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Initiation of Septoria Nodorum Blotch Epidemics in Winter Wheat by Seedborne Stagonospora nodorum. D. Shah, Former graduate student, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908; G. C. Bergstrom(2), and P. P. Ueng(3). (2)Associate professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908; (3)Research plant pathologist, USDA-ARS, Plant Molecular Biology Laboratory, Beltsville, MD 20705. Phytopathology 85:452-457. Accepted for publication 15 December 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/Phyto-85-452.

To determine the potential contribution of seedborne Stagonospora nodorum to Septoria nodorum blotch epidemics, field plots that were isolated 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 199091, and 0.0, 0.5, 3, 11, 14, 19.5, 25.5, and 29% in 199192. In 199091, a season mildly conducive to Septoria nodorum blotch, even plots sown to seed with less than 1% infection by S. nodorum developed epidemics. Seed infection level had a significant effect on disease incidence 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 percentage of harvested seed infected by S. nodorum (P < 0.05). The above relationships of disease and seed infection level were nonlinear and asymptotic. In 199192, a season more conducive to Septoria nodorum blotch, epidemics were initiated in plots with seed infection levels as low as 0.5%. Seed infection level had a significant effect on disease incidence at the two-leaves-unfolded stage (P < 0.05), but not later that season. The contribution of seedborne S. nodorum to epidemic initiation also was assessed, in 199091, 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 with 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 for 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. nodorum could initiate epidemics of Septoria nodorum blotch in new locations and could provide for year-to-year perpetuation of these populations.

Additional keywords: Leptosphaeria nodorum, Phaeosphaeria nodorum, Septoria nodorum, Triticum aestivum.