Effects of Interacting Populations ofAlternaria solani, Verticillium dahliae,and the Potato Leafhopper (Empoasca fabae) on Potato Yield

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Abstract
Yield reduction in potato caused by early blight (Alternaria solani). Verticillium wilt (V. dahliae), and potato leafhopper (Empoasca fabae) was studied on three cultivars, Russet Burbank, Norland, and Red Pontiac, in factorially arranged field experiments in 1983 and 1984. Early blight epidemics were initiated by inoculation, and the fungicide chlorothalonil was used to develop varying levels of disease. A Verticillium wilt treatment was introduced either by inoculating stems at flowering (1983) or by applying a conidial suspension to the seed piece (1984). Varying levels of potato leafhopper nymphs were developed by managing the natural population with insecticides. In 1983, main effects of cultivar, leafhopper, early blight, and Verticillium wilt on yield were significant (P = 0.05). In addition, cultivar .times. leafhopper, cultivar .times. early blight, leafhopper .times. early blight, and leafhopper .times. early blight .times. Verticillium wilt interactions were significant (P = 0.05). Maximum yield reduction averaged across cultivars by each pest alone was 54, 31 and 12% for leafhopper, early blight, and Verticillium wilt, respectively. Maximum yield loss by any combination of pests was 63%. Significant effects on yield in 1984 were cultivar, leafhopper, early blight, and cultivar .times. early blight (P = 0.05). Maximum yield reductions averaged across cultivar by early blight and leafhoppers alone were 18 and 15%, respectively. In combination, early blight and leafhoppers reduced yield a maximum of 26%. Cultivar .times. early blight interactions were related to greater yield losses on late-maturing cultivars (Russet Burbank and Red Pontiac) than on an early-maturing cultivar (Norland). Pest .times. pest interactions showed that yield losses by solitary pests were not additive. Differences in results between 1983 and 1984 were due to differences in the range of pest levels attained.