The amount of Gaeumannomyces graminis var. tritici
inoculum was quantified by slot-blot hybridization assay, using a specific
probe, pG158. Estimates of the amount of G. graminis var.
tritici DNA in naturally infested and inoculated soils were made
and correlated with take-all disease severity as determined by a
conventional soil bioassay (expressed as a percentage of
seminal roots with G. graminis var. tritici lesions).
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 yi=53·078 log
where yi=the predicted value of the disease
severity (%) and xi=amount of G.
graminis var. tritici DNA (ng) in 100 g soil. The
DNA-based assay for quantifying the amount of G. graminis var.
tritici has been further developed and improved in terms of the
sampling techniques and efficiency. Soils of different types were collected
to a depth of 10 cm and organic matter between
0·5–1·4 mm in size was separated from this soil. Using
a slot-blot hybridization assay, it was found that G. graminis
var. tritici was
mostly present in the organic matter fraction greater than a size of
0·5 mm and in soil at a depth of 5 cm. This rapid and reliable
DNA-based assay is being used in conjunction with a model of take-all disease
development to predict disease levels in the field due
to G. graminis var. tritici.