Distribution of Mating Types of Sphaerotheca fuliginea in the United States. M. T. McGrath, Department of Plant Pathology, Long Island HorticulturalResearch Laboratory, Cornell University, 3059 Sound Avenue, Riverhead, New York 11901-1098. H. Staniszewska, and N. Shishkoff, Department of Plant Pathology, Long Island HorticulturalResearch Laboratory, Cornell University, 3059 Sound Avenue, Riverhead, New York 11901-1098, and GeorgeCasella, Department of Plant Breeding and Biometry, Cornell University, 320 Warren Hall, Ithaca, New York14853. Plant Dis. 80:1098-1102. Accepted for publication 22 June 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1098.

Mating type of Sphaeroiheca fuliginea isolates was determined on summer squash leaves in detached leaf culture. Both mating types were found in all eight states examined (Arizona, California, Florida, Georgia, New York, North Carolina, Texas, and Virginia) and in 18 of the 27 fields examined in 1993 and 1994; therefore, the absence of one mating type is not the sole explanation for the rare sightings of cleistothecia in most areas of the United States. Cleistothecia were found on leaves collected in Michigan, indicating that both mating types also were present in this state. One mating type occurred more frequently (≥70% of the isolates tested) in 15 of these 18 fields. In 1993, 82% of the 132 isolates tested belonged to MAT1-2; whereas in 1994, only 31% of 155 isolates were MAT1-2. The frequencies of isolates resistant to triadimefon (50 ?g/ml) were 0 and 63% for MATI-1 and MATI-2, respectively, in 1993 and 25 and 62%, respectively, in 1994. Triadimefon-resistant isolates of each mating type were found in the same fields in California, Georgia, and Texas in 1994. All isolates shared identical alleles at the mating incompatibility locus. This provides strong evidence that 5. fuliginea does not possess multiple alleles at the single mating locus.

Keyword(s): cucurbits, powdery mildew, teleomorph