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Bean, Snap (Phaseolus vulgaris) - Strategies for Controlling White and Gray Mold in the Same Field
Cause The fungus, Botrytis cinerea (teleomorph: Botryotinia fuckeliana), is an opportunistic pathogen that easily invades weak, damaged, or senescing tissue. All aboveground portions of the bean plant can be attacked but often disease starts on senescing flowers or cotyledons. Flower infections are important for subsequent spread to leaves and pods. Spores (conidia) are produced throughout the growing season under a wide range of temperatures and humidity and stem infections are a durable inoculum source, producing conidia throughout bloom. Cool temperatures (68°F is optimal but the fungus can grow between 50F° and 80°F), 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 longer time. Durable sclerotia overwinter in soil. Gray mold is more severe when beans follow beans. Gray mold occurs in southern Idaho at trace levels, unlike in the Willamette Valley of Oregon where it causes significant losses.
Symptoms The initial symptom is a dark, water-soaked, slimy lesion, with concentric zonation and sometimes a yellowish margin. Long brown streaks form on stems and petioles. As tissues dry out, gray, powdery spore masses form. Black sclerotia may also develop.
Cultural control
- Rotate out of beans to cereals and corn for at least 2 years.
- If the placement of irrigation equipment allows, orientating the bean rows in the direction of prevailing winds helps control gray mold as well as white mold.
- Limit irrigation during and after bloom. Irrigate in the morning so plants do not stay wet more than 12 hours. In some areas, growers may be able to irrigate heavily before bloom and as little as possible thereafter.
- Avoid-overfertilization with nitrogen.
Chemical control
- Cannonball WP (Group 12) at 7 oz/A when 10% of the plants have at least one (1) open blossom. Preharvest interval is 7 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 on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Fontelis at 14 to 30 fl oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Luna Privilege at 6.84 fl oz/A on 14-day intervals. Do not apply within 14 days of harvest. 12-hr reentry.
- Chlorothalonil products (Group M5) also are registered but do not control gray mold as well as dicarboximides and are ineffective against white mold; however, they may be useful if resistance to other fungicides is a problem.
- Bonide Fung-onil is available for home gardens. H
- Bravo Ultrex at up to 2.7 lb/A. Do not apply within 7 days of harvest. Do not graze treated areas or feed treated plant parts to livestock. 12-hr reentry.
- Echo 720 at 3 pints/A on 7-day intervals when disease threatens. Preharvest interval is 7 days. 12-hr reentry.
- Cueva (Group M1) at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied up to the day of harvest. Poor control as a stand-alone product. 4-hr reentry. O
- Iprodione products (Group 2) such as Rovral 4 Flowable at 1.5 to 2 pints/A when 10% of plants have at least one open bloom and again 5 to 7 days later or up to peak bloom if conditions are favorable for disease. Do not allow foraging for 14 days after last application. Do not feed dry bean hay to livestock until 45 days after last application. 24-hr reentry.
- Omega 500F (Group 29) at 0.5 to 0.85 pint/A when 10% to 30% of the plants have at least one (1) open blossom and if needed again 7 to 10 days later. Do not apply within 14 days of harvest. 12-hr reentry or 72-hr reentry for high exposure activities.
- 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
- Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A on 7-day intervals. Do not apply within 7 days of harvest. 12-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 Group 7 + 11 fungicides are available for use. Do not make more than two (2) applications per season.
- Priaxor Xemium Brand at 4 to 8 fl oz/A. Preharvest interval is 7 days. 12-hr reentry
Biological control
- 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 0.5 to 1 teaspoon/gal water. H O
- Double Nickel LC at 0.5 to to 4.5 pints/A at planting and again at cultivation, can repeat on 10- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Heads Up at 0.035 oz/34 fl oz water/360 lb seed will stimulates pest suppression system of plants. Efficacy unknown. 12-hr reentry. O
- Prestop WG on 3- to 6- week intervals. 4-hr reentry. 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
- Serenade Opti at 14 to 20 oz/A on 7- to 10-day intervals. Applications can be made up to and the day of harvest. 4-hr reentry. O
- Stargus at 2 to 4 quarts/A plus a nonionic surfactant on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
References Hall, R. 1991. Compendium of Bean Diseases. St. Paul, MN: APS Press.
Johnson, K.B., Sawyer, T.L., and Powelson, M.L. 1994. Frequency of benzimidazole- and dicarboximide-resistant strains of Botrytis cinerea in western Oregon small fruit and snap bean plantings. Plant Disease 78:572-577.