Cause Botrytis cinerea, (teleomorph: Botryotinia fuckeliana), a fungus that is pathogenic on many plants. This organism can survive on many hosts or on plant debris. It is an opportunistic pathogen and its spores (conidia) easily invade weak, damaged, or senescing tissue when wet. Conidia are produced throughout the growing season under a wide range of temperatures and humidity but cool temperatures, high humidity, and free water on plant surfaces favor the disease, as do close plant spacing and irrigation practices that keep plants wet for a long time. Downy mildew infection of leaves or Pythium infection of lower leaves or stems will make tissues susceptible to gray mold.
Symptoms The initial symptom is a dark, water-soaked lesion. A soft rot of infected leaves and stems occur as the pathogen grows into healthy tissues. Seedlings will fall over if infection girdles the stem. Any time before harvest, leaves may wilt, beginning with the oldest leaves and continuing until the plant collapses. Gray, powdery spore masses form, particularly on shaded leaves. Black sclerotia may also develop.
Cultural control
- Avoid injury to plant tissues.
- Limit irrigation so plant leaves do not remain wet; avoid overhead irrigation.
- Control other pathogens that infect leaves or lower stem tissues.
- Minimize crop debris/residues at planting.
- In greenhouse production, reduce the relative humidity.
Chemical control
- Cannonball WP (Group 12) at 7 oz/A on 7- to 10-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Carboxamide (Group 7) formulations are registered for use. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action.
- Endura at 8 to 11 oz/A. Preharvest interval is 14 days. 12-hr reentry.
- Fontelis at 14 to 24 fl oz/A on 7- to 14-day intervals. Preharvest interval is 3 days. 12-hr reentry.
- OSO 5% SC (Group 19) at 3.75 to 13 fl oz/A on 7- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Rovral 4 Flowable (Group 2) at 1.5 to 2 pints/A in at least 40 gal/A water. Preharvest interval is 14 days. 24-hr reentry.
- Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O
- Premixes of fungicides are available.
- Miravis Prime (Group 7 + 12) at 13.4 fl oz/A on 7- to 10-day intervals. No more than two (2) sequential applications. Can be applied day of harvest. 12-hr reentry.
- Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A on 7- to 10-day intervals. No more than two (2) sequential applications. Can be applied day of harvest. 12-hr reentry.
Biological control Efficacy unknown in Oregon.
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 4-hr reentry. O
- Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
- Romeo at 0.45 to 0.68 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 4-hr reentry. O
Reference Davis, R.M., Subbarao, K.V., Raid, R.N., and Kurtz, E.A. 1997. Compendium of Lettuce Diseases. St. Paul, MN: APS Press.