INFLUENCE OF CONTINUOUS CROPPING OF SEVERAL POTATO CLONES ON THE EPIDEMIOLOGY OF VERTICILLIUM WILT OF POTATO

Two field studies with potato were conducted to study the effects of continuous cropping of Verticillium-resistant potato clones on the epidemiology of Verticillium wilt of potato. Investigations focused on effects of continuous cropping of different clones on wilt incidence, Verticillium dahliae reproduction within the host, changes in the soilborne inoculum density, crop yields, and effects on succeeding crops of susceptible cultivars. Each study involved 5 yr of continuous cropping with potato. Five cultivars (and/or clones) with different levels of resistance to Verticillium wilt were planted in the first study. These were Russet Burbank and Butte (susceptible), Targhee (resistant), and A66107-51 (-51) and A68113-4 (highly resistant). The second study included the susceptible cultivar Russet Burbank, the resistant clone -51, a nonhost (corn), and a fallow treatment. There was no evidence for change in the relative resistance of the cultivars over the course of these studies, nor was there evidence that an extremely virulent strain of V. dahliae was enhanced selectively from the indigenous population. All resistant clones remained nearly symptomless and resistant to stem colonization throughout our investigations. The first study, conducted from 1977 to 1982, demonstrated that V. dahliae inoculum densities in the soil were negatively correlated with the degree of resistance of the cultivar grown the previous year. Where highly resistant cultivars were grown for five seasons, inoculum densities of V. dahliae were 60-70% lower than in plots where susceptible cultivars were grown. The second study, conducted from 1983 to 1988, confirmed that increases in V. dahliae inoculum densities were related to cultivar susceptibility. Where the resistant -51 clone was grown, the increase of V. dahliae populations was delayed by 1 yr, and populations increased to only 60% of those which developed with the susceptible Russet Burbank. Although V. dahliae inoculum densities increased with the cropping of potato clones, no changes in inoculum density occurred in either corn or fallow plots. Inoculum densities in plots with the highly resistant clones decreased to the point that neither the susceptible Russet Burbank nor a highly susceptible potato clone (NDA8694-3) exhibited substantial Verticillium wilt in a subsequent cropping season. When resistant potato clones are integrated into a potato production program, they appear to limit Verticillium wilt severity both during the years in which they are grown and in following crops of susceptible cultivars.