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Ecology and Epidemiology

Characterization of Resistance to Early Blight in Three Potato Cultivars: Incubation Period, Lesion Expansion Rate, and Spore Production. 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, New York 14853. Phytopathology 79:511-517. Accepted for publication 6 September 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-511.

Early blight incubation periods (degree-hours from inoculation until the lesion radius reaches 0.35 mm) and lesion expansion rates (micrometers of lesion radius per degree-hour during leaf wetness) were measured on intact plants of field-grown potato cultivars Kennebec, Norchip, and Rosa at about 18, 30, and 45 days after emergence. Incubation periods were shorter and lesion expansion rates were greater on lower leaves and on older plants. During leaf wetness, lesion expansion increased in a linear manner over a range of 6 to 24 hr and in response to temperatures ranging from 9 to 27 C. Incubation periods and lesion expansion rates were described by a linear function of leaf position, plant physiological age (Pdays after emergence), and interaction of leaf position and plant age. Incubation periods on 18-day-old Norchip were shorter than on Kennebec and Rosa; 45-day-old plants were no longer markedly different. Lesion expansion rates were lowest on Rosa. Cultivar differences became less apparent as the plants grew older. Spore production per square millimeter of lesion was measured from individual lesions on leaves harvested from field-grown plants at 74 and 92 days after planting. Spore production increased as a linear function of lesion area. Plant age (Pdays after emergence) and leaf position did not have a significant effect on spore production. Spore production was lowest on Kennebec; Norchip and Rosa were not significantly different. Spore production was not correlated with the maturity class of the cultivar.