Rhizoctonia Root Rot

Rhizoctonia solani species complex

Rhizoctonia Root Rot is a soilborne fungal disease complex caused chiefly by host-associated groups within Rhizoctonia solani. It affects seedlings, vegetables, ornamentals, turf, and field crops, but symptom expression and aggressiveness depend on anastomosis group, host, temperature, and production system. Roots and lower stems develop firm tan to reddish-brown lesions, pruning of feeder roots, damping-off, or girdling cankers. Unlike water molds, Rhizoctonia does not produce motile zoospores and disease can be severe without standing water, although wet compacted media and slow growth increase vulnerability. The fungus is common in soil, so laboratory recovery must be interpreted with active lesions and host context.

Identification

Operational role: soilborne fungal root and lower-stem disease. Inspect recently affected plants for discrete sunken lesions, reddish-brown cortical decay, fine-root loss, or a dry constriction at the soil line. In seedlings, post-emergence damping-off may leave a wiry stem. Microscopy may reveal right-angle hyphal branching with constrictions, but culture and anastomosis-group or molecular testing are needed for stronger attribution. Submit whole plants with intact roots and medium, choosing early lesions rather than completely rotted tissue. Compare with Pythium/Phytophthora, Fusarium, black root rot, southern blight, fertilizer salts, cold media, and mechanical stem injury.

Symptoms / Signs

Germinating seeds may rot before emergence, seedlings may topple, and established plants may remain stunted with sparse, discolored roots. Root lesions are often firm and sharply delimited rather than initially watery. Basal stems can develop tan or reddish-brown cankers that girdle tissue. In some hosts Rhizoctonia causes web blight or aerial symptoms when contaminated media splashes into the canopy. Distribution may follow reused trays, contaminated benches, dense plugs, or patches of slow-draining substrate. Complete root collapse invites secondary organisms that obscure the original lesion.

Life Cycle / Process

R. solani survives as sclerotia and persistent mycelium in soil, media, pots, benches, crop debris, and infected plants. Hyphae grow through moist substrate to contact susceptible roots or stems; wounds are not always required. The pathogen generally spreads locally through contaminated material, tools, water splash, and plant-to-plant contact rather than through a large airborne spore phase. Temperature optima vary among anastomosis groups. Disease intensifies when roots grow slowly because of cool, wet, compacted, saline, or poorly aerated conditions, giving the fungus more time to colonize tissues.

Prevention

Use pathogen-free media, clean trays and containers, sanitized benches, and healthy propagation material. Avoid reusing contaminated substrate. Space plants and irrigate so foliage and crown surfaces dry while roots receive adequate water. Maintain media porosity, temperature, pH, and fertility for rapid establishment. Keep hose ends and tools off the floor, and prevent runoff from diseased lots entering clean areas. Seed or transplant treatments may protect during establishment when labeled, but sanitation and root-zone design remain essential.

Management / Control

Remove severely affected plants and adjacent contaminated medium. Reduce excess irrigation, compaction, and prolonged surface wetness without drought-stressing healthy roots. Clean and disinfest trays, benches, and tools after removing organic debris. Confirm Rhizoctonia before choosing a fungicide because products active against oomycetes may not control it, and sensitivity differs among isolates. Apply only products labeled for the crop, site, and Rhizoctonia disease, with rotation among permitted modes of action. In fields, rotation, residue management, planting into suitable soil temperature, and improved drainage can reduce loss but may not eradicate inoculum.

Procedure Steps

1. Map affected plants and note tray, bench, bed, irrigation, and substrate relationships. 2. Collect whole early-symptom plants with roots and attached medium. 3. Photograph basal stem lesions and wash only a subset of roots gently for examination. 4. Compare lesion firmness and pattern with Pythium, Phytophthora, Fusarium, and abiotic root injury. 5. Submit active lesion margins for isolation or molecular confirmation. 6. Remove affected plants and contaminated substrate, then clean and disinfect production surfaces. 7. Correct root-zone conditions and use only label-supported Rhizoctonia products when treatment is justified.

Safety Notes

Wear gloves when handling diseased substrate and eye protection when applying disinfectants. Remove organic debris before using sanitizers and never mix incompatible chemicals. Observe pesticide re-entry intervals and protect drains and surface water. Dust from dry media can irritate lungs; dampen carefully or use appropriate respiratory protection during cleanup.