Effect of Temperature and Photoperiod on Resistance to Purple Leaf Spot in Orchardgrass1

Abstract

Experiments were conducted to determine whether resistance of orchardgrass (Dactylis glomerata L.) to purple leaf spot (caused by Stagonospora arenaria Sacc.) is environmentally stable. Regrowth leaves of 60 greenhouse‐grown genotypes were produced at four environmentally different dates (March, June, October, December) and were artificially inoculated. Spot size and spot frequency were scored on scales of 1 (no disease) to 8 (very severe disease). Plants scored <3.5 for either criterion were classified resistant. Only two of the 60 genotypes were resistant at all dates. Five genotypes were resistant at one or more dates, but moderately susceptible on at least one date. A significant genotype ✕ environment variance indicated resistance of these five genotypes was unstable. Three genotypes that were classified unstable and one genotype each that was classified resistant, intermediate, or susceptible, were then grown in combinations of temperature and photoperiods as follows: i) warm, short day; ii) warm, long day; iii) cool, short day; and iv) cool, long day. They were inoculated and incubated 48 h in the dark at 22°C and returned to their respective greenhouse or growth chamber environments in three trials. All genotypes formed moderately‐large to large spots in warm, short days, and all but the susceptible genotype formed small spots in cool, long days. The genotypes showed significant interactions with temperature or photoperiod for spot size and frequency. Repetitive artificial inoculations of genotypes during greenhouse screening programs may contribute to germplasm improvement by detecting and eliminating environmentally unstable genotypes.