Rhizoctonia Web Blight Information
Causal Agents
Rhizoctonia solani
Hosts
Azalea
Begonia
Dahlia
Dianthus
Goldenrod
Holly
Hydrangea
Juniper
Stonecrop
Rosemary
Verbena
Zinnia
Symptoms & Signs
Web blight gets its name from the copious amounts of aerial mycelium, that look like spider webs spanning between foliage, that the fungus produces. Initial plant symptoms appear as small brown to black lesions. Symptoms rapidly develop into full blighting and die back of foliage. Defoliation may occur, but the fungus can produce so much aerial mycelium that blighted leaves are stuck on the plant.
When Does it Happen?
Rhizoctonia is a soilborne fungus that produces hardened, resilient survival structures, called sclerotia, that allow it to survive in the soil for years without a host. Aerial mycelium can be spread between plants by wind, and water splash may move soil onto the plant canopy. Favorable air temperatures for web bight range from 68 to 90° F, with an optimal temperature range of 75 to 86° F. High humidity and poor air circulation favor disease development.
Cultural Management
Web blight can be successfully managed through cultural practices that promote proper air circulation and reduce relative humidity. Prune dense canopies and do not place plants too close together. Irrigation should be applied early in the morning, and overhead irrigation should be avoided if possible. In landscape settings, mulch can be used to prevent soil being splashed onto foliage from rain or irrigation. Do not re-use potting soil. Do not re-use pots or flats if possible, otherwise thoroughly wash and sanitize between uses. Only use healthy stock. Remove and destroy infected plant material as soon as symptoms are evident. Remove plant debris. Scouting should be done weekly during favorable environmental conditions since web blight can develop rapidly.
Chemical Management
Preventative fungicide applications can be used to protect plants during periods of high heat and humidity Immediately discard infected plants in greenhouses and treat nearby plants. Curative applications are not as effective as preventative applications due to the rapid progression of web bight.
