MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Early symptoms of Botrytis bunch rot (gray mold), caused by the fungus Botrytis cinerea, have been showing up in grape clusters in some locations. However, in many cases, it was found to be associated with grape berry moth infestation. The entry point and tunnels created by the larva allow entry of Botrytis into the berry. So check the affected berries closely and look for the tell-tale entry hole and webbing. You may see a larva upon opening up the frass and berries. However, frequent precipitation and high humidity do enhance Botrytis and growers should be prepared, particularly if it rains heavily in the weeks before harvest. Tight-clustered varieties, such as Pinot Noir, Pinot Gris, Vignoles, etc. are most seriously affected. Botrytis bunch rot may be confused with sour rot, which is caused by bacteria and yeasts. The main difference is that clusters with sour rot smell distinctly like vinegar and do not support the gray sporulation typical of Botrytis.

Botrytis biology

Botrytis cinerea is a “weak” pathogen that primarily attacks highly succulent, dead, injured, or senescent tissues such as wilting blossom parts and ripening fruit. The fungus thrives in high humidity and still air (optimum temperature: 59-77°F). Grape berries are most susceptible to infection after veraison. However, if Botrytis spores are available and wet conditions prevail, berries can become infected anytime after bloom. Infection occurs through scars left by the fallen caps or by contact with sporulating floral debris. Infections often remain latent (dormant) until the fruit ripens or may not progress at all. However, the few that do activate can lead to rapid disease spread within the cluster as berries become highly susceptible upon ripening. Controlling infections at bloom provides no benefit if post-veraison weather is dry and doesn't support further disease development, but can pay significant dividends if the weather turns wet before harvest. In most years, fungicide applications at veraison and pre-harvest are more beneficial than earlier applications.

Factors that favor the disease

Factors that cause latent infections to activate are poorly understood, although high humidity and tissues with elevated nitrogen levels appear to promote this process. Cluster compactness also has a pronounced effect on disease development, due largely to rapid berry-to-berry spread. In addition, berries in tight clusters often crack due to pressure within the cluster, providing moisture and nutrients for growth as well as an entry point for the fungus. Insect or other injury, e.g., grape berry moth holes, can also lead to Botrytis as well as sour rot infection. Research in New York has shown that late powdery mildew infections (barely visible with the naked eye) of the berries can also predispose them to rots.

Control options

Promoting good air circulation by canopy management and leaf pulling is an important cultural option for managing Botrytis bunch rot. In past trials in Michigan, leaf removal has been one of the best treatments for control of bunch rots (Botrytis and sour rot) and comparable to fungicide treatments. Avoid excessive leaf pulling, as berries may suffer from sun scald when suddenly exposed to 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. Sun-scalded berries tend to dry up rather than rot. There are some products available that reduce sun damage to fruit crops: Purshade (calcium carbonate) and Surround (kaolin clay), but they have not been tested on grapes in Michigan as far as I know. There are currently some excellent fungicides available for control of Botrytis bunch rot.

• Elevate (Hydroxyanilides; locally systemic; 0-day PHI): good to excellent preventive and limited post-infection activity.
• Vangard (Anilinopyrimidines; systemic, 7-day PHI): good to excellent preventive and post-infection activity.
• Scala (Analinopyrimidines; systemic; 7-day PHI): good to excellent preventive and post-infection activity.
• Endura (Carboxamides; systemic; 14-day PHI): good to excellent preventive and post-infection activity. Use at 8-oz rate for Botrytis control.
• Rovral (Dicarboximides; locally systemic; 7-day PHI): moderate to good preventive 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.
• Pristine (strobilurins: systemic, 14-day PHI); good preventive and post-infection activity but only at the high rate (18.5-23 ox/acre).
• Topsin M (Benzimidazoles; systemic; 14-day PHI): moderate preventive and post-infection activity.
• Serenade (Biological control agent; protectant; 0-day PHI): fair to moderate preventive activity. Organic formulation can be used in organic vineyards.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.