Rhizoctonia Rot

Rhizoctonia is a soil-inhabiting fungus that survives between seasons as tough, dark brown to black resting bodies called sclerotia, and as fine strands of mycelium in soil, thatch and plant debris. In humid, warm conditions you may see a fine, cobweb-like grey to brown mycelium spreading across affected turf or over the soil surface early in the morning while leaves are still wet, though this is often gone by midday. On vegetable and ornamental seedlings, the fungus produces a reddish-brown, sunken lesion at the soil line that girdles and collapses the stem.

This fungus doesn't produce airborne spores in the way many other fungal diseases do. Instead it spreads through direct contact between roots, through movement of infected soil, water, mowing equipment and infected plant material, and via sclerotia that can persist in soil for years waiting for the right conditions.

In Australian conditions, Rhizoctonia is most active during warm, humid weather, typically from late spring through to autumn, with outbreaks often following periods of high overnight humidity, poor air movement and extended leaf wetness. It affects a very wide host range, from lawns (where it's commonly known as brown patch) to strawberries, potatoes, beans, lettuce and many ornamental seedlings, making it one of the most common and widespread soil fungi in home gardens across the country.

Management relies on reducing the conditions that favour the fungus rather than eliminating it entirely, since it's a normal part of most garden soils. This means improving drainage and air circulation, avoiding overwatering (especially late-afternoon or evening watering that leaves foliage wet overnight), not over-fertilising with nitrogen which produces soft, susceptible growth, avoiding mowing turf too low or too frequently during outbreaks, and practising crop rotation and good sanitation in vegetable beds to reduce sclerotia build-up.