Black root rot

Cause

The fungus Thielaviopsis basicola.

Symptoms

In the early stages, brown lesions appear on the plant roots then increase in size and turn black. Large areas, or all, of the roots are affected. Small roots are often completely destroyed.

The disease can occur at any stage of plant growth, from seedlings in containers to plants ready for market. Infected plants may be severely stunted.

How it spreads

The fungus may occur naturally in Queensland soils. Alternatively, it may be introduced with peat moss because Thielaviopsis species have been identified in some sources of commercially available peat.

Lettuce seedlings on affected properties during the early 1990s were produced in seedling mixtures containing peat. This explains the early seedling infections and the wide distribution of the pathogen in South East Queensland. Peat is also suspected as the source of infection for hydroponically-grown plants.

Once detected in soil, or established in a hydroponic system, the fungus is likely to remain there for a long time. Crops replanted in the same soil may be diseased and unmarketable.

Poor drainage and soil temperatures of 17 to 25°C favour the disease.

It affects a wide range of crops and survives in the soil for long periods. Common sowthistle is a weed host.

Crops affected

Wide range of crops including lettuce, bean, watermelon, rockmelon, cucumber, tobacco, sweet pea and pansy.

The common milkthistle, Sonchus oleraceus, is a natural host. Other hosts include soybean, cowpea, clover and lucerne.

Control

Plant varieties with resistance to the disease. Avoid double cropping lettuce or rotating with susceptible crops such as soybean, cowpea, clover or lucerne. Improve soil drainage. Soil fumigation gives short-term control.

Young seedlings can become infected through the use of seedling trays contaminated with the fungus. It is important to thoroughly wash trays and to decontaminate with a disinfectant solution such as chlorine or a quaternary ammonium compound. Plastic trays are easier to clean than polystyrene.

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.