Considerations for Verticillium wilt resistance evaluation in potato

Verticillium wilt (Vw), caused by the soilborne fungi Verticillium dahliae and V albo-atrum, is an important disease of potato (Solarium tuberosum?). Host plant resistance is a promising method of Vw control. Culture-based methods that quantify the pathogen in host tissue often are used for Vw resistance screening. To evaluate the processing time, accuracy, and precision of these methods, 46 clones were planted in a field naturally infested with V dahliae to collect data on visual disease symptoms, pathogen colonization, and yield. In 2002, disease severity explained 43.4% of the variability of yield loss, but the linear relationship between stem colonization and yield loss was not significant. In 2003, stem colonization explained 57.5% of the variability of yield loss, whereas disease severity explained 1.7% of the variability of yield loss. Correlations comparing clone ranks from repeated pathogen measurements indicated that culturing sap from individual stems or bulked stems generated more repeatable clone rankings than culturing dried stems. Clone rankings were more repeatable between years if pathogen measurements were made earlier in the growing season, The results indicate a need to characterize the effect of the environment on the relationship among pathogen population sizes in planta, disease symptoms, and yield loss.