Aug 5, 2015

Protect hail-damaged clusters from Botrytis infection

Recent hail storms have caused serious damage to grapes in northwest Michigan. Canes have been gouged and blown off the trellis, leaves are tattered and berries have been split open. Wounds on leaves and canes will probably not have any major further consequences unless crown gall bacteria present on the vines take advantage of the wounds to enter canes, therefore keep an eye out for possible crown gall development. However, injured berries are easy prey for rot organisms, particularly the fungus Botrytis cinerea, which causes Botrytis bunch rot.

While the injured berries themselves may not be salvageable and will eventually shrivel up, we don’t want to invite Botrytis to establish itself in the cluster as it may cause problems later during fruit ripening. Botrytis is somewhat of an opportunistic pathogen, invading weakened, damaged or senescent plant tissues. The spores are ubiquitous in the air as the fungus is capable of abundant sporulation on dying plant tissues under humid conditions and the spores are easily windborne. The fungus may also already be present in the clusters as latent infections since bloom. It is therefore advisable to apply fungicides to damaged clusters as soon as possible, especially if rain or humid weather is in the forecast.

The following fungicides are registered for bunch rot control on grapes. Systemic fungicides are recommended for better coverage and some “back action” in damaged grapes. Of the fungicides listed below, Luna Tranquility is probably the strongest material at this time with two active ingredients with Botrytis-specific activity. Use the highest labeled rate for curative activity and alternate fungicides in different chemical groups for fungicide resistance management. Remember that the biologicals are protectants only and have moderate activity against Botrytis while the conventional fungicides have good to excellent activity against Botrytis. Read the label for tank-mix compatibilities, recommendations for adjuvants and restrictions on the number of sprays per season. For efficacy ratings and additional information, see the “2015 Michigan Fruit Management Guide” (Michigan State University Extension Bulletin E0154).

— By Annemiek Schilder, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences