SELECTION FOR DECREASED SENSITIVITY TO PROPICONAZOLE IN EXPERIMENTAL FIELD POPULATIONS OF STAGONOSPORA-NODORUM (SYN-SEPTORIA-NODORUM)

Selection of Stagonospora nodorum (syn. Septoria nodorum) isolates wit h decreased sensitivity to propiconazole was attempted using foliar ap plications of propiconazole to experimental wheat plots inoculated wit h S. nodorum isolates in 1989 and 1990. The isolates evaluated represe nted the range of variation in sensitivity to propiconazole present in S. nodorum populations in commercial wheat fields in New York State d uring 1985-1989. Changes in the sensitivity of S. nodorum to propicona zole in inoculated field plots were determined by testing isolates col lected before and after the application of propiconazole using trap pl ants (1989) or by direct sampling of infected leaves (1990). Significa nt differences in propiconazole-sensitivity phenotype were observed in 1989 but not in 1990 between plots treated with the commercial rate o f propiconazole and plots receiving no propiconazole treatment. Applic ations of double the commercial rate of propiconazole in 1990 failed t o select for isolates with decreased sensitivity to propiconazole. The selection of S. nodorum isolates with decreased sensitivity to propic onazole in 1989, but not in 1990, may have resulted from the manner in which the plots were sampled in each year. Very small changes in mean propiconazole sensitivity phenotype were detected in response to prop iconazole treatment in both experiments. This was likely a result of l imited variability in propiconazole-sensitivity phenotypes of the eval uated isolates or an insufficient number of reproductive generations o ccurring during these experiments.