruit is questionable). On berries of V. vinifera and certain hybrid cultivars, infections can continue to occur until bunch closure or slightly thereafter. Such midsummer infection usually results in the development of sparse, inconspicuous infections that can be especially important as entry points for Botrytis and sour rot organisms that become apparent at harvest, or for spoilage microorganisms (e.g., Brettanomyces) that reduce wine quality.
Leaf infections that occur beyond the fruit set period are much less serious on Concord and similar cultivars than on V. vinifera and susceptible hybrids. On Concord, such infections appear to have relatively little effect on yield and Brix levels for low to moderately cropped vines, although they can suppress both Brix levels and yield on more heavily cropped vines, particularly in years with poor ripening conditions. Thus, on this variety, the need for fungicide sprays after fruit set should be heavily influenced by both crop size and weather factors. On V. vinifera and highly susceptible hybrid cultivars, continued suppression of foliar mildew is required throughout the summer to avoid poor ripening, premature defoliation, and reduced winter hardiness. Also, good management of leaf disease throughout the season significantly reduces disease pressure the following year, by limiting the number of cleistothecia (fungal fruiting structures) that form, overwinter, and initiate infection in the spring. Limiting the level of overwintering inoculum has been shown to have a particularly positive impact on the control of cluster infections the following season.
For effective management of powdery mildew, sprays may be required as early as 3- to 5-inch shoot growth on V. vinifera and highly susceptible hybrid cultivars, depending on rainfall and temperature in the early season (cold nights retard disease development), and the level of disease control the previous year. Sprays should not be delayed beyond the immediate prebloom stage (or earlier if the disease is seen to be active) on Concord or other moderately susceptible cultivars. High disease levels the previous year increase early-season disease pressure, and consequently, the relative importance of early-season sprays.
To protect against powdery mildew developing on the fruit, management programs should be at their peak from just before bloom through fruit set, emphasizing the use of effective fungicides, full rates, appropriate spray intervals, and superior spray coverage (every row, proper speed, sufficient water volume). The risk of berry infection is particularly high when days and nights remain warm during this period (see Table 3.1.3) and/or weather is cloudy and wet, and spray programs may need to be especially “tight” under these circumstances. Protection of Concord berries is not required after fruit are 0.25-inch in diameter, although continued foliar protection may be beneficial under high-crop or poor-ripening conditions. For V. vinifera and susceptible hybrids, it is important to maintain excellent protection of the clusters through the bunch closure period, since powdery mildew infections at this time can promote the later development of bunch rots and/or wine spoilage. Maintenance programs to protect foliage throughout the summer are necessary for attaining maximum fruit and vine quality on V. vinifera and susceptible hybrid cultivars. In years or locations where several weeks may elapse between harvest and frost, additional postharvest protection of the foliage may benefit vines of highly susceptible V. vinifera cultivars, especially if disease is active in the vineyard and the weather is reasonably warm. See Table 3.1.2 for varietal susceptibility to this disease.
RIPE
ROT (see “Summer rots”).
SOUR ROT develops on injured berries when the weather is wet during the pre-harvest period. Affected berries typically are colonized by a mix of various wound-invading fungi and bacteria and can give off a strong smell of vinegar, the result of infection by a specific group of bacteria. Diseased berries drip juice and spores or cells of the sour rot microorganisms onto nearby healthy berries, which in turn become infected through any wounds that might be available (rain cracks, bird or insect damage, Botrytis or powdery mildew infections, etc.).
Although it is almost impossible, under wet conditions, to stop sour rot once it has become established, controlling other causes of injury beforehand will greatly reduce the probability of the disease getting started in the first place. Experience has shown that excellent control of powdery mildew and, especially, Botrytis are two measures that significantly minimize sour rot development. There is also some indication that broad-spectrum fungicides used for Botrytis control (e.g., Flint, Pristine) may provide some additional control of the wound-invading sour rot fungi; however, any product that gives good Botrytis control will help greatly to prevent sour rot.
