Botrytis Bunch Rot

Prior to fruit ripening, early-season shoot blight may occur following prolonged warm moist conditions caused by frequent spring rains. Patches of soft brown tissue develop resulting in the death of the infected plant part. Infections often occur in leaf axils causing shoots to wilt or break off. At veraison, individually infected berries in a cluster turn brown on white cultivars or reddish in red and black cultivars. If temperatures are moderate, moisture is high, and wind speed is low, epidermal cracks will form in which fungal growth produces mycelium and spores, resulting in the char­acteristic gray, velvety appearance of infected berries.

The fungus overwinters as sclerotia most commonly in berry mummies on the ground or left hanging on the vine and in canes. After rain or irrigation the sclerotia germinate and produce spores, that are moved by air currents or splashing rain. Infections require free water for a definite period of time depending on temperature. Flowers can become infected through the stigma and scar tissue on the receptacle (tip of the pedicel) left by the detachment of the calyptra during bloom; generally the fungus then becomes dormant until late in the season when sugar concentration increases in the infected berry. The fungus then resumes growth and spreads throughout the berry. Infected berries split and leak, thus allowing the pathogen to grow and sporulate on berry surfaces and spread to adjoining berries by mid-season. Spores from infected fruit can directly infect intact, ripe berries as harvest approaches. Late-season infections are most severe when relative humidity exceeds 92%, 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, machinery, etc. may become infected at any time after the fruit begins to ripen because the juice in the berry can provide the necessary water and nutrients for fungal growth.

Successful management of Botrytis bunch rot can be achieved through the use of several strategies. By employing cultural control methods, properly applying fungicides, and using resistant cultivars when practical, the disease can be managed.

Designing vineyards to the anticipated vigor of the site conditions will produce balanced canopies with moderate shoot vigor that optimizes leaf and cluster exposure and can reduce the conditions that promote Botrytis bunch rot. Canopy management practices such as shoot thinning, hedging, and leaf removal can be used to manage canopy density when appropriate. Removal of basal leaves immediately after berry set has resulted in significantly reduced incidence and severity of disease. In warmer growing areas, be careful not to remove excessive numbers of leaves, which can lead to sunburned fruit. This condition is made worse when leaves are removed later in the season especially on canopies with southern and western afternoon exposures. If leaves are removed at fruit set, the berries acclimate readily to the sunlight and develop a thicker cuticle that helps prevent sunburn as well as Botrytis infection.

The efficacy of a fungicide depends on getting good coverage, and coverage is affected by the canopy and stage of growth. If leaves are not removed and the interior of the canopy remains dry in the spring, one fungicide application should be made sometime between bloom and pea-size berries. Otherwise, apply sprays before rainfall especially at bloom or after veraison. Grapes expected to be harvested late in the season should be treated prior to bunch closure and as needed prior to harvest for table grapes that will be stored for an extended period.

Avoid unnecessary irrigation or nitrogen fertilization that may promote excessive canopy growth. Manage insect populations that feed and produce entry wounds that promote Botrytis infections.

Canopy management and other cultural control methods along with sprays of Organic JMS Stylet Oil and Serenade are organically acceptable methods.

Look for flagging shoot tips or entire shoots or numerous brown irregular lesions on leaves in the spring during rapid shoot growth especially when warm moist conditions occur. If the entire shoot is flagging, look for a hole at the base, which could indicate feeding by branch and twig borer.

If basal leaves are not removed, apply fungicides before rain in northern central valley and coastal production areas to prevent flower infections. Research data shows a trend toward better control if fungicides are applied at bloom, preclose, and veraison. If leaf removal is practiced, then sprays can be limited to applications prior to wet weather during bloom (or none if no rain occurs). Thorough coverage is essential for all fungicide treatments.

A fungicide application may also be warranted if a major rain is expected late in the season when grapes are nearly mature. Alternating fungicides with different modes of action within the season and/or between seasons is essential to prevent pathogen populations from developing resistance to classes of fungicides.

At harvest, survey vineyards for Botrytis symptoms to assess the current season's management program and to plan for next year.

For more information on choosing a fungicide for grapes, refer to General Properties of Fungicides used in Grapes, Fungicide Efficacy—Conventional Products, Fungicide Efficacy—Biologicals and Natural Controls, Treatment Timing for Key Diseases, and Fungicide Resistance Management.

• Photos of flagging symptoms
• Photos of fruit damage at harvest