With the 2014 wine grape harvest just getting underway in some areas, it’s a good time to check clusters for signs of certain disease and insect damage. Results can help you evaluate just how well your management program controlled the problems this season and give you time to make any needed adjustments to limit such threats to next year’s crop.
Pest management guidelines for grapes at harvest, part of a year-round IPM program described on the University of California IPM Online web site – www.ipm.ucdavis.edu – includes a section on Damage to Fruit at Harvest. It features photos of damage to wine grapes and links to detailed information on identifying, managing, monitoring and treating the following diseases and insects.
Berries infected with Botrytis bunch rot turn brown, split, and leak. As the disease progresses, the infection spreads to other berries.
Late-season infections are most severe when relative humidity exceeds 92 percent, free moisture is present on the fruit surface and temperatures are in the 58° to 82°F range. Berries that have been damaged by insects, birds, and machinery may become infected at any time after the fruit begins to ripen, because the juice in the berry can provide the water and nutrients needed for fungal growth.
This disease is more prevalent in areas with consistently high summer temperatures, such as the Central Valley, and in areas with heavy spring rainfall. Generally, plantings that are 10 years of age or older are affected. However, measles has been seen on fruit and foliage on younger vines.
Look for small, round, dark spots, each bordered by a brown-purple ring on the berries. These spots may appear at any time between fruit set and ripening.
Affected leaves display small, chlorotic interveinal areas that enlarge and dry out. Foliage symptoms are frequently called "esca." In red varieties, dark red margins surround the dead interveinal areas.
Severely-affected leaves may drop and canes may dieback from the tips. Symptoms may occur at any time during the growing season but are most prevalent during July and August. In severely affected vines, the berries often crack and dry on the vine or are subject to spoilage
Two signs of this disease are rotting fruit clusters during veraison and the presence of vinegar flies (also called fruit flies) - small, yellowish flies that are commonly attracted to fermenting fruit of all kinds. Others include masses of black, brown, or green spores that develop on the surface of infected berries.
As berries ripen and sugar content exceeds 8 percent, injured fruit becomes increasingly susceptible to invasion by a wide variety of naturally-occurring fungi. Invasion occurs at the point of injury caused by insect or bird feeding, mechanical or growth cracks, or lesions resulting from powdery mildew or black measles (esca). The resulting rot can be severe as it progresses beyond the original injury.
Bunch rot often culminates in sour rot, especially in the central and southern San Joaquin Valley. Sour rot is caused by a variety of microorganisms, including acetic acid bacteria, which are spread by vinegar flies attracted to the rotting clusters.
Sunburned fruit resulting in the loss of quality can be a result of defoliation by western grape leaf skeletonizer. Because hairs on the bodies of larvae can irritate the skin if they are brushed against, this pest can cause problems for workers at harvest. Defoliation after harvest may weaken vines by affecting stored reserves.
The metallic bluish or greenish black western grapeleaf skeletonizer moths are not long-distance fliers. As a result, this pest has been slow to spread in California since its first appearance in the 1940s and does not occur in all grape-production areas.
The first- through the early fourth-instar larvae feed on the lower leaf surface, leaving only the veins and upper cuticle. This gives leaves a whitish paperlike appearance; eventually the entire leaf turns brown. The late fourth- and all fifth-stage larvae skeletonize the leaves, leaving only the larger veins. When abundant, larvae can defoliate vines by July.
When vines are severely defoliated, larvae will then feed on grape clusters, which can result in bunch rot.
Look for rotting and raisining (drying) of Thompson seedless grapes, which follows chewing damage by omnivorous leafroller larvae.
This insect can cause serious damage in California's Central Valley and inner coastal vineyards. Although it does feed on leaves, flowers, and developing berries, the most significant damage occurs after veraison when feeding allows rot organisms to enter fruit at the damage sites.
Infestations of the grape mealybug, which can transmit grape viruses, have been on the rise in recent years in the San Joaquin Valley and North Coast. Susceptibility to mealybug damage varies by variety. It is worse on varieties that produce clusters close to the base of the shoot because the fruit often touches old wood.
Honeydew and white wax on grape vines often indicate a vine mealybug infestation. This particular pest produces more honeydew than other mealybugs. This is especially noticeable when no ants are present. During the summer look for honeydew exudates on the clusters, trunk and cordons. These exudates will resemble melted candle wax, if the infestation is severe, and basal leaves will appear shiny and sticky. Sooty mold will grow on the honeydew, and permanent parts of the vine will appear black in fall and winter.
Also look for fallen leaves beneath the canopy in July and August. To locate less severe infestations, look for all stages of the insect under the bark mainly at the graft union, on trunk pruning wounds and below the base of the spur.
The presence of ants moving up and down the vine may indicate the presence of Pseudococcus mealybugs, vine mealybug or European fruit lecanium scale.
Halo spots appear as small scars around sites where thrips punctured the grape to lay eggs. Feeding by these small insects (.04 inches long), with distinctive feathery wings produces a starfish pattern of scarring on berries.
Grape thrips and western flower thrips are the most important species causing damage on grapes. Both species may be found in most grape-growing areas. Grape thrips populations usually reach their greatest numbers in July, coinciding with peak vine growth. As vine growth slows, the numbers of thrips decreases. Western flower thrips populations peak in May, during grape bloom, when winter plant hosts dry up.
In general, thrips are a minor problem on wine and raisin grapes in California with the exception of large populations on emerging shoots in cool-growing regions.
While summer damage of leaves by thrips is common, it is not considered a problem for most varieties. However, a heavy grape thrips population can be a problem in Salvadors.
Pest management guidelines for grapes at harvest, part of a year-round IPM program, are described on the UC IPM Online web site. The Damage to Fruit at Harvest page provides links to more detailed information on identifying, managing, monitoring and treating the above diseases and insects.