Plant Pathology
Factsheet

Rhizoctonia fungi, particularly the fungus Rhizoctonia solani, are found in most agricultural soils in Kentucky. Rhizoctonia fungi are natural soil inhabitants that colonize and live on dead organic matter. Under the right environmental conditions, they are often able to attack living plants, including alfalfa.

Under warm, wet conditions, Rhizoctonia fungi can cause just about every conceivable type of alfalfa disease. Important examples are listed below.

SEED ROT AND SEEDLING DAMPING OFF
Seedlings can be very susceptible to infection by these fungi, and may die and wither away quickly.

ROOT CANKER AND ROT
Taproots may exhibit brown to black cankers when invaded by R. solani. Another soilborne fungus, Rhizoctonia crocorum, causes decayed roots to appear cinnamon brown to violet in color.

CROWN ROT
R. solani can invade crowns and cause a fatal crown rot. Infected tissues appear dark brown to black.

STEM CANKER
Discrete light brown cankers, similar to anthracnose lesions, can result from infection of lower stems. Foliage of infected stems wilts and turns yellow, sometimes with reddening.

WEB BLIGHT
Infection of foliage can cause leaves and stems to collapse in a watersoaked mass, which becomes light brown when it dries. During morning dews, cobwebby fungal strands may be evident on infected tissues.

Rhizoctonia diseases of alfalfa are very common in Kentucky, as our warm, humid climate is very favorable for these fungi. Poor drainage and excessive growth can also favor these diseases. Another factor that favors Rhizoctonia diseases is undecomposed organic matter, particularly if it is high in nitrogen. For example, Rhizoctonia diseases are sometimes more severe where a sod is plowed under, or where manure is spread during warm, humid weather. Note that Apron seed treatment fungicide does not have any activity against Rhizoctonia fungi.

MANAGEMENT
Grow adapted, improved varieties. Pay attention to the UK Agronomy variety trials. Commercial alfalfa varieties are not specifically bred for Rhizoctonia resistance. However, the varieties that consistently yield well during the warm, humid conditions common in Kentucky will likely tolerate occasional outbreaks of Rhizoctonia diseases.

Site selection and preparation. Plant into a well-drained, fine but firm seedbed. See the UK Extension publication AGR-76, "Alfalfa, Queen of the Forages", for information on site selection and preparation.

Harvest management. Harvest no later than first flower. Postponing cutting creates a more humid environment where Rhizoctonia fungi can thrive. Also, remove hay promptly, as the swath may trap moisture that activates Rhizoctonia fungi. In circumstances where the hay has not cured completely but warm, wet weather is expected, it may be best to treat with a hay preservative and bale early. See the UK extension publication ID-46, "Hay Preservatives", for more information.

Minimize crown injury. Injuries to the crown provide infection sites for Rhizoctonia fungi. Avoid practices that injure crowns such as grazing or using heavy equipment when the soil is wet.