Identification of IPM strategies for Pythium induced root rots in Apiacae vegetable crops
This study identified several new pathogens from roots of Apiaceae. In a study of root rots over 3 years of trials in sandy and sandy loam soils in Victoria, Pythium was found to be the most common group on diseased roots of autumn sown parsley and parsnip. As parsnip roots matured, other common pathogens invaded. More than 11 different species of Pythium were identified, some of which were previously not know to occur on parsley and parsnips in Australia. A species of Phoma, not seen before on these crops, was also identified. This suggests that these diseases have a complex of causes, induced by Pythium. This information will help better target control treatments. Two disease management options that provided disease control and improved yields were fungicide treatments and varieties. Metalaxyl, applied to the soil, reduced the severity of root rot in parsley and canker in parsley, resulting in improved yields. This fungicide is active against the ‘water mould’ pathogens such as Pythium. However, results were not consistent. Fungicides were not effective in one of two trials in parsley and one of four trials in parsnip. Heavier soil types appeared to be a major constraint. Some parsnip cultivars evaluated in the field proved to be much less susceptible to canker, returning up to 3 times more marketable crop than the standard variety. These varieties may be an option for cultivation when the disease risk is very high. Three different biological control agents, Bacillus subtilis, Streptomyces lydicus and Pythium oligandrum, did not provide any significant control of root rot in field grown parsley, parsnip or hydroponically grown coriander. There was evidence that Bacillus subtilis treatments stimulated the growth of parsnip and coriander. Cultural control treatments of hilling soil over the parsley and parsnip crowns, and blanketing soil with organic mulch, did not reduce root disease, although both hilling and mulch treatments stimulated plant growth in some trials. Managing diseases in autumn sown crops where disease risk is high, particularly for long growing crops such as parsnip, is particularly challenging. Future research should focus on understanding the importance of the newly discovered pathogen in the diseases complex. Research should also be directed at improving the application and timing of fungicides and biological control agents to ensure that the highest concentration of active material occurs in the root zone at the time when the risk of infection is greatest. Recommendations for control of these Pythium induced root rots include rotation with non-Apiacaea crops (e.g. broccoli), selection of fields/beds with relatively good drainage (e.g. avoid the heavier water logged soil), choose varieties that are less prone to disease, and early application of fungicides such as metalaxyl, preferably in the granule form to ensure a good distribution in soil.