Rhizoctonia root and stem rot is widespread on snap beans in Florida and occurs annually since the pathogen is found in all soils. Beans are commonly infected by Rhizoctonia spp., but seedlings of many other plant species also are susceptible to these fungi.
The disease causes an annual yield loss of approximately 10%, although individual fields may experience up to 100% infection rates. Stand losses of up to 75% have been reported.
Rhizoctonia can cause stem lesions on seedlings before or after emergence. Infections of seedling stems are most common near the soil surface. Stem infections typically occur near the soil surface and result in some shade of brown, red, or orange discoloration. Older lesions will appear sunken with less red color and may eventually rot the entire outer portion of the stem, thus causing the plant to fall over.
Small black-brown sclerotia may form on or just under the surface of older lesions. Severe infections can cause plant stunting and death.
In general, seedlings and young plants are highly susceptible to infection while the disease is seldom a problem on older plants.
Survival And Spread
The fungus survives in infected plant debris, and inoculum concentrations in the soil are increased by continually cropping fields to susceptible crops such as beans and potatoes.
Rhizoctonia is spread within and between fields by irrigation water and soil movement.
Disease development can occur over a wide range of soil types, pH, temperature, moisture, and fertility. Optimum soil temperatures for development of the disease range from 75°F to 85°F.
Control of Rhizoctonia root and stem rot in beans is difficult. Although control of Rhizoctonia spp. is achieved in a number of commercial crops, such as tomatoes, peppers, and strawberries through preplant fumigation, soil fumigation is seldom used with beans due to cost.
As a result, control of Rhizoctonia root rot in beans is more difficult and erratic than in fumigated crops. In the absence of a fumigation, growers must utilize several control measures to attain maximum control if soil fumigation is not used.
A major goal in managing Rhizoctonia in beans is to establish a fast growing seedling that reduces the “hazard time” as young tender plants are more susceptible than older plants.
There are several measures that will assist in this regard. Use only healthy, disease-free seed. Poor quality seed will germinate slowly, which offers a distinct advantage to Rhizoctonia.
Rhizoctonia root and stem rot also can be reduced by shallow seeding, planting in warm soils suitable for rapid germination, avoidance of high seeding rates, and rotation of beans with non-host crops.
Since the Rhizoctonia pathogen typically survives between crops on infected plant debris, sanitation is important and all residues should be disked under and allowed to decompose.
A number of fungicides can control Rhizoctonia on young bean plants if applied as a seed treatment or an in furrow application at seeding.