VIEW ARTICLE

Ecology and Epidemiology

Effect of Interaction of Inoculum Dose, Cultivar, and Geographic Location on the Magnitude of Bacterial Ring Rot Symptom Expression in Potato. A. A. G. Westra, Research support specialist, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; S. A. Slack, Uihlein professor of plant pathology, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 84:228-235. Accepted for publication 19 November 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-228.

The effect of the interaction of inoculum dose of Clavibacter michiganensis subsp. sepedonicus, potato cultivar, and geographic location on the magnitude of bacterial ring rot symptom expression was investigated by planting an inoculum dose × cultivar factorial experiment in several potato-growing regions of the United States. Inoculum dose was positively correlated with the expression of both foliar and external tuber symptoms of bacterial ring rot and negatively correlated with total yield and vine length. The magnitude of the response to inoculum dose was modulated primarily by its interaction with cultivar and location. Three-way interaction between inoculum dose, cultivar, and location was generally not significant. The relative effect of cultivar on the inoculum dose response for foliar symptom expression, vine length, and yield remained constant (Russet Burbank > Norchip ≥ Norland), irrespective of geographic location, whereas its effect on tuber symptom expression was variable. The relative effect of location on dose response was more variable than that of cultivar but followed the general pattern: Washington ≥ Maine, New York, Oregon > Colorado, Wisconsin ≥ North Dakota. Variability in the effect of location on dose response may indicate that environmental conditions specific to each location influence the expression of bacterial ring rot symptoms. Variation in the effect of location on the different disease response variables was also observed within a single growing season. This, combined with the poor correlation observed among these variables, indicated that environmental conditions may affect these responses differentially and that they are likely to be of little value as predictors of one another.