Aggressiveness of Stagonospora nodorum Isolates from Perennial Grasses on Wheat

Isolates of Stagonospora nodorum from 13 perennial grasses were tested on detached wheat leaves. Grass isolates that were consistently associated with large or small lesions were statistically differentiated. Some grass isolates were detected that were associated with large lesions on wheat (high symptom severity), but most isolates produced small lesions (mild level of symptom severity). The ability of grass isolates to cause significant differences in symptom severity, as measured by lesion length, was interpreted as differences in aggressiveness. Grass isolates that were associated with high symptom severity were found to be similar to selected wheat isolates in their level of aggressiveness. Results were confirmed with inoculations of seedling wheat plants in the glasshouse. Wheat cultivars were statistically differentiated from one another with all types of isolates. The cultivar × isolate interactions were, generally, not significant with unselected isolates, indicating a low possibility of specificity. In contrast, when isolates associated with large and small lesions were compared, the cultivar × isolate interaction was significant in most experiments, but the magnitude of the mean square for the interaction was low compared to the mean squares of the main effects. The ability of S. nodorum to infect perennial grasses would improve the likelihood of survival of this fungus between wheat crops. Because most isolates are not associated with severe symptom severity, their potential effect on wheat is probably less than would be expected considering the potential number of isolates available from the grasses.