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 50° 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.
Necrotic lesions on the pods and stem.
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.
Deep plowing will bury sclerotia in infected debris or weeds but plowing the field in later years may return viable sclerotia to the surface where they can produce apothecia or conidia.
Chemical
control:
Chlorothalonil products 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.
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 pt/A on 7-day intervals when disease threatens. Preharvest interval is 7 days. 12-hr reentry.
Endura at 8 to 11 oz/A. To limit the potential for development of resistance, do not make more than two (2) applications of Endura per season. The preharvest interval is 7 days. 12-hr reentry.
Nevado 4F 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.
Rovral 4 Flowable at 1.5 to 2 pints/A. Not recommended in Idaho due to lower level of control. Apply at 1 to 10% of plants have at least one open bloom and again 5 to 7 days later. No more than two (2) applications per season season and apply no later than peak bloom. Do not allow foraging for 14 days after last application. Do not feed dry bean hay to livestock until 45 days after last applica-tion. 24-hr reentry.
Switch 62.5WG at 11 to 14 oz/A on 7-day intervals. Do not apply within 7 days of harvest. 12-hr reentry.
References:
Hall, R. 1991. Compendium of Bean Diseases. St. Paul, MN: APS Press.
Johnson, K.B., T. L. Sawyer, and M. L. Powelson. 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.
