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Characterization of Resistance to Early Blight in Three Potato Cultivars: Receptivity. J. R. Pelletier, Agriculture Canada, Research Station, P.O. Box 457, St.-Jean-sur-Richelieu, Quebec, Canada J3B 6Z8; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 80:361-366. Accepted for publication 4 October 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-361.

Receptivity of potato leaves (number of lesions per germinated conidium) to Alternaria solani was measured at 6- to 12-day intervals in 1984 and 1985 on detached leaves of field plot-grown potato plants, cultivars Kennebec, Norchip, and Rosa. Receptivity decreased with height in the canopy (measured in 15-cm strata) but increased over time. Changes in receptivity were described by a segmented linear function of plant physiological age, stratum, and age-stratum interaction. Physiological age was defined as the square of Pdays (minimum, optimum, and maximum temperatures: 7, 21, and 30 C, respectively) accumulated since emergence, divided by the centimeters of precipitation accumulated since planting. Dynamics of receptivity of Norchip (susceptible) were significantly different from those of Kennebec and Rosa (moderately and highly resistant, respectively). Whole-plant receptivity, calculated by weighting the receptivity of each stratum by its proportional leaf area, displayed trends over time that were similar to the receptivities of individual strata. Whole-plant receptivities of the three cultivars were significantly different. Changes in receptivity were poorly correlated with leaf area, shoot dry weights, and tuberization.

Additional keywords: infection efficiency, quantitative inoculation.