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Inheritance of Brown Leaf Spot Resistance in Smooth Bromegrass¹

Smooth bromegrass (Bromus inermis Leyss.) cultivars are susceptible to brown leaf spot (Pyrenophora bromi [Died.] Drechs.); however, some plants have smaller lesions than others. We studied inheritance of lesion size because there was no previous information on inheritance of resistance to brown leaf spot. Ten parent clones differing in lesion size were reciprocally single crossed with parent clone 11. The 10 parental clones were also polycrossed. Greenhouse-grown parents and progeny were inoculated with conidia of P. bromi and rated for lesion size in November 1979. Regrowth was inoculated and rated in January 1980. Reciprocals were not statistically different. Four parental clones were more resistant than parent 11 in at least one of the two trials, but the F₁ progeny of two of these clones were significantly more susceptible than the midparent (mean of clone 11 and the single cross parent). Five other parental clones were more susceptible than clone 11, and F₁ progeny of two of them were more susceptible than the midparent. Polycross progeny means did not differ from polycross midparent means or were slightly lower in 2 of 20 observations. The results indicated that lesion size is regulated by multiple genes, and that susceptibility may be dominant or epistatic to resistance. There was a significant entry x date interaction believed to be attributable to environment before and after inoculation. Developing populations with high levels of resistance to brown leaf spot will be difficult.

Key Words: Forage grass • Disease resistance • Bromus inermis • Drechslera bromi • Helminthosporium bromi • Pyrenophora bromi

² Research agronomist, research plant pathologists, and research geneticist, respectively, U.S. Regional Pasture Research Laboratory, University Park, PA 16802.

Received for publication June 18, 1982.