INITIATION OF SEPTORIA-NODORUM BLOTCH EPIDEMICS IN WINTER-WHEAT BY SEED-BORNE STAGONOSPORA-NODORUM

To determine the potential contribution of seedborne Stagonospora nodo rum to Septoria nodorum blotch epidemics, field plots that were isolat ed from other wheat plants or residues were sown to winter wheat with seed infection levels by S. nodorum of <1, 3, 10, 20, 30, and 40% in 1 990-91, and 0.0, 0.5, 3, 11, 14, 19.5, 25.5, and 29% in 1991-92. In 19 90-91, a season mildly conducive to Septoria nodorum blotch, even plot s sown to seed with less than 1% infection by S, nodorum developed epi demics. Seed infection level had a significant effect on disease incid ence and severity at the main shoot and three tillers growth stage (P < 0.0001), on the F-5 leaf position at first node stage (P < 0.05), on the F-1 leaf position at late milk stage (P < 0.1), and on the percen tage of harvested seed infected by S. nodorum (P < 0.05). The above re lationships of disease and seed infection level were nonlinear and asy mptotic. In 1991-92, a season more conducive to Septoria nodorum blotc h, epidemics were initiated in plots with seed infection levels as low as 0.5%. Seed infection level had a significant effect on disease inc idence at the two-leaves-unfolded stage (P < 0.05), but not later that season. The contribution of seedborne S. nodorum to epidemic initiati on also was assessed, in 1990-91, by following two isolates (with DNA fingerprints distinguishable from each other and background isolates) of S. nodorum from infected seed through the crop canopy. Isolates wit h DNA fingerprints identical to those of the respective seed isolates were recovered from F-5 leaves and from harvested seed. These results showed that seedborne S. nodorum was at least partially responsible fo r initiation of Septoria nodorum blotch on the foliage. Moreover, the presence of the same isolates in the seed used for sowing and the seed harvested gave corroborative evidence that seed populations of S. nod orum could initiate epidemics of Septoria nodorum blotch in new locati ons and could provide for year-to-year perpetuation of these populatio ns.