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Resistance

Field Resistance of Crisphead Lettuce to Bremia lactucae. 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 74:149-152. Accepted for publication 16 March 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-149.

Field resistance to Bremia lactucae (causal agent of downy mildew of lettuce) was observed in several crisphead lettuce cultivars in experimental plantings within commercial lettuce fields in Oswego County, New York, during 1980 and 1981. In 1980 three crisphead lettuce cultivars (Ithaca, Mesa 659, and Iceberg) were measured for severity of downy mildew with a key specifically designed for this purpose. Plants of Ithaca and Mesa 659 were more susceptible to mildew than Iceberg, but Ithaca was more susceptible than Mesa 659. In 1981, plants of Florida breeding line FL 49015 were the most resistant; these data were obtained with a graphics digitizer used to measure leaf and lesion area of leaf images in a similar experiment with six cultivars of lettuce (none with any known specific resistance factors). Similar readings of disease severity resulted from using either total lesion area or the percent area infected. Among the remaining cultivars, the average severity of disease developed on leaves of the New York State cultivars (Ithaca and Minetto) and the Wisconsin cultivars (Greenlakes and Montello) were almost the same. Minetto was much more resistant to downy mildew than was Ithaca. Plants of cultivar Ithaca from Foundation stock seed were more susceptible and developed larger lesions than those of cultivar Ithaca grown from commercial seed.