Reactions of Isolates from Matings of Races 1 and 23N of Exserohilum turcicum . P. FALLAH MOGHADDAM, Department of Plant Pathology, University of Illinois, Urbana 61801. J. K. PATAKY, Department of Plant Pathology, University of Illinois, Urbana 61801. Plant Dis. 78:767-771. Accepted for publication 11 April 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0767.

Races 1 and 23N of Exserohilum turcicum isolated from corn leaves infected with northern leaf blight that were collected in 1990 from Belle Glade, Florida, were crossed in culture on johnsongrass stems. Pseudothecia, which formed after 21-25 days in darkness, were crushed and streaked on water agar. Ascospores were isolated and transferred to lactose-casein hydrolysate agar. Parental isolates and isolates from 112 ascospore progeny were assayed twice in the greenhouse for resistant and susceptible reactions on corn inbred lines with the genes Ht1, Ht2, Ht3, and HtN and susceptible inbred checks. Reactions of the 112 isolates were not independent (1:1 ratio of similardissimilar reactions) on plants with the genes Ht2 and Ht3 or the genes Ht1 and HtN, i.e., reactions were similar on Ht2 and Ht3 genotypes for 189 of 224 assays and different on Htl and HtN genotypes for 159 of 224 assays. Eight combinations of virulence were observed in both greenhouse trials; however, reactions of 72 of the 112 isolates differed between trials. Races I, 23N, and 123 were most frequent among the 112 isolates based on greenhouse assays. When seven isolates that were virulent on Ht1, Ht2, Ht3 and/or HtN in the greenhouse were assayed in the field, only four were virulent on Ht1 and all isolates were avirulent on Ht2, Ht3, and HtN.