Virulence Factors ofBremia lactucaein New York

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Abstract
Field collections of B. lactucae from the lettuce [Lactuca sativa] cv. 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.