Identification of and Cultivar Reactions to a New Race (Race 4) of Peronospora farinosa f. sp. spinaciae on Spinach in the United States. L. P. Brandenberger, Graduate Research Assistant, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. J. C. Correll, and T. E. Morelock. Assistant Professor, Department of Plant Pathology, and Professor, Department of Horticulture and Forestry, University of Arkansas, Fayetteville 72701. Plant Dis. 75:630-634. Accepted for publication 21 December 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0630.

Race 4 of Peronospora farinosa f. sp. spinaciae was identified on spinach in California and Texas. Differential cultivars used for race identification included Viroflay (susceptible to races 1, 2, and 3), Nores (resistant to races 1 and 2), Califlay (resistant to races 1 and 3), and Polka and St. Helens (resistant to races 1, 2, and 3). All five differentials were susceptible to race 4. All five isolates recovered from spinach in California were identified as race 4. A sugar beet isolate from California did not infect any of the spinach differentials. Of the five isolates recovered from spinach in Texas, two were identified as race 4 and three were identified as race 3. Inoculation tests on 29 commercial cultivars and five Arkansas breeding lines were conducted in controlled environmental chambers with an isolate of race 4 from California and an isolate of race 3 from Washington. Of the 29 cultivars tested, 18 have demonstrated or reported resistance to races 1, 2, and 3; seven have reported resistance to races 1 and 2; and two have reported resistance to races 1 and 3. All 19 cultivars resistant to race 3 tested were resistant to race 3 in our inoculation tests. Arkansas cultivars and breeding lines with reported polygenic resistance to race 3 had susceptible reactions on cotyledons and intermediate reactions on true leaves in race 3 inoculation tests. All cultivars and breeding lines tested were susceptible to race 4.