CORRELATION OF TAKE-ALL DISEASE SEVERITY AND INOCULUM LEVEL OF GAEUMANNOMYCES-GRAMINIS VAR. TRITICI USING A SLOT-BLOT HYBRIDIZATION ASSAY

The amount of Gaeumannomyces graminis var. tritici inoculum was quanti fied by slot-blot hybridization assay, using a specific DNA probe, pG1 58. Estimates of the amount of G. graminis var. tritici DNA in natural ly infested and inoculated soils were made and correlated with take-al l disease severity as determined by a conventional soil bioassay (expr essed as a percentage of seminal roots with G. graminis var. tritici l esions). Disease severity in the glasshouse assay and DNA levels were highly correlated. The level of take-all disease severity in naturally infested soil can be predicted using the equation y(i) = 53.078 log ( x(i)) + 88.866, where y(i) = the predicted value of the disease severi ty (%) and x(i) = amount of G. graminis var. tritici DNA (ng) in 100g soil. The DNA-based assay for quantifying the amount of G. graminis va r. tritici has been further developed and improved in terms of the sam pling techniques and efficiency. Soils of different types were collect ed to a depth of 10 cm and organic matter between 0.5-1.4 mm in size w as separated from this soil. Using a slot-blot hybridization assay, it was found that G. graminis var. tritici was mostly present in the org anic matter fraction greater than a size of 0.5 mm and in soil at a de pth of 5 cm. This rapid and reliable DNA-based assay is being used in conjunction with a model of Lake-all disease development to predict di sease levels in the field due to G. graminis var. tritici.