INFLUENCE OF CULTIVAR RESISTANCE TO VERTICILLIUM SPP ON POTATO EARLY DYING

Microplot studies were conducted over 3 yr to examine the influence of Verticillium dahliae (VD) and Pratylenchus penetrans (PP) on the Vert icillium-susceptible cultivars, Kennebec and Superior, and the resista nt cultivars, Reddale and Russette. V. dahliae alone negatively affect ed yield in 2 of the 3 yr for Kennebec and Superior, but showed no eff ect on yield in Russette and Reddale in any year. An interaction betwe en V. dahliae and P. penetrans resulted in higher yield losses in 1 of the 3 yr for Kennebec, Reddale and Russette. Regression equations wer e developed for each cultivar from data averaged over 3 yr which indic ated that, on average, the product of VD and PP density terms can he u sed to describe yield losses in Verticillium resistant cultivars. With the susceptible cultivars, a quadratic function with VD and either th e VD x PP interaction or PP alone was the best descriptor of average y ield losses. Incidence of senescence (a measure of potato early dying [PED]) was related to densities of V. dahliae and P. penetrans by fitt ing quadratic regression models with the terms: time (T),VD, PP, T-2, VD X T, VD2, PP x T, VD x PP, and PP2. By finding which terms were sig nificant and comparing parameter estimates for these terms among culti vars, information was gained on both the effect of V. dahliae and P. p enetrans on FED incidence and the rate of increase of FED incidence ov er time. A VD x FP interaction for Kennebec and Russette in 1989 and K ennebec in 1991 was associated with earlier, high incidence of FED sym ptoms. The synergistic action of V; dahliae and P. penetrans could cau se proportionally higher yield losses, although much higher yield loss es were associated with the susceptible cultivars. Susceptible cultiva rs generally had higher senescence incidence and a higher rate of sene scence increase, early in the season, compared to resistant cultivars.