Response of wheat and barley isolates of Septoria nodorum to passage through barley and wheat cultivars

A single-spore wheat isolate of S. nodorum was passaged once through barley and a barley isolate passaged once through wheat. Reisolations from barley leaves were designated as either 'original-type wheat' (OTW) and resembled the wheat isolate used for inoculations in morphology, or 'sector-type barley' (STB) resembling the known morphology of barley isolates. Reisolations from wheat were similarly designated either OTB or STW. Sub-cultures of STB were significantly more pathogenic to barley in a glasshouse experiment than OTW, and vice versa. Similarly STW was significantly more pathogenic to wheat than OTB, and vice versa.
A single-spore wheat isolate was also passaged 0, 1, 2, 3 or 4 times through barley and a barley isolate passaged similarly through wheat. Disease assessment and yield data from a field experiment showed that the pathogenicity of the passaged wheat isolate remained largely unaltered from that of the original isolate. However, the barley isolate responded after the third wheat passage by causing significantly less disease and significantly higher yields in barley than the unpassaged isolate; conversely, on wheat, disease levels significantly increased and yield slgnficantly decreased after the second wheat passage.
Data obtained are discussed in the context of cultural observations made on colony morphology during host passage sequences and possible genetic mechanisms responsible.