Current Virulence of Pyrenophora teres on Barley in Western Australia

Net blotch, caused by Pyrenophora teres Drechs. (anamorph: Drechslera teres (Sacc.) Shoemaker), is a prominent foliar disease of barley (Hordeum vulgare L. emend. Bowden) in Western Australia (19,46), as well as elsewhere in the world (13,28,31,34,39,41). Two types of leaf symptoms are associated with the net blotch disease: the net type, caused by P. teres f. teres, which causes horizontal and vertical crisscrossed dark brown venation that sometimes turns chlorotic; and the spot type, caused by P. teres f. maculata, which causes dark brown circular or elliptical spots accompanied by chlorosis of the surrounding leaf tissue (20,32). Both pathogens reduce yield (7,11,22,23,27,35,36), mainly through reduced grain size, which may impair the brewing quality of malting barley. Yield loss experiments conducted in Western Australia demonstrated a 17% yield decrease in barley cultivar Beecher due to net type net blotch (30). In a more detailed study, cultivar Dampier had a mean yield reduction of 21% with a maximum loss of 37% (16). For spot type, yield losses varied from 3 to 22% in cultivars Beecher and O’Conner in Western Australia (17). Studies on the occurrence and distribution of different virulence types of P. teres are essential to identify useful forms of resistance and assist in the development of future barley breeding strategies. Pon (26) first reported the variation in virulence in P. teres f. teres isolates in the United States. Since then, various studies have been conducted to understand the pathogenic variation and prevalence of pathotypes of P. teres around the globe using a range of differential barley lines (1– 3,9,10,12–15,20,25,29,34,39,40) and also restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers (21,24,45). In Western Australia where net blotch is endemic, three pathotypes of P. teres f. teres were reported by Khan and Boyd (18). The occurrence and virulence of P. teres f. maculata was reported by Khan and Tekauz (20). A change in virulence was observed with the introduction of new barley cultivars in Western Australia. The incidence of net type net blotch declined in the 1970s, and by the early 1980s, 73% of P. teres f. teres isolates were avirulent on the previously susceptible cultivars Beecher and Atlas (15). The moderately resistant cultivars Stirling and O’Connor became susceptible a decade after their release in Western Australia, indicating the ability of the pathogen population to change with cultivars. The change in virulence, adoption of high-yield production methods, and wide use of conservation tillage practices have contributed to the reemergence of net blotch as a threat to barley production. The objective of this study was to determine the virulence spectrum of current populations and assess possible changes in P. teres over the 19 years since it was last assessed by Khan (15) in Western Australia.