Rhizoctonia Root and Stem Rot: Symptoms, Conditions, and Control

Cross-wall near base of branch

Pathogen Biology

Rhizoctonia is a soil-borne fungus causing root rots, stem rots, damping-off, and occasional leaf blight. It thrives in warm (70–90°F), moderately moist soils. The fungus persists as hyphae and sclerotia; spore production is rare, occurring only in its sexual stage. Young hyphae are colorless, turning brown with age. Under magnification, hyphal branches form at 90° angles with a distinct cross-wall and constriction at the base of each branch.

Symptoms

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

Lesions expand into dry, sunken cankers that may girdle the plant. Soil particles cling to cankered areas due to coarse, brown mycelium. Leaves touching soil can be infected; under high humidity, the fungus webs to neighboring leaves and rots them rapidly.

Conditions Favoring Growth

• Warm soil temperatures: 12–32°C (70–90°F)
• Even, moderate soil moisture (65% saturation); higher moisture favors growth if plants are injured

Control Measures

• Proper soil pasteurization eliminates Rhizoctonia from potting mixes.
• Encourage prompt growth – older plants are more resistant.
• Fungicides can be effective. Contact Cooperative Extension for current recommendations.

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