The pathogenic fungus, Rhizoctonia, is known to cause root rots, stem rots, damping-off and, in some cases, a blight of leaves. It is a soil-borne fungus favored by warm, moderately moist soil conditions.

Cross-wall near base of branch

The fungus is able to persist in the soil as hyphae and sclerotia. It generally does not produce any spores except in its sexual stage, which occurs very rarely. Once the fungus begins to grow in the plant the infected areas decay quickly, causing brown to reddish brown lesions to form at or just below the soil-line. If conditions remain favorable for disease development, the lesions will enlarge, forming dry sunken cankers which may eventually girdle the plant. The young hyphae are colorless but turn brown with age. Soil particles often cling to the cankered areas of the plant when removed from the soil because of the coarse, brown mycelium. Leaves touching the soil can be infected. The fungus will web to neighboring leaves quickly under high humidity conditions and will rot leaves quickly. Under magnification, hyphal branches are at 90 degree angles to the parent hypha and there is a cross-wall and a constriction of the cell at the base of each branch.

Symptoms

• Plants wilt during midday
• Stems rot at soil line with brown to reddish brown lesions

Conditions Favoring Rhizoctonia Growth

• Warm soil temperatures, 12 to 32C (70 to 90F)
• Even, moderate soil moisture (65% soil saturation) unless plant is injured, then higher soil moisture favors growth

Control

• Proper soil pasteurization eliminated Rhizoctonia from the potting mix.
• Encourage prompt growth (older plants are more resistant)
• Fungicides can be used effectively to manage this fungus. Contact Cooperative Extension to obtain information on what fungicides are currently recommended.

Rhizoctonia hypha with right-angle branching Rhizoctonia webbing on infected poinsettia stem and roots.