In Australia’s main tomato growing areas around Bowen and Bundaberg in Queensland, soil-born diseases often cause heavy crop losses. Many growers cope with these disease problems by fumigating soil with methyl bromide or methan sodium, or by applying nematicides such as fenamiphos or oxamyl. However, the decision to use these chemicals is not always made on a rational basis. Fumigants and nematicides are often applied routinely as an insurance against losses, even though the casual pathogens are not always present at levels that will cause economic damage.

In a project funded by the Queensland tomato industry and Horticulture Australia, Dr Graham Stirling from Biological Crop Protection and a team of pest management consultants from all tomato-growing areas of Queensland developed procedures that will enable growers to reliably predict whether particular diseases will cause problems in their next tomato crop. Information on cropping history, previous disease incidence, soil texture and planting date was used to calculate a Hazard Index from each of the diseases that are likely to occur in a field. The research team then went on to show that this index provided a useful numerical assessment of disease risk. Losses from root-know nematode and fusarium wilt, for example, were only observed in fields with a Hazard Index of more than 40 and 30, respectively.

Another way of improving the prediction process is to collect soil samples prior to planting, determine the levels of pathogens in the sample and then use the results to make an informed management decision. In this component of the work, Dr Stirling collaborated with Dr Kathy Ophel-Keller and Dr Alan McKay from the South Australian Research and Development Institute in Adelaide, who have used DNA technologies to develop a root disease testing service for the Australian cereal industry. The aim was to see whether similar procedures could be used in horticulture. The results were encouraging, as it was possible to detect and quantify root-knot nematode and the fungal pathogens responsible for fusarium and verticillium wilt in soil from tomato fields using DNA techniques. The amount of pathogen DNA was related to disease incidence in the field, demonstrating that the test will be useful for predictive purposes.