First report of apothecia of Tapesia yallundae occurring on the wild grass Holcus lanatus (Yorkshire fog) in New Zealand

Tapesia yallundae is a causal agent of eyespot, a damaging stem base disease of cereal crops. The pathogen has also been detected on many wild and cultivated grasses even if characteristic eyespot symptoms are not visible (Lucas et al., 2000). The pathogen exhibits a heterothallic mating system and apothecia of the sexual stage have been detected on straw stubble in many European countries, Australasia, Southern Africa and North America. During field studies in New Zealand in 1995 apothecia resembling those of T. yallundae were discovered on decaying stem bases of Holcus lanatus (Yorkshire Fog) at the margin of one wheat field near Carterton, Wairarapa, North Island (Fig. 1).

To verify that apothecia were indeed T. yallundae, ascospore discharge was induced and single ascospore cultures established. Resultant colonies were confirmed as T. yallundae on the basis of conidia and colony morphology, and growth rate (2.3 ± 0.1 mm/day) on PDA (Dyer et al., 1996). Isolates from H. lanatus also produced a 1.05 kb PCR product characteristic of T. yallundae following amplification with the species specific primers Ty16F and Ty16R (Nicholson et al., 1997) (Fig. 2). No product was obtained with primers specific to the closely related species T. acuformis. Furthermore, amplification with RAPD primer OPA-10 resulted in the production of 0.68 kb and 1.32 kb bands, again characteristic of T. yallundae (Dyer et al., 1996) (Fig. 2). Finally, inoculation of wheat seedlings (cv. Avalon) with conidial suspensions resulted in production of characteristic eyespot lesions (Fig. 3).

This represents the first report of apothecia of T. yallundae occurring on H. lanatus and the first evidence of infection of H. lanatus by T. yallundae. The sexual stage of T. yallundae has also been detected on the wild grasses Bromus diandrus and Hordeum leporinum (barley grass) (Wallwork, 1987). Should the sexual cycle occur regularly on wild grasses then these species may provide an important reservoir of eyespot disease able to infect nearby cereal crops as a result of the dispersal of air borne ascospores. This problem may be exacerbated by the recent establishment of grassy islands ('beetle banks') as wildlife refuges in UK crops.

Studies were supported by the BBSRC and British Council.