Persistence of DNA of Gaeumannomyces graminis var. tritici in soil as measured by a DNA-based assay

There are an increasing number of assays available for fungal plant pathogens based on DNA technology. We have developed such an assay for Gaeumannomyces graminis var. tritici (Ggt) in soil, using slot-blot hybridisation. To ensure the validity of DNA-based soil assays for the fungus, it is important to determine the stability of Ggt DNA in soil. This study was undertaken to quantify the DNA degradation of dead Ggt in soil using a DNA-based assay. Mycelia were killed using various treatments, then DNA was extracted and estimated by a slot-blot hybridisation technique using the specific Ggt DNA probe, pG158. Mycelia were also killed using a fungicide (triadimefon) at a concentration of 150-250 microg ml(-1). The amount of detectable DNA of Ggt, killed using triadimefon, declined by 82-93%. Inoculum in the form of diseased wheat roots, artificially inoculated ryegrass seed, particulate soil organic matter and whole soil was killed using heat-treatment. The amount of detectable DNA of Ggt declined markedly (90%) in both heat-treated roots and inoculated ryegrass seeds, and declined by 50% in both treated soil and soil organic matter. The rate of DNA degradation of Ggt in soil varied with the type of inoculum. The amount of detectable DNA of Ggt in dead mycelia declined by 99.8% after 4 days of incubation in soil. No DNA was detected after 8 days of incubation. In contrast, Ggt DNA in live mycelia declined by 70% after 8 days of incubation and declined to 10% of original DNA level after 32 days. In ground ryegrass seed inoculum, DNA in both killed and live Ggt declined by 50% after 8 days. In diseased roots, DNA from both live and killed Ggt did not appear to decline over 16 days. Estimates of the amount of Ggt in the soil using a DNA-based assay reflect both live and dead populations of the fungus. The rate of breakdown of DNA of the dead fungus is very high and the presence of dead fungi in roots probably a rare event so the DNA from dead fungus probably contributes little to the total DNA level.