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Genetics

Heterothallism in Sclerospora graminicola. R. W. Michelmore, Visiting scientist, Millet Improvement Program, International Crops Research Institute for the Semi-Arid Tropics, ICRISAT, P.O., Patancheru, 502 324, Andhra Pradesh, India, Present address: Department of Vegetable Crops, University of California, Davis 95616; M. N. Pawar(2), and R. J. Williams(3). (2)(3)Research technician, and principal pathologist, respectively, Millet Improvement Program, International Crops Research Institute for the Semi-Arid Tropics, ICRISAT, P.O., Patancheru, 502 324, Andhra Pradesh, India. Phytopathology 72:1368-1372. Accepted for publication 25 February 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1368.

Six isolates of the pearl millet downy mildew (DM) pathogen, Sclerospora graminicola, were established and maintained in plants of a susceptible pearl millet (Pennisetum americanum) cultivar. Sporangia were originally collected from field grown plants that showed no evidence of sexual reproduction. The isolates were used singly and in all paired combinations to inoculate seedlings of a highly DM-susceptible pearl millet cultivar. Mature oospores were observed in seedlings inoculated with certain specific pairs of isolates. Inoculation with single isolates usually did not result in oospore production. The abundance of oospores varied, but large numbers of oospores were found in seedlings that had shown little or no asexual sporulation, nor extensive chlorosis. The six isolates were subcultured individually for four asexual generations, and no oospores were detected in any of the plants infected with five of the isolates. A few oospores were found infrequently in plants infected with one of the isolates. These results demonstrate the existence of heterothallism in S. graminicola. The isolates could be assigned to one of two sexual compatibility types that have been designated G₁ and G₂. Scanning electron micrographs showed hyphae of two morphological types, similar to those observed preceding the formation of gametangia in the heterothallic Bremia lactucae. The patterns of asexual and sexual sporulation of S. graminicola can be explained in terms of differential colonization of the apices of young host plants. The consequences of heterothallism to pathogenic variability and to breeding for resistance are discussed.