Relationship between environmental variables and the prevalence and isolation frequency of leaf-spotting pathogens in spring wheat

The prevalence of stagonospora nodorum blotch (SNB), septoria tritici blotch (STB), septoria avenae blotch (SAB), tan spot, and spot blotch, and the frequency of isolation of the pathogens that cause these diseases, were monitored from 1989 to 1995 in southern Manitoba. From 123 to 185 fields of wheat grown to cultivars belonging to Canada Western Red Spring, Canada Prairie Spring, and Canada Western Amber Durum wheat classes were surveyed each year. Infected leaf tissue was collected, and later incubated to promote conidiation for species identification. Environmental data were obtained from the weather station closest to each field. Prevalence and frequency of isolation of Pyrenophora tritici-repentis (tan spot) was higher in durum wheat cultivars, and prevalence of Phaeosphaeria nodorum (SNB) lower, than in cultivars of the other two wheat classes. High prevalence and frequency of isolation of P. nodorum were associated with years of high rainfall, and isolation frequency of Cochliobolus sativus (spot blotch) was higher in years with high daily minimum temperatures (warm nights). Within-year analysis showed that frequency of isolation of P. nodorum was correlated both with rain and high temperatures. Isolations of Mycosphaerella graminicola (STB) were negatively correlated with frequency of rain during the 7 days before sampling. Leptosphaeria avenae f. sp. triticea (SAB) isolations were positively correlated with temperature for the 14-day period before sampling and with frequency of rain in the 7 days before sampling. Isolations of P. tritici-repentis were negatively correlated with rain, and isolations of C. sativus were positively correlated with temperature. In part, the environmental data help explain the variability in annual levels of leaf-spotting diseases of wheat in Manitoba. The significance of this to breeding programs is discussed.