Botrytis blight, or gray mold—Botrytis and Botryotinia spp.
Gray mold is named for the brown, gray, or tan fungal spores that develop on infected tissue when conditions are humid or moist. The tiny, stalked spore-forming structures give rotted tissues a fuzzy appearance when viewed with the naked eye. Various Botrytis and Botryotinia species cause gray mold, most commonly Botrytis cinerea.
Disease often presents initially as tiny, almost translucent spots. These infected spots turn brown and appear water soaked. Infected fruit, leaves, petals, and succulent stems become dark, soft, and wilted and die. Succulent tissue of most plants is susceptible to Botrytis blight. Landscape hosts include azalea, bird of paradise, cacti, coast redwood, fuchsia, giant sequoia, hydrangea, rhododendron, and rose.
Spores germinate and produce new infections only after plants have been continuously wet for 6 or more consecutive hours or when relative humidity has been higher than about 90% for 6 or more hours. In greenhouses or outdoors in coastal areas Botrytis and Botryotinia diseases can be a problem almost any time of year. Away from the coast, gray mold generally is a problem only during the late fall through early spring rainy season. However, Botrytis blight can occur anywhere plants are frequently wetted by overhead irrigation, especially those growing under crowded and shaded conditions where air circulation is poor.
Azalea petal blight and rhododendron petal blight and camellia petal blight when these hosts are involved can resemble Botrytis blight. However, Botrytis blight is a drier rot that also affects dying or inactive green tissue. Azalea petal blight, camellia petal blight, and rhododendron petal blight infect and kill only blossoms.
When humidity is high or plants are wet for at least 6 hours, declining or dying, soft tissues can become infected by airborne spores of Botrytis and Botryotinia species. Healthy, actively growing green plant parts are seldom infected directly by these fungi. However, once petals or dead or weakened tissues are infected the fungus can move to invade adjacent, healthy green tissue that contacts diseased tissues. Gray mold fungi do not infect woody parts.
Botrytis and Botryotinia species can grow on almost any moist or decaying herbaceous vegetation. Weeds and plant debris are common sources of gray mold spores, which are produced in enormous numbers and readily spread in air to nearby plants. Gray mold is particularly troublesome under high humidity and moderate temperatures (70° to 77°F), but it is also active over a broader temperature range.
Infection by Botrytis and Botryotinia species causes herbaceous plant parts to decay, discolor, and wilt. Affected plants develop spotted flower petals and leaves and infected buds can fail to open. When infected tissues are wet, commonly grayish fungal spores develop on the decaying tissue and give the tissues a fuzzy or woolly appearance. Infected terminals may die back. On roses large, diffuse, target-like lesions can form on green canes.
Provide proper cultural care to keep plants vigorous. Use good sanitation practices and modify environmental conditions where feasible. Remove and dispose of old blossoms, declining green plant tissue, plant debris, fallen leaves, and weeds. Avoid wetting foliage, such as by using drip or microsprinkler irrigation. Improve air circulation around plants. For example, adequately space plants, prune canopies to eliminate some branches and foliage, and cut back nearby and overhead vegetation.
Where gray mold has been a problem, the fungicides chlorothalonil, thiophanate methyl, or triforine can be applied before disease develops. Fungicides are only preventative and generally are not very effective in landscapes when conditions (e.g., susceptible plant material and wet conditions) favor gray mold development. Many gray mold populations are resistant to certain fungicides.
Adapted from Integrated Pest Management for Floriculture and Nurseries and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Grayish, fuzzy fungal growth of Botrytis cinerea.
Diseased strawberry due to Botrytis blight.
Water-soaked lesions on oleander blossom with Botrytis blight.
Dead, brown, rose blossoms killed by gray mold fungus.
Brown, necrotic foliage due to gray mold.