Rhizoctonia

Rhizoctonia solani is common in most soils and is able to cause some form of disease in almost all cultivated plants. Different 'strains' of R. solani, called anastomosis groups, have been recognised. The anastomosis groups differ in their host range and pathogenicity.

The most common diseases are damping-off of seedlings, root and stem rots, stem cankers and fruit rot. The diseases often have common names that describe symptoms on the particular host, for example base rot of lettuce, black scurf of potato tubers, crater rot of beetroot, and wire stem of tomato and crucifer seedlings.

Cause

The fungus Rhizoctonia solani.

Symptoms

Symptoms vary according to the host and the plant part affected. Damping-off of seedlings is probably the most common disease caused by R. solani.

Infection of stems causes dark-coloured cankers or rotting at the base of the stem. On plants growing close to the ground (e.g. lettuce), the fungus attacks the older leaves in contact with the soil, causing brown lesions on the leafstalks and leaves. The disease may progress to involve the whole plant, causing wilting and death.

On fleshy, succulent stems, roots and storage organs, the fungus causes brown, rotten areas or sunken cankers that may be covered by fungal mycelium. Infection of potato tubers causes black scurf in which small, dark, flat sclerotia resembling specks of dirt occur on the tuber surface and which are not removed by washing.

Fruit growing near the ground can be infected, developing firm, water-soaked areas that become sunken and often crack open.

Rhizoctonia is a common cause of death in young olive nursery plants. It does not appear to be a problem in mature trees.

How it spreads

Rhizoctonia solani is present in most soils and, once introduced, remains there indefinitely. Its distribution in soils is often patchy, but it is found predominantly in the upper 15 to 20cm of the soil profile as mycelium or sclerotia, or in organic debris.

The anastomosis groups differ in host range, types of diseases caused and the ability to compete and survive in soil. The anastomosis groups present in a soil are strongly influenced by previous cropping history.

R. solani occurs with soil movement, in water, and on contaminated tools and plant parts. Rhizoctonia diseases are more severe in soils that are moderately wet rather than waterlogged or dry.

Crops affected

Almost all cultivated plants.

Control

• Grow seedlings in soil-less potting mixes or in sterilised or pasteurised soil.
• Treat seed with the recommended fungicide before planting.
• Maintain optimum plant growth and avoid injuring plants, as wounds provide a means of entry for the fungus.
• Where possible, avoid soil contact of fruit by using plastic mulch or by staking plants.
• Ensure crop residues decompose thoroughly before replanting an area.
• Applications of fungicides to the soil provide control of some diseases caused by R. solani.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.