Botrytis bunch rot of grapes, or gray mold—Botrytis cinerea
Decay of grape berries and other plant parts occurs after they become infected with the B. cinerea fungus. Various other bacteria and fungi can also rot grapes. But unlike B. cinerea, these other microorganisms cannot directly infect healthy berries. They require berries to be diseased from B. cinerea or otherwise injured before they can infect and develop.
Identification
Botrytis can infect and decay flowers, fruit, leaves, and occasionally stems. Prior to fruit ripening, early-season shoot blight may occur following prolonged warm, moist conditions from spring rains. Green plant parts commonly turn brown when infected. Infections often occur in leaf axils causing shoots to wilt or break off.
Infected plant parts are usually those on or near the soil and in the dampest or densest areas of the canopy. When humidity is high or plants are wet, infections develop into brown to gray, fungal spores that to the naked eye appear fuzzy. Infected berries may turn brown or reddish, shrivel, and decay. Black, flattened sclerotia, which are hardened masses of fungal mycelia about 1/8 inch in diameter, can develop in or on decayed plant parts.
Life cycle
Injured berries are susceptible to direct infection by B. cinerea and various other microorganisms. Botrytis and various other decay microorganisms can enter through plant parts damaged by birds, insects (e.g., caterpillars chewing), mechanical (physical) impact, and other pathogens (e.g., powdery mildew). Healthy berries become susceptible to B. cinerea infection and disease once they begin to ripen.
The Botrytis fungus overwinters as sclerotia mostly in or on infected canes and berry mummies on the ground or left on vines. When conditions are wet the sclerotia germinate and produce infectious spores. Botrytis spores readily infect flowers and plant parts that are senescing (declining and dying) or injured, especially plant parts in contact with damp soil or within dense foliage. Note that gray mold has numerous other hosts from which the fungus can spread.
Healthy, green tissues become infected if they are in contact with other plant parts that are infected with Botrytis or when spring rains are frequent. Infections require free water for a minimum period of time that varies with the temperature. Grape flowers that become infected are a source of the pathogen that remains dormant until fruit begin to mature. Once berries start to ripen, B. cinerea resumes growth and healthy berries can become infected and develop bunch rot.
Damage
Most any green or succulent plant part is susceptible to gray mold disease after wet conditions. Infection of flowers and berries are the important damage because this reduces fruit yield, and diseased berries are inedible. However, for making certain types of wine, Botrytis-infected berries are highly desirable.
Solutions
Because Botrytis infection and disease develop after wet conditions, hasten the drying of plant parts by improving air flow and increasing grapevines exposure to light. Methods include to
- Avoid crowding of plants.
- Control nearby weeds.
- Keep any groundcover low and back some from the vines.
- Prune off leaves around grape bunches soon after bloom. If leaves are removed at fruit set, the berries acclimate to the greater sunlight exposure and develop a thicker cuticle that helps prevent sunburn as well as Botrytis infection.
- Use a training (trellis) system to keep leaves and vines off the soil.
Prevent sprinkler water from wetting leaves and vines. Use drip, flood, or furrow irrigation. Direct any hand watering to soil beneath canopies. Avoid overirrigation and the application of excessive nitrogen fertilizer because these encourage excess growth of succulent leaves and stems susceptible to gray mold. Remove diseased plant parts as soon as they appear and dispose of them away from valued plants.
Pick grapes as soon as they are ripe. Harvest fruit early if humid or rainy weather is expected. Pick and dispose of any old fruit clusters because the fungus can persist and overwinter on berry mummies.
Fungicide application is generally not warranted if no fog or rain occur during bloom and leaves will be pruned off around bunches shortly after bloom. If moist conditions are expected during bloom before fog or rain occur, a preventive fungicide can be sprayed. To potentially be effective the fungicide must thoroughly cover the plant. Potentially effective products include Bacillus subtilis, chlorothalonil, copper fungicides, horticultural oil, neem oil, and thiophanate methyl. If wet conditions persist, more than one application may be warranted. Note that some populations of B. cinerea are resistant to certain fungicides. If more than one application is being made, rotate to a different fungicide. No fungicides are recommended for bunch rot later in the growing season.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources also available as an eBook and Pest Management Guidelines: Grapes and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
A spring infection of Botrytis on a grape leaf.
Late-season Botrytis disease of grape berries.
Gray mold spores that developed after a Botrytis-infected grape became wet.
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