How to Manage Pests
UC Pest Management Guidelines
Botrytis Bunch Rot
Pathogen: Botrytis cinerea
(Reviewed 6/06, updated 2/14)
In this Guideline:
Early-season shoot blight may occur following frequent spring rains. Flowers can become infected 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.
COMMENTS ON THE DISEASE
The fungus overwinters as sclerotia in berry mummies on the ground or left hanging on the vine and in canes. Germination and spore production occur in spring. Infections require free water for a definite period of time depending on temperature. Infections may occur during bloom should rains occur; preclose rachis infections often occur on Chardonnay. 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. The efficacy of a fungicide depends on getting good coverage, and coverage is affected by the canopy and stage of growth. By employing cultural control methods, properly applying fungicides, and using resistant varieties, the disease can be managed.
Excellent control has been achieved using canopy management and leaf removal in particular. Removal of basal leaves or basal lateral shoots at or 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. If leaves are removed at cluster set, the berries acclimate readily to the sunlight and develop a thick cuticle that helps prevent sunburn as well as Botrytis infection.
On cordon-trained vines, only remove leaves from the side of the vine that receives morning sun. Do not remove lateral shoots. If leaves are not removed and weather is dry in 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.
Northern and coastal production areas
Remove leaves or lateral shoots around clusters beginning at late bloom and continue to berry set.
Remove leaves (from bloom to berry set) or hedge (mid-season) to open canopy.
Organically Acceptable Methods
Canopy management and other cultural control methods along with sprays of Organic JMS Stylet Oil and Serenade are organically acceptable methods.
Monitoring and Treatment Decisions
Look for flagging shoot tips or entire shoots during rapid shoot growth. If you see flagging, attempt to break or cut the shoot in the region between the flaccid area and the adjacent area with normal turgor. Brown discoloration on the cut surface is evidence of Botrytis.
If the entire shoot is involved, 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 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 one application if wet weather occurs 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. Otherwise, management of Botrytis bunch rot following bloom generally relies on proper cultural practices as outlined in SUMMER BUNCH ROT.
At harvest, check table grape for Botrytis symptoms to assess this year's management program and to plan for next year.
UC IPM Pest Management Guidelines: Grape
W. D. Gubler, Plant Pathology, UC Davis