Aggressiveness of Stagonospora nodorum Isolates Obtained from Wheat in the Northern Great Plains

Isolates of Stagonospora nodorum, obtained from diseased wheat leaves collected in fields in Montana, North Dakota, and South Dakota, were tested on detached seedling leaves of wheat and found to be pathogenic. Differences among isolates chosen at random were detected in the first phase of the inoculations. Isolates associated with severe and mild symptom severity, as measured by lesion size, were selected in phase two. Isolates associated with severe and mild symptom severity were statistically differentiated in phase three. The ability of isolates to cause significant differences in symptom severity was interpreted as differences in aggressiveness. The differences among isolates detected with detached leaf experiments was confirmed with glasshouse inoculations of seedling wheat plants. The high aggressive isolates consistently produced higher symptom severity on wheat seedlings, as measured by percentage necrosis, than low aggressive isolates. Cultivars were consistently differentiated with isolates associated with different levels of aggressiveness. With unselected isolates, the cultivar × isolate interaction was nonsignificant, indicating a lack of specificity. With high and low aggressive isolates, the cultivar × isolate interaction was significant. Considering that the magnitude of the mean squares for interactions were rather low compared to the main effects, it is speculated either that specificity is detected with the present isolates but only at a low level, or that specificity is apparent only when isolate extremes are tested. With studying isolates in phases rather than random comparisons, fungal isolates associated the high and low aggressiveness can be identified and, once identified, isolates from different hosts or geographical areas can be compared in an efficient and meaningful manner.