How to Identify Gray Mold (Botrytis cineraria) in Snap Beans Dan McGrath and Ken Johnson, Oregon State University

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1 Background In the past, one or two applications of Ronilan (BASF) fungicide was an effective method of controlling both gray mold (Botrytis cineraria) and white mold (Sclerotinia sclerotiorum) in snap beans. Starting in 2006, Ronilan will no longer be available. Oregon snap bean growers will often need to use tank mixes of two fungicides to control bean mold. None of the currently available alternatives to Ronilan effectively controls both fungi. This will significantly increase the cost of mold control in snap beans. One means of controlling costs is to identify bean plantings with very low risk of gray mold where a single fungicide may be adequate. This approach is not without risk. Over time, this strategy may lead to a fungal shift, stimulating the development of gray mold populations that are highly resistant to available fungicides. Gray mold forms soil borne sclerotia in crop residue that give rise to gray mold spores. Gray mold also infects injury plant materials, blossoms of many plant species, and decaying fruit. These become additional sources of gray mold spores. The wind carries gray mold spores a significant distance. Gray mold, therefore, can occur at the landscape-scale during wet summers. Intense gray mold summers occur once or twice per decade. There are many factors that determine risk of gray mold in snap beans including: field history, mold history, bean variety, weather patterns, and canopy closure. The presence of actively sporeforming gray mold point sources in a bean planting indicates high risk of mold infection during blossoming and pod formation. Scout your fields to determine if they are present. To find out more about gray mold, visit the PNW Disease Management Handbook on the web at: Identification of Gray Mold in the Field Gray mold forms millions of tiny, gray spores that appear and act like gray dust when you disturb them. Gray mold infects injured plants. Look gray mold point sources in wheel tracks and other areas of the field where you expect plant injury. Under very wet conditions, gray mold infects seedling cotyledons and stems at the base of the developing plants (Figures One to Three). If you see much of this in a planting prior to bloom, the field is at very high risk of gray mold infection of the blossoms and pods. Gray mold infected blossoms drop on the soil. Spores from decaying blossoms can infect subsequent blossoms and other tissues. (Figures Four & Five) Gray mold infects black berry blossoms and decaying fruit at the edge of the field. These spore sources may infect late plantings. (Figures Six & Seven) Gray mold forms sclerotia in crop residues. Sclerotia over winter in the soil and may become spore sources for subsequent crops. (Figures Eight and Nine) Page 1 of 7

2 Figure One: Gray mold on cotyledons and seedling stems. Page 2 of 7

3 Figure Two: Gray mold forming spores on dead seedling. Figure Three: Gray mold on decapitated seedling. Page 3 of 7

4 Figure Four: Gray mold blossoms infect other plant tissues. Figure Five: Gray mold on bean leaves is rare. Page 4 of 7

5 Figure Five: Black berries at the field edge may be a source of gray mold spores late during the growing season. Figure Six: Gray mold on decaying blackberries. Page 5 of 7

6 Figure Eight: Gray mold sclerotia in bean stems. Page 6 of 7

7 Figure Nine: Gray mold spores on gray mold sclerotia. Page 7 of 7