The epidemiology of leaf spot disease in a native prairie. II. Airborne spore populations of Pyrenophora tritici-repentis

A Burkard 7-day volumetric spore sampler was operated continuously from April 14 to November 23, 1970, and from April 21 to September 29, 1971, at Matador, Saskatchewan, in a native prairie dominated by Agropyron dasystachyum and A. smithii. Both ascospores and conidia of Pyrenophora tritici-repentis were trapped, but the counts of conidia greatly exceeded those of ascospores. The peak catch of ascospores occurred before that of conidia in both years. The total numbers of conidia per month were similar in both years except in June, when six times as many were trapped in 1971 as in 1970. Most conidia and ascospores are believed to have originated on dead infected grasses since P. tritici-repentis was shown not to sporulate on lesions on live Agropyron leaves. There was a marked diurnal periodicity in the trapping of conidia, with the peak usually at about 1200 h. This periodicity probably reflected the effects of light on sporulation and of wind speed on spore dispersal. However, simple meteorological explanations could not be found for the wide fluctuations (0–707) that occurred in the daily total catches of conidia during the midsummer period.