SUMMER ROTS is a term sometimes used for two similar diseases (ripe rot and bitter rot) common in more southern, humid production regions, although they occasionally occur in NY and PA (especially Long Island and southeast PA). Bitter rot is the more regular threat in the mid-Atlantic region, but appears to occur only sporadically in New York, presumably due to the somewhat cooler temperatures farther north. Nevertheless, it now appears that this disease may occur more frequently than previously thought. Usually, symptoms first occur after veraison, as the bitter rot fungus moves into the berry from the pedicel and turns the diseased portion brown (on white varieties) or a dull purple. Once the berry is completely rotted, it becomes covered with numerous raised black pustules (the fungal fruiting bodies, called acervuli). Within a few days, diseased berries soften and may drop; others shrivel into firmly attached mummies that resemble those caused by black rot and Phomopsis. In addition to the timing of symptom appearance (most or all black rot infections should be apparent at or before veraison), black rot and bitter rot also can be distinguished by (i) the appearance of the fungal fruiting bodies on infected fruit (those of black rot are relatively small, round and uniform in size, whereas those of bitter rot are irregular and variable in size, often larger and more pronounced); and (ii) the tendency of fruit infected with bitter rot to leave hands sooty black if handled when wet (whereas those infected with black rot will leaves hands clean). Berries infected with Phomopsis also tend to appear during the preharvest period. However, significant Phomopsis fruit rot typically occurs in association with significant infections of the rachis, and of the shoots and petioles (leaf stems) near the bottom three to five leaf positions on the shoots bearing the diseased berries. An absence of these other symptoms suggests that Phomopsis is not the cause of multiple berry infections.
Ripe rot tends to predominate further south, although it has been documented as far north as New England. Symptoms do not develop until after veraison and become increasingly prevalent by harvest. Infected fruit initially develop circular, reddish brown lesions on their skin, which eventually expand to affect the entire berry. Under humid conditions, small “dots” of slimy, salmon-colored spores may develop across the rotten berry, and serve to spread the disease to healthy fruit if rains continue. Infected fruit shrivel and mummify, and may either remain attached or fall to the ground. No foliar symptoms are produced.
Both diseases are favored by abundant, warm rains (77° to 86°F while berries remain wet is optimum) between veraison and harvest, although initial infections can occur much earlier and remain “dormant” until this time. Captan and the strobilurin fungicides are particularly useful for control of these diseases in regions where they are common, or to the north if the weather is unusually wet and warm. Susceptibility to both diseases increases with fruit maturity, so fruit become especially vulnerable when harvest is delayed.
The bitter rot fungus colonizes rotten berries and other dead tissues of the grapevine (fallen leaves and berries, damaged shoots, necrotic bark), where it overwinters and produces spores the following spring. After flowering, some spores are moved by splashing raindrops onto the pedicels (individual stems) of the developing berries, where they germinate and cause a latent (dormant) infection. When the berries mature, the fungus resumes growth and advances into them, causing the fruit to rot. The acervuli that cover the diseased berries contain abundant spores, which spread the disease as they are splashed onto healthy berries during subsequent rains. Infection occurs through any type of injury, including rain cracking, insect damage, or bird injury.
The ripe rot fungus overwinters in mummified fruit, infected pedicels, and dead bark and cankers. Spores are produced from these sites in the spring and are distributed by splashing and blowing rain. Fruit may be infected at any stage of their development, but the fruit remain symptomless (infections remain latent) until the berries begin to ripen. During warm rainy periods, the salmon-colored spores produced upon diseased fruit can spread the disease to additional berries, which become increasingly susceptible to infection as they ripen. Frequent rains during the pre-harvest period can result in severe crop loss once the disease becomes active and begins to spread, especially if it is not managed.
Cultural practices, such as pruning out dead spurs, removing overwintered mummies, and removing weak or dead cordons, are important to help reduce the inoculum of both diseases in the vineyard. Both are frequently controlled in the early- to mid-summer by fungicide sprays targeted against other diseases, such as downy mildew and black rot. However, not all fungicides applied for control of these latter diseases will provide control of bitter rot and ripe rot; check current recommendations to determine which products may be appropriate. Similarly, with the exception of Flint and Pristine, most fungicides used for Botrytis management (Elevate, Scala, Rovral, Vangard) provide little control of bitter rot or ripe rot. Thus, exclusive use of such botrytis-specific fungicides during wet preharvest seasons can lead to outbreaks of these other diseases in regions where they are not common and, therefore, are not consciously managed. Sprays targeted against bitter rot and/or ripe rot may be needed in the late season if the weather is warm and wet, especially if the diseases are observed in the vineyard or have occurred there in the past. In southerly regions where the diseases are consistent problems, it is typically necessary to apply protectant fungicides on a 2-week schedule from bloom until harvest, except during periods of drought. Because fruit are especially vulnerable in their final stages of ripening, pre-harvest sprays are particularly important where these diseases are active. French-American hybrids are generally more resistant to bitter rot than are cultivars of V. vinifera, within which Chardonnay is among the more susceptible of the widely-grown cultivars.
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