Black root rot of lettuce

Reviewed by Heidi Martin, Department of Primary Industries and Fisheries.

The disease

The disease is caused by the fungus  Thielaviopsis basicola. It attacks the roots of lettuce, beans, watermelons, rockmelons, cucumbers, tobacco, sweet peas and pansy. The common milkthistle, Sonchus oleraceus, is a natural host.

In the early stages, brown lesions appear on the roots of lettuce plants 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 lettuce plants may be severely stunted.

Figure 1. Plants inoculated with the black root rot fungus (right) and uninoculated plant (left)

How is it spread?

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. Lettuce replanted in the same soil may be diseased and unmarketable.

Resistant varieties

Some varieties are much more resistant than others. Growers should discuss their variety requirements with seed company representatives to determine the latest information on resistant varieties.

On farms with a history of the disease, we recommend that growers test varieties of head lettuce to identify the varieties most resistant and suitable for their situation.

Nursery hygiene

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.

Fungicides

There is no fungicide registered to control black root rot on lettuce.

DPI&F information and services

• To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call, from 8 am to 6 pm Monday to Friday (excluding public holidays). E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.

• Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.

Industry links

Industry links is a page of links to various sites of interest to horticultural growers.

Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

Last reviewed 22 October 2007