Don’t Let Bunch Rots Ruin Your Grapes

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

If humid conditions persist, Botrytis pressure may be high this year. Remove leaves from around grape clusters and protect clusters with effective fungicides.

This 2014 growing season, Botrytis bunch rot may be a bigger menace than usual due to prevailing moisture and high humidity. Botrytis bunch rot is caused by the fungus Botrytis cinerea and is the most common cause of pre-harvest losses of wine grapes. Another problem may be sour rot, caused by acetic acid bacteria, yeasts and certain fungi. Sour rot can explode after heavy rainfall during fruit ripening, resulting in swelling and bursting of berries, especially near woods. Fruit flies get in on the melee and help spread the microbes on their bodies. Botrytis and sour rot can be present in the same cluster, in addition to Phomopsis and black rot, so it is important to distinguish the culprits. Bunch rot often begins in one or a few berries, and can consume most of the cluster under the right conditions. Sometimes when rot is prevalent or developing faster than expected, the harvest date has to be moved up to avoid excessive losses.

There are various fungicides available for control of Botrytis bunch rot while very few aid in sour rot control. Sour rot is best controlled by leaf pulling around the clusters between bunch closure and veraison to reduce humidity and increase sun exposure. While biocontrol agents like Serenade (Bacillus subtilis), Actinovate (Streptomyces lydicus) and Blightban (Pseudomonas fluorescens) may help by competing with sour rot microbes, leaf pulling is the main control method. Growers in Ontario apply potassium metabisulfite to “dry out” the clusters and kill yeasts and bacteria.

Early symptoms of Botrytis bunch rot in grapes.

Mid-symptoms of Botrytis bunch rot in grapes.    

For Botrytis treatments, Michigan State University Extension advises growers to make sure to alternate fungicides in different chemical classes to avoid fungicide resistance development. The Fungicide Resistance Action Committee (FRAC) code indicates the mode of action; any fungicides with the same FRAC number share the same mode of action. A conventional standard that has been quite effective in our fungicide efficacy trials is a Pristine/Vangard alternation: bloom (Pristine), bunch closure (Vangard), veraison (Pristine) and two weeks before harvest (Vangard). Adding another chemical class (e.g., Elevate) would be even better from a fungicide resistance management perspective. Biological control agents and reduced-risk products also can be used, particularly during lower risk periods (e.g., bunch closure) or when approaching harvest to avoid fungicide residues.