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Resistance

Virulence Factors of Bremia lactucae in New York. J. E. Yuen, Graduate research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; J. W. Lorbeer, professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 72:1363-1367. Accepted for publication 15 March 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1363.

Field collections of Bremia lactucae from the lettuce cultivars Ithaca, Minetto, and A-1 were determined to have virulence factors 2 and 7 when tested on 5-day-old cotyledons of 21 differential lettuce cultivars grown on vermiculite in a controlled environment chamber. A field experiment consisting of 11 of the 21 differential cultivars was conducted in Oswego County, New York. Plants possessing resistance factor 7 became infected first. Eventually all plants of cultivars possessing the resistance factors 5, 6, 7, 8, and 10 became infected. B. lactucae isolates collected from the planting were tested on 5-day-old cotyledons of the 21 differential cultivars grown in the controlled environment chamber, which confirmed the presence of virulence factors 5, 6, 7, 8, and 10 and also revealed the presence of virulence factors 1, 2, and 4. Virulence factors 9 and 11 may have been present, since 12–54% of the plants possessing the corresponding resistance factors became infected.