Field Resistance of Crisphead Lettuce toBremia lactucae

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Abstract
Field resistance to B. lactucae (causal agent of downy mildew of lettuce) was observed in several crisphead lettuce culitvars in experimental plantings within commercial lettuce fields in Oswego County, New York [USA] 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 6 cultivars of lettuce (none with any known speciifc resistance factors). Simliar 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 ''Ithaca'' from Foundation stock seed were more susceptible and developed larger lesions than those of ''Ithaca'' grown from commercial seed.