DNA polymorphism and host range in the take-all fungus, Gaeumannomyces graminis

Thirty-five isolates of Gaeumannomyces graminis were tested for ability to infect wheat, rye and oats, and for DNA polymorphisms using nuclear ribosom al DNA (rDNA) RFLP patterns and RAPD analysis. In general, the cereal-attac king isolates could be readily assigned to the rye-attacking (R) or non-rye -attacking (N) subgroups of var. tritici, or to var, avenae, on the basis o f either of these molecular approaches. A small number of isolates gave ano malous rDNA RFLP patterns, but could nevertheless be assigned to one of the three groups by RAPD analysis. Two related G. graminis var. tritici isolat es (T1-1 and T1-2) clearly grouped with the N rather than the R var. tritic i subgroup on the basis of molecular analysis but were pathogenic to rye, i ndicating that the ability to infect this host may have arisen more than on ce. An earlier phylogenetic study of Gaeumannomyces involving DNA sequence analysis of the internal transcribed spacers of the rDNA indicated that sev eral oat-infecting isolates originally classified as G. graminis var. triti ci could be grouped with var, avenae isolates. From the rDNA RFLP and RAPD analysis described here, however, these isolates appear to be intermediate between var. avenae and var. tritici, although it is not clear whether they represent evidence of inter-varietal sexual hybridization.