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Effects of Wetting Period on Resistance to Leaf Spotting of Wheat by Leptosphaeria microscopica with Conidial stage Phaeoseptoria urvilleana. R. M. Hosford, Jr., Professor, Department of Plant Pathology, North Dakota State University, Fargo, ND 58102; Phytopathology 68:908-912. Accepted for publication 29 November 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-908.

A fungus was isolated from leaf spots on wheat and found producing conidia resembling those of Phaeoseptoria urvilleana on wheat stubble in western North Dakota. On potato-dextrose agar it produced ascospores resembling Leptosphaeria microscopica and conidia resembling Phaeoseptoria urvilleana. This fungus was incubated on all of the following cultivars in all of the following sunlit wet periods. It caused no leaf spotting on any cultivar following postinoculation wet (mist) periods of 24, 36, or 42 hr. Following wet periods of 48 hr or longer it caused severe leaf spotting on spring wheat cultivars Waldron and Red River 68. Following 72- and 91-hr wet periods it caused severe spotting on ND495 spring wheat and slight spotting on Ward durum; no leaf spotting developed on Chris and Duri spring wheats, Wells and Hercules durum, WW8 awned winter wheat, Larker barley, Caribou rye, or Lodi oats. This appeared to be the first report of leaf spotting caused by L. microscopica. The fungus required a long postinoculation wet period to cause spotting. Expression of varietal resistance to leaf spotting was associated with the duration of wet period. This phenomenon of varietal resistance related to the duration of postinoculation wet period now has been detected with seven wheat leaf-spotting fungi. Compilation of the literature suggests that among four wheat leaf-spotting Leptosphaeria spp. with similar three-septate ascospores, increasing pathogenicity to more wheat cultivars is coupled to decreasing pycnidiospore size, septation, and pigmentation.

Additional keywords: Hendersonia species, Leptosphaeria avenaria Weber f. sp. triticea T. Johnson, L. eustomoides Sacc., L. nodorum Müller, L. tritici (Gar.) Pass., Phaeosphaeria Speg., Septoria avenae Frank f. sp. triticea T. Johnson, S. nodorum (Berk.) Berk., Phaeoseptoria airae (Grove) Sprague, P. festucae Sprague.