Effect of Recurrent Phenotypic Selection on Resistance to Purple Leaf Spot in Orchardgrass1

Abstract

Genetic resistance to important diseases of forage grasses is considered to be the only practical means of reducing economic losses in yield and quality. The objective of this research was to determine the effectiveness of recurrent phenotypic selection as a method of breeding for resistance to purple leaf spot caused by Stagonospora arenaria Sacc. in orchardgrass (Dactylis glomerata L.). The initial selections were made from a source population of 28 cultivars (cycle 0). Four cycles of selection and two cycle‐comparison tests were conducted in the greenhouse. In a test comparing cycles 0, 1, 2, and 3, there were significant differences in size and frequency of purple leaf spots among entries within each cycle and among cycle means. Mean spot size and frequency for cycle 3 were markedly lower than for any previous cycle. In a four‐cycle comparison test, mean ratings for spot size and frequency were lower for cycles 3 and 4 than for previous cycles. The mean spot size and spot frequency ratings for cycle 4 were significantly lower than for cycle 3. The percentage of plants in the different severity classes shifted progressively towards higher percentages with lower ratings in each successive cycle. However, even in cycle 4, 7 to 10% of the progeny were rated susceptible. The correlation between spot size and spot frequency rating (3 df) was 0.97, significant at P < 0.01. There was a consistent significant effect of scorer on spot size rating for all tests, and in most instances on spot frequency rating. Methods of inoculation and incubation have been developed that were effective and reliable in screening for resistance to purple leaf spot. Four cycles of selection effectively increased the level of resistance to purple leaf spot in orchardgrass.