13 IREC Farmers' Newsletter No. 198 — Spring 2017 per gram of soil. Populations of 600–700 spores per gram of soil have been found in 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 black root rot. Initial investigation A field in the MIA was identified as having reduced cotton yields due to black root rot. The field had a long cropping history of black root rot host crops, mostly ‘back to back’ cotton and one faba bean crop. Previous research in 2000 in northern NSW by NSW Agriculture researcher David Nehl showed that summer flooding of fields was very effective in reducing black root rot spore loads to very low levels (Table 1.) Flooding of the field before planting cotton is a method used to control soil-borne diseases of cotton in parts of California. Growers there report that the severity of black root rot is reduced substantially in the next four cotton crops after flooding. Ambient temperature is an important factor. Flooding is most effective during summer when there is a minimum of 30 days with maximum air temperature at 30 °C or more. One way to achieve summer flooding in California was to grow a rice crop in the field. This was also an option in the MIA field due to the bankless layout and good water availability in the 2016–17 season. Soil samples to 15 cm were collected and sent to a specialist microbiology laboratory before and after the rice crop. Testing for black root rot showed that the spore level dropped from 180 spores/g of soil to 65 spores/g of soil. This test has the potential to be developed as a predictive tool for growers and consultants. Cotton seedlings were grown in glasshouses in the two soil samples at Yanco (NSW DPI) and the reduction in severity and incidence of black rot root symptoms was confirmed (see photos above). Biofumigant crops 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. Cotton seedlings grown in soil that had previously grown cotton (left), showing more severe symptoms of black root rot than seedlings grown in soil that had previously grown rice (right). Table 1. Decline in the population of Thielaviopsis basicola after summer flooding of a field at Merah North (west of Wee Waa) for 57 days, February and March 2000. Sampling position 50 m from head ditch 50 m from tail drain Water depth (cm) 30 55 Soil temperature max* (°C) 26.2 25.6 Soil temperature min* (°C) 23.8 24.1 Population before (spores/g soil) 359 485 Population after (spores/g soil) 8 19 Population reduction (%) 98% 96% *10 cm below the top of the bed