Barley (Hordeum vulgare), an important crop in Uruguay, is vulnerable to several foliar diseases. In early November 2001, leaves displaying unusual lesions were observed in a commercial barley crop and research plots in the Paysandu District of western Uruguay. Lesions were small, elliptical, tan-colored to necrotic, approximately 4 × 1 mm, and had a narrow, dark brown margin. Older lesions occasionally were surrounded by a narrow (0.5 mm wide) chlorotic halo and some lesions had a white dot in the center. Sections of affected leaf tissue were placed in a moist chamber at 20°C with a 12 h light/dark cycle for 4 days to promote sporulation of a putative fungal pathogen. Conidia subsequently developing from infected tissue were large (250 to 325 μm long) and were identified as Drechslera gigantea (2). Koch's postulates were verified by growing the fungus on 10% V8 juice agar, and the aqueous suspension was adjusted to 3 × 104conidia per ml. Lower concentrations were tested but they were not great enough to prevent disease escape. A De Vilbiss atomizer was used to spray 2-week-old barley seedlings kept in a growth chamber at 18°C with a 17/7 h light/dark cycle. Inoculated plants were kept in the dark for 24 h at 100% relative humidity and then returned to the previous conditions. Twenty seedlings of each of two Canadian, one American, and eight barley genotypes from Uruguay were used for the study. The experiment was conducted twice including water controls each time. Small lesions appeared on all leaves of all inoculated plants on 10 cultivars 4 days after inoculation, and after 9 days they were fully developed and visually the same as those on plants in the field. Control plants did not display symptoms. D. gigantea was reisolated from lesions on all diseased leaves. A 0 to 5 disease infection scale, based on lesion size, was developed for which infection types 0, 1, and 2 were considered as immune, resistant, and moderately resistant phenotypes, respectively, type 3 as an intermediate phenotype, and 4 and 5 as moderately susceptible and susceptible phenotypes, respectively. The most susceptible infection type was observed on ‘Dayman’ barley, the same cultivar in which the disease was first noted and from which the pathogen was isolated. Cv. Herta, once grown in western Canada, also was susceptible, whereas the cvs. Bowman (United States) and McGwire (Canada) were moderately resistant. Only one cultivar (Uruguay) did not show any symptoms. Several Uruguayan experimental barley lines exhibited moderately resistant or intermediate phenotypes. D. gigantea has been reported as causing an “eyespot” disease on grasses (2) and rice (1). Further studies with different inoculation protocols and host development stages as well as sampling of a broader range of barley germ plasm is needed to assess the current and potential impact of this fungus. To our knowledge, this is the first report of D. gigantea as a pathogen of barley in Uruguay, and possibly the first report of this species as a barley pathogen globally.
References: (1) S.-W. Ahn. Plant Dis. 64:878, 1980. (2) C. Drechsler. J. Agric. Res. 24:641,1923.
