Botrytis cinerea is a fungal pathogen that affects many plant species, including wine grapes, where it commonly know as Botrytis bunch rot (BBR) or gray mold. As one of the most important late-season rots, BBR typically develops close to harvest, but it can also occur earlier during mid-season under favorable conditions.
- Grape flowers are considered the “gateway” for Botrytis infection, but the pathogen can also find its way to infect the clusters later in the season from bunch closing to harvest.
- Infections experienced at bloom (through senescing blossom parts) remain latent (dormant) until some of them resume activity and rot the berries as they begin to ripen.
- Factors such as high humidity and physical damage (due to high-speed winds, insect, bird, or diffuse colonization of powdery mildew, etc.) during the pre-harvest period may promote activation.
- Infections occurring closer to harvest do not undergo the latent period, and can readily and rapidly cause symptoms on berries.
- Leaf and woody tissue infections are rare, and typically do not serve as inoculum for secondary infections, thus, disease management including fungicide applications should target clusters.
- Extended moisture and relatively cool temperatures (60 to 75F) favor Botrytis infections.
- Serious BBR losses can result from spread during post-veraison/pre-harvest period, after berries begin ripening and become highly susceptible to Botrytis and other rots.
- Pre-harvest spread may be increased with high N content of berries (high soil N and or foliar urea applications). Be more diligent with your Botrytis scouting and management if you apply post-veraison N.
- Varieties with tight/compact clusters have greater susceptibility due to limited spray penetrations and coverage on clusters. Reducing cluster compactness has been shown to reduce infections.
- Shoot thinning and fruit zone leaf removal are helpful cultural practices that opens up the canopy to air flow (reducing the risk of having extended wet periods) and improving fungicide spray coverage. Fruit thinnig will also help reduce cluster-to-cluster spread.
- Bloom sprays are targeted to limit the establishment of latent infections. These are especially critical during seasons with extended wet weather during the bloom period.
- Bunch closure and veraison sprays are targeted to limit the establishment of new primary infections and prevent berry to berry spread.
- A pre-harvest application is critical to prevent spread especially under wet/humid conditions.
Botrytis can be managed with fungicides, however, the pathogen has tremendous ability to develop resistance to these fungicides. It is thus important to select fungicides that have no or less resistance issues and closely follow guidelines for fungicide resistance management. Below are a list of single-site fungicides commonly used, grouped by frequency of resistance detected in local populations.
High frequency of resistance detected
- The strobies (FRAC 11) only have suppression activity against BBR. It is not advisable to use this group for Botrytis control regardless of resistance.
- Vangard (FRAC 9) has high volatility and its use during summer is less effective.
- Scala (FRAC 9—the same chemical family as Vanguard).
- T-methyl (FRAC 1) and other products in this group.
Medium frequency of resistance detected
- Elevate (a.i. fenhexamid, FRAC 17)
- Rovral (FRAC 2). Other products in this group include Iprodione 4L, Nevado, and Meteor.
- Endura and Pristine (FRAC 7). The fungicides within FRAC 7 lack complete cross-resistance, meaning that fungal isolates resistant to Endura or Pristine are not necessarily resistant to other FRAC 7 fungicides.
Low frequency of resistance detected (<15%)
- Switch or Miravis Prime (a.i. fludioxonil, FRAC 12).
- Luna Experience and Luna Sensation (a.i. fluopyram, FRAC 7).
- Kenja and Fervent (a.i. isofetamid, FRAC 7).
Note: Please remember that all single-site fungicides are prone to resistance development. Try not use the same fungicide group more than twice a season. For a complete list of active ingredients and products registered for Botrytis and other disease management, referring to the MyIPM app (download for free from App Store or Google Play)
Recommendations for Resistance Management
- Avoid using fungicides to which resistance in Botrytis is widespread. Based on our findings, FRAC 7 (newer ones) and FRAC 12 have much less resistance issues than other groups.
- Captan has some efficacy, which does not select for resistance and can be used alone when the disease pressure is relatively low.
- The key to managing resistance issues is spraying less single-site fungicides. When weather is relatively dry, consider extending spray intervals. The best fungicides can not beat dry weather!
Additional Resources for Botrytis and General Grape Disease Management
- Grapes: Diseases and Insects in Vineyards (pdf)
Horticultural & Forest Crops 2022, Virginia Tech
Timely Viticulture is designed to give those in the Maryland grape industry a timely reminder on procedures or topics they should be considering in the vineyard.