Foci of stagonospora nodorum blotch in winter wheat before canopy development

Stagonospora nodorum blotch (SNB) often develops explosively on upper leave s and glumes of wheat. Inoculum for late season infections may arise from e arly disease foci in the lower canopy or from recent immigration of wind-di spersed ascospores. Research was conducted to determine if foci of SNB are present and secondary spread has occurred in fields before tiller elongatio n. We determined the incidence of infection by Stagonospora nodorum for pla nts sampled at the mid-tillering stage in 96 1-m(2) quadrats in each of two fields. Isolates of S. nodorum were recovered from 32 quadrats, one per in fected plant where possible. Multilocus restriction fragment length polymor phism haplotypes were determined for each isolate. Of 55 isolates collected from one field, there were 22 distinct haplotypes. Diseased plants were ag gregated in both fields; aggregates sometimes extended to adjacent quadrats . Plants within aggregates were often infected by the same haplotype, sugge sting that secondary spread had occurred. Foci overlapped because some aggr egates were infected by more than one haplotype. Our results show that gene rically diverse populations of S. nodorum were already established in field s before canopy development and were comprised of sometimes overlapping foc i undergoing clonal expansion.