Rhizoctonia Rot

Rhizoctonia is a soil-inhabiting fungal pathogen present in most Australian soils and potting media. Rather than producing visible spores in the way many other fungi do, it survives and spreads mainly as mycelium and as small, irregular, dark brown to black sclerotia (resting structures) in soil, thatch or plant debris. On affected tissue the mycelium can sometimes be seen as fine, brown, cobweb-like threads across the plant surface or soil in humid conditions, particularly in the early morning before dew dries. The fungus does not form a distinct fruiting body that is easy to spot with the naked eye, so diagnosis relies heavily on symptom pattern and conditions rather than seeing the organism itself.

In turf, Rhizoctonia solani is the main species responsible for brown patch, which develops as roughly circular patches of blighted grass ranging from a few centimetres to several metres across. In nursery and production horticulture, various Rhizoctonia species cause damping off of seedlings, stem and crown rot at or near the soil line, and root rot in pots and field-grown plants.

The disease is favoured by warm, humid weather with extended periods of leaf or soil surface wetness, typically in late spring through autumn in most of Australia, though outbreaks can occur whenever warm, moist conditions persist. Overwatering, poor drainage, excess nitrogen, dense canopy or thatch, and reused or contaminated potting media all increase risk. The fungus can persist for extended periods in soil and organic matter between susceptible crops or turf renovations.

Management is based on cultural control: improving drainage and airflow, avoiding overwatering (especially watering late in the day), reducing thatch, moderating nitrogen inputs, using clean or pasteurised potting media, and avoiding overcrowded seedling trays. Fungicide treatments registered for Rhizoctonia can support control but work best alongside these cultural measures rather than as a standalone fix.