A new variant of Puccinia striiformis causing stripe rust on barley and wild Hordeum species in Australia

Stripe rust of wheat (caused by Puccinia striiformis Westend. f. sp. tritici, Pst) was first detected in Australia in 1979 and progressively evolved from a single progenitor pathotype through single step mutational events for pathogenicity (Wellings & McIntosh, 1990). Annual surveys of the pathogen population have monitored these events to provide advice to growers and wheat breeders concerned with the selection, release and management of stripe rust resistant wheat cultivars.

During the survey in 1998, stripe rust samples were collected from wheat and grasses (principally barley grass Hordeum spp.) and occasionally barley. These collections were expected to yield various pathotypes of Pst. However, the samples were unusual in two respects. Firstly, approximately half were collected from barley grass, whereas samples from previous surveys were predominately from wheat. Secondly, approximately 30% of samples from barley grass were highly avirulent on seedlings of most wheat differentials, partially virulent on the wheat differential Chinese 166, and pathogenic on certain barleys, notably cv. Skiff. One sample collected from Skiff in a field trial had the same features.

The initial evidence suggested these isolates were P. striiformis f. sp. hordei (Psh). However, seedling tests of Australian barley cultivars revealed that only a small group was affected. Earlier tests in Mexico with Psh Race 24 had shown that the majority of Australian barleys were susceptible.

Current evidence suggests that this "barley grass stripe rust" (BGYR) may represent a new forma specialis. This rust resembles pathotype CDL 21 of Pst described by Line & Qayoum (1991). However, preliminary isozyme data indicated that BGYR contrasts with Pst at the Pgm locus. Although single step loss of virulence in Pst has been observed on several occasions (Wellings, unpublished), the pathogenicity contrast between current Pst and BGYR collections suggests that the latter represents a new introduction into Australia. This rust continued to predominate over Pst on wild Hordeum spp. in 1999 and infected early sown barley in trial plots and farmers' fields in New South Wales and Victoria. The potential economic impact of this new variant of Puccinia striiformis to barley will be explored in field trials in 2000. It appears that BGYR is not an immediate threat to wheat.