Black Root rot (Thielaviopsis basicola) BRR is a major threat to cotton production in southern and central NSW. It has a wide host range of more than 230 species of plants (Pereg 2013). The fungus survives for long periods in the soil as resistant resting spores. Infection of cotton is favoured by soil temperatures below 20 °C, which is normal at planting time in these cooler regions. Research in the USA has shown that severe disease symptoms result when the population of the black root rot fungus reaches 100 spores/g. It delays crop development in a region which already has a constrained growing season.

Populations of 600–700 spores per gram of soil have been found in some Australian cotton fields. Black root rot fungus does not kill seedlings by itself, however severe infection will render cotton more susceptible to other seedling diseases such as Pythium and Rhizoctonia. Stand losses of 30% or more are common from combinations of these seedling diseases. Seedlings affected by black root rot are stunted and slow growing. In effect, the disease ‘steals’ time from the crop leading to delayed maturity and yield loss. As weather conditions and temperatures improve, infected cotton crops will recover but in poor establishment conditions, yield reductions of 25–50% have been attributed to severe black root rot. The use of Woolly pod vetch as a green manure crop has been shown in previous research to reduce the impact of BRR in fields. This project aims to investigate the use of a bio fumigant crop such as Woolly Pod Vetch based on previous research. The concept of biofumigation involves planting a crop that releases compounds that are toxic to pests or pathogens in the soil. It involves growing and harvesting the biofumigant plant as either a rotation crop or as a sacrificial crop that is sprayed out and incorporated (brown manuring) or freshly incorporated (green manuring) into the soil prior to planting cotton. The effectiveness of biofumigation relies on the bulk of the crop being incorporated at least four weeks before planting cotton to allow breakdown of the material so there are no phytotoxic effects on the following cotton crop. A number of crop types have been trialed over the years as biofumigant crops including woolly pod vetch, mustard, canola and fodder radish. Three seasons of trials on different fields in northern NSW resulted in a 28–70% reduction in black root rot disease severity from Indian mustard and a 24–61% reduction from woolly pod vetch (Nehl 2004 )