Botrytis bunch rot, caused by the fungus Botrytis cinerea, is a common problem wherever grapes are grown. The disease can cause serious losses in both yield and quality when weather conditions favour the disease.
Ripening grapes are affected by a rot which may progress to infect whole clusters. With sufficient rain and humidity, berries split open and develop a greyish mold on the surface. Affected berries may shrivel in the dry Okanagan climate.
Botrytis may cause girdling lesions on the pedicel or rachis, leading to drying of clusters or portions of clusters and premature cluster drop.
Botrytis infects grape shoots, flowers, leaves and fruit under a range of temperature and humidity conditions. Slow drying conditions together with high humidity (90% or better) favour the development of botrytis. Infection is optimal at 15-20°C with free water or over 90% humidity.
| Temperature (°C) | Amount of time (hours) |
|---|---|
| 10 | 30+ |
| 15.5 | 18+ |
| 22.5 | 15+ |
| 26.5 | 22+ |
| 39 | 35+ |
Grape cultivars with dense canopies, thin skins and/or tight clusters are more susceptible to botrytis bunch rot.
| Very susceptible | Susceptible | Moderately susceptible | Least susceptible |
|---|---|---|---|
| Auxerrois Bacchus Chardonnay Chasselas Chenin Blanc Ehrenfelser Gamay Noir Madeleine Angevine Madeleine Sylvaner Optima Pinot Blanc Riesling Semillon Siegerebe Zinfandel |
Barbera Kerner Lemberger Malbec Muscat Ottonel Pearl of Csaba Pinot Gris Pinot Meunier Pinot Noir Sangiovese Sauvignon Blanc |
Ortega Scheurebe Schoenburger Syrah Vidal Viognier |
Cabernet Franc Cabernet Sauvignon Chancellor Foch Merlot Petit Verdot |
Botrytis overwinters as mycelium or sclerotia on bark and as mycelium in dormant buds. It may also overwinter in mummified fruit. In the spring, conidia (spores) are produced which can infect leaves and young clusters before bloom.
The fungus may infect blossoms leading to fruit infection, but the fungus becomes inactive (latent) in the fruit possibly due to low sugar and high acid contents. It becomes active again when the berries begin to soften. Fruit infection usually begins in berries with 5-8% sugar (at veraison), with berries remaining susceptible up to harvest and also post-harvest.
Abundant conidia are produced on infected fruit, leading to secondary spread to adjacent berries in a cluster and to nearby clusters. Spread of the spores is aided by summer rains, overhead irrigation, heavy dew and juice from split berries. Spores are also carried by the wind. Mechanical damage caused by birds or insects creates entry points for the fungus and can greatly increase losses due to bunch rot.
During the growing season, take the following measures:
During the dormant season, knock mummified fruit and infected prunings to the ground and cover with soil or, alternatively, burn.
Correct identification of botrytis bunch rot prior to applying fungicide sprays is important. Botrytis can be confused with other rots such as sour rot. Application of botrytis fungicides where sour rot is present may increase the level of sour rot infection.
The number of spray applications necessary to control bunch rot depends upon disease pressure in the vineyard and weather conditions, as well as other factors, such as susceptibility. Fewer applications may be needed if weather is very dry and/or disease pressure is low. Use a full program in vineyards where bunch rot was a serious problem in the previous year and where sanitation measures were not taken after harvest. The number of sprays can be reduced or eliminated in vineyards with a history of low disease levels and for all vineyards in dry years.
Key timings for botrytis fungicide applications are bloom, just before bunch closing, veraison and pre-harvest. Botrytis control becomes more difficult as the grapevine matures because heavy canopy growth and bunch closing make it difficult to place the fungicide where it is needed. Under typical Okanagan weather patterns, high risk periods for botrytis infection are during bloom and from 1-2 weeks after veraison to harvest. These growth stages often coincide with rainy periods during June and September.
Be cautious about adding spreaders or stickers to fungicide sprays, as some spray adjuvants have been shown to increase the development of bunch rot on grape clusters. It is believed that spray adjuvants remove the natural wax protection on grape berries.
For more information about chemical control, see the Pesticides page.
