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Ministry of Agriculture and Lands

Botrytis Bunch Rot of Grape (Botrytis cinerea)

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.

Symptoms:

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.

Discoloured, shrivelled berries caused by Botrytis Bunch Rot. Photo courtesy P.Sholberg, Agriculture & AgriFood Canada.
Botrytis cinerea sporulating on grape berries. Photo courtesy P.Sholberg, Agriculture & AgriFood Canada.

Infection and Spread:

Botrytis infects grape shoots, flowers, leaves and fruit under a range of temperature and humidity conditions (Table 1). Infection is optimal at 15-20 oC with free water or over 90% humidity.  Grape cultivars with dense canopies, thin skins, and/or tight clusters are more susceptible to botrytis bunch rot (Table 2).

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 to 8 percent 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.

Table 1.  Time/temperature relationships for botrytis bunch rot infection

Temperature Amount of Time
10 oC 30 hours or more
15.5 oC 18 hours or more
22.5 oC 15 hours or more
26.5 oC 22 hours or more
39 oC 35 hours or more

Slow drying conditions together with high humidity (90% or better) favour the development of Botrytis. Table 1 indicates the temperature required plus the corresponding period that slow drying conditions must exist for bunch rot infection to occur.

Cultural Control of Botrytis Bunch Rot:

During the growing season:

  • Avoid overhead irrigation and keep irrigation periods as short as possible.
  • Prevent excessive vine growth by judicious use of water and fertilizer.
  • Canopy management, including shoot thinning and leaf removal before fruit set will reduce botrytis bunch rot. Remove leaves and lateral shoots located opposite, one node above, and one node below each fruit cluster.
  • Prevent berry damage by effective control of powdery mildew early in the growing season.
  • Minimize berry damage by birds and insects.

During the dormant season:

  • Knock mummified fruit and infected prunings to the ground and cover with soil, or alternatively burn.

Table 2. Susceptibility of grape varieties in British Columbia to botrytis bunch rot

Variety Susceptible
  Highly Very Moderately Slightly
Auxerrois x      
Bacchus x      
Barbera   x    
Cabernet Franc       x
Cabernet Sauvignon       x
Chancellor       x
Chardonnay x      
Chasselas x      
Chenin Blanc x      
Ehrenfelser x      
Foch       x
Gamay Noir x      
Gewurztraminer       x
Kerner   x    
Lemberger   x    
Madeleine Angevine x      
Madeleine Sylvaner x      
Malbec   x    
Merlot       x
Muscat Ottonel   x    
Optima x      
Ortega     x  
Pearl of Csaba   x    
Petit Verdot       x
Pinot Blanc x      
Pinot Gris   x    
Pinot Meunier   x    
Pinot Noir   x    
Riesling x      
Sangiovese   x    
Sauvignon Blanc   x    
Scheurebe     x  
Schoenburger     x  
Semillon x      
Siegerebe x      
Syrah     x  
Vidal     x  
Viognier     x  
Zinfandel x      

Chemical Control of Botrytis Bunch Rot:

Fungicides registered for the control of Botrytis include Rovral (iprodione), Vangard (cyprodinil), Elevate (fenhexamid) and Scala (pyrimethanil) (see table 3). These fungicides represent 3 different chemical classes.  Note that Scala and Vangard belong to the same class, with the same mode of action. Alternate fungicides from different classes to help prevent the development of resistance. Do not apply more than 2 sequential sprays of the same fungicide. Note that none of these fungicides provides any significant control of powdery mildew. A new biofungicide, Serenade Max, is also available for suppression of powdery mildew and bunch rot. This product is OMRI approved for use in organic vineyards. It is considered a weak botrytis control product.

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.

Table 3. Fungicides registered for bunch rot control on grape

Fungicide Chemical
 Group		 Active ingredient Rate/ha Rate/acre Max. sprays per season Pre-harvest Interval
Rovral 2 iprodione 50% 1.5 kg/ha 600 g/acre 2 7
Elevate 17 fenhexamid 50% 1.12 kg/ha 450 g/acre 3 7
Scala 9 pyrimethanil 400g/L 2.0 L/ha 810 mL/acre 3 7
Vangard 9 cyprodinil 75% 750 g/ha 300 g/acre 2 7
Serenade MAX* NC Bacillus subtilis 3-6 kg/ha 1.2-2.4 kg/acre   0
*Serenade Max is a biofungicide, registered for disease suppression only. For best results do not tank mix with other products or fertilizers.

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 to 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.

December 2008