Relationships among measurements of fitness and disease severity in Phytophthora infestans

Several methods of estimating pathogen fitness and disease severity in the Phytophthora infestans Solanum tuberosum pathosystem were investigated. Components of fitness measured in a growth chamber were compared with area under the disease progress curve (AUDPC) measured in small field plots. The closest relationship with AUDPC was for sporulation capacity (coefficient of determination, r ²= 0.86)and the least close relationship was with infection frequency ( r ²= 001). Two different methods were compared for calculating the relative fitnesses of two isolates: (i) apparent infection rates and (ii) changes in frequency in mixed epidemics. The values obtained by the two methods were not contradictory (0.98 and 0.93 units per day for the less fit isolate, against 1 for the more fit isolate), but had wide and overlapping confidence intervals.