Influence of cultivar resistance toVerticillium SPP. On potato early dying

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Microplot studies were conducted over 3 yr to examine the influence ofVerticillium dahliae (VD) andPratylenchus penetrans (PP) on theVerticillium-susceptible cultivars, Kennebec and Superior, and the resistant cultivars, Reddale and Russette.V. dahliae alone negatively affected yield in 2 of the 3 yr for Kennebec and Superior, but showed no effect on yield in Russette and Reddale in any year. An interaction betweenV. dahliae andP. penetrans resulted in higher yield losses in 1 of the 3 yr for Kennebec, Reddale and Russette. Regression equations were developed for each cultivar from data averaged over 3 yr which indicated that, on average, the product of VD and PP density terms can be used to describe yield losses inVerticillium resistant cultivars. With the susceptible cultivars, a quadratic function with VD and either the VD x PP interaction or PP alone was the best descriptor of average yield losses. Incidence of senescence (a measure of potato early dying [PED] ) was related to densities ofV. dahliae andP. penetrans by fitting quadratic regression models with the terms: time (T), VD, PP, T2, VD x T, VD2, PP x T, VD x PP, and PP2. By finding which terms were significant and comparing parameter estimates for these terms among cultivars, information was gained on both the effect ofV. dahliae andP. penetrans on PED incidence and the rate of increase of PED incidence over time. A VD x PP interaction for Kennebec and Russette in 1989 and Kennebec in 1991 was associated with earlier, high incidence of PED symptoms. The synergistic action ofV. dahliae andP. penetrans could cause proportionally higher yield losses, although much higher yield losses were associated with the susceptible cultivars. Susceptible cultivars generally had higher senescence incidence and a higher rate of senescence increase, early in the season, compared to resistant cultivars.