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 fenamipho s. A loss in efficacy with prolonged use of fenamiphos may be due to enhanc ed biodegradation by soil microbes. A simple and inexpensive method was dev eloped to test soil for enhanced biodegradation by comparing nematode recov ery from unsterile or sterile soil which had been treated with fenamiphos a t 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 ino culated with R. similis 2 days later. Recovery of R. similis alter 7 days f rom the corn roots was then used as an indicator of enhanced biodegradation . A soil with a long history of fenamiphos application, and reported to con tain fenamiphos-degrading microorganisms, reduced the chemical's active tim e in unsterile soil to 2 weeks. Some soils which had not previously been ex posed to fenamiphos contained microorganisms which rapidly degraded fenamip hos. A soil with enhanced biodegradation of fenamiphos did not prematurely degrade cadusafos. Hence, if fenamiphos is rapidly degraded in soil, rotati on with cadusafos may allow nematode management. Comparison of the corn see dling bioassay with a bioassay using Rhizoctonia solani and Aphelenchus ave nae confirmed enhanced biodegradation. The corn seedling bioassay is relati vely easy to establish and uses the same nematode species which attacks ban ana plants.