There are several late-season bunch rots that can pop up in wine grapes in Michigan if and when the rains return. The most common of these is Botrytis bunch rot caused by Botrytis cinerea, the same fungus that causes gray mold in strawberries. Tight-clustered varieties such as Pinot gris, Pinot noir, Chardonnay, Aurore and Vignoles are particularly susceptible. Another, more sporadic disease is sour rot, which is a wet rot that smells distinctly of vinegar due to the presence of acetic acid bacteria. Often, other organisms are also involved in sour rot, including various yeasts and fungi. Damage can be extensive because infections that begin in a single berry can rapidly spread to adjacent berries and destroy most or all of a cluster.

While under cool, dry conditions, Botrytis bunch rot sometimes can be beneficial for wine quality (“noble rot”), but sour rot is always undesirable. Bunch rot often begins in one or a few berries, usually at the site of an injury or a quiescent flower infection, and then spreads rapidly throughout the cluster.

Factors that favor disease development

Injury to the berries and environmental conditions are the primary factors influencing bunch rot development. As berries ripen and sugar content increases, injured fruit becomes increasingly susceptible to bunch rot pathogens. Other than Botrytis cinerea, which can directly penetrate intact berry skins under conditions of prolonged moisture or very high humidity, most other bunch rot organisms are opportunistic pathogens that live on plant surfaces and can only cause infections if they gain entry to the berry through wounds. Examples are injuries from berry splitting caused by growth pressure in tight-clustered grape varieties as well as insect, bird and hail injury.

Entry holes created by grape berry moth larvae are a common cause of bunch rot as well. In addition, infection by the powdery mildew fungus creates small, dead spots on the berry skin, which can lead to cracking of the berry and possible invasion by bunch rot pathogens. Recent studies have shown that these microbes may also be able to enter openings or cracks between the berry stem and berry. Fruit flies that are attracted to rotting or overripe fruit may also play a role in development and spread of sour rot.

In addition, wet weather during fruit ripening also favors bunch rot by causing expansion and splitting of berries as they take up water and providing moisture for growth of fungi and bacteria. Dense canopies and high humidity can also enhance conditions for bunch rot development.

Management strategies

Promoting good air circulation and sunlight exposure within the grapevine canopy also reduces the risk of bunch rot, such as by leaf pulling in the fruit zone, shoot positioning, shoot thinning and hedging. Limit excessive vegetative growth by balance-pruning and avoiding excess nitrogen fertilization.

For sour rot as well as Botrytis bunch rot control, it is not too late to pull leaves to expose the cluster to sunlight and airflow. This is, indeed, one of the best control tactics. Avoid excessive leaf pulling, as berries may suffer from sun scald when suddenly exposed to strong sunlight and high temperatures. Sun scalding is usually restricted to the sides of the berries exposed to the sun and will appear like browning and collapsing (flattening) of the affected berry surface and underlying tissues. Sun-scalded berries may dry up in dry weather, but may also split at the site of the injury after rains occur.

There are currently numerous fungicides available for control of Botrytis bunch rot, while few aid in sour rot control. Sour rot is best controlled by leaf pulling and application of Serenade is complementary. Make sure to alternate fungicides with different modes of action to avoid fungicide resistance development.

  • Elevate (fenhexamid; Hydroxyanilides; locally systemic; 0-day PHI): good to excellent preventive and limited post-infection activity.
  • Endura (boscalid; Carboxamides; systemic; 14-day PHI): good to excellent preventive and post-infection activity. Use at 8-oz rate for Botrytis control.
  • Inspire Super (difenoconazole + cyprodinil; Sterol inhibitors and analinopirimidines; systemic; 14-day PHI): it is mainly the cyprodinil component that provides Botrytis control: preventive and post-infection activity. Good to excellent preventive and post-infection activity.
  • Luna Experience (fluopyram + tebuconazole; pyridinyl ethyl-benzamides + sterol inhibitors; systemic; 14-day PHI, 10-day REI for leaf pulling, trying and training in wine grapes): good to excellent preventive and post-infection activity.
  • Luna Privilege (fluopyram; pyridinyl ethyl-benzamides; systemic; 7-day PHI, 10-day REI for leaf pulling, trying and training in wine grapes): good to excellent preventive and post-infection activity.
  • Pristine (pyraclostrobin + boscalid; Strobilurins: systemic; 14-day PHI): good preventive and post-infection activity, but only at the high rate (18.5-23 oz acre).
  • Rovral (iprodione; Dicarboximides; locally systemic; 7-day PHI): moderate to good preventive and limited post-infection activity; activity is improved by addition of oil or non-ionic spray adjuvant. Some vineyards may have resistant strains if Rovral was used a lot in the past.
  • Scala (pyrimethanil; Analinopyrimidines; systemic; 7-day PHI): good to excellent preventive and post-infection activity.
  • Serenade (Bacillus subtilis; Biological control agent; protectant; 0-day PHI): fair to moderate preventive activity. Organic formulation can be used in organic vineyards.
  • Switch (cyprodinil and fludioxonil; Analinopirimidines and phenylpyrroles; systemic; 14-day PHI): provides good Botrytis control; preventive and post-infection activity.
  • Topsin M (thiophanate methyl; Benzimidazoles; systemic; 14-day PHI): good preventive and post-infection activity.
  • Vangard (cyprodinil; Anilinopyrimidines; systemic; 7-day PHI): good to excellent preventive and post-infection activity.