Bioassay for enhanced biodegradation of nematicides in soil

In the Australian banana industry, burrowing nematode (Radopholus similis) is controlled by routine use of chemical nematicides particularly fenamiphos. A loss in efficacy with prolonged use of fenamiphos may be due to enhanced biodegradation by soil microbes. A simple and inexpensive method was developed to test soil for enhanced biodegradation by comparing nematode recovery from unsterile or sterile soil which had been treated with fenamiphos at 10 mu g/g soil with nematode recovery from untreated soil. Every 2 weeks, for 8 weeks, corn seedlings were planted in subsamples of the soil and inoculated with R. similis 2 days later. Recovery of R. similis alter 7 days from the corn roots was then used as an indicator of enhanced biodegradation. A soil with a long history of fenamiphos application, and reported to contain fenamiphos-degrading microorganisms, reduced the chemical's active time in unsterile soil to 2 weeks. Some soils which had not previously been exposed to fenamiphos contained microorganisms which rapidly degraded fenamiphos. A soil with enhanced biodegradation of fenamiphos did not prematurely degrade cadusafos. Hence, if fenamiphos is rapidly degraded in soil, rotation with cadusafos may allow nematode management. Comparison of the corn seedling bioassay with a bioassay using Rhizoctonia solani and Aphelenchus avenae confirmed enhanced biodegradation. The corn seedling bioassay is relatively easy to establish and uses the same nematode species which attacks banana plants.