Recurrent Phenotypic Selection for Resistance to Brown Leaf Spot in Smooth Bromegrass1

Abstract

Smooth bromegrass (Bromus inermis Leyss.) cultivars are susceptible to brown leaf spot [caused by Pyrenophora bromi (Died.) Drechs.]; however, there is variation among plants within cultivars for lesion size. The objective of this research was to determine the effectiveness of recurrent phenotj pic selection as a method of breeding for resistance to brown leaf spot. The source population (Cycle 0) was 93 plant introduction lines and five cultivars. Four cycles of selection for reduced lesion size were conducted using artificial inoculation in the greenhouse. A test comparing the four cycles and the check cultivars Saratoga and Baylor was conducted in the greenhouse. Mean lesion size, lesion frequency, and overall disease severity were significantly lower for Cycle 4 than for previous cycles. The percentage of plants in each severity class shifted to higher percentages in lower rating classes in Cycle 4 compared to previous cycles and check cultivars. However, even in Cycle 4 some plants were rated as moderately susceptible. The correlations between lesion size and lesion frequency, lesion size and overall severity, and lesion frequency and overall severity (476 df) were 0.64, 0.90, and 0.83, respectively (P<0.01). Methods of inoculation and incubation have been developed that were effective and reliable in screening for resistance to brown leaf spot. Four cycles of selection resulted in a significant increase in the level of resistance to brown leaf spot in smooth bromegrass.