Comparing different epidemiological models in field evaluations of selected genotypes from Solanum tuberosum CIP population A for resistance to Phytophthora infestans (Mont.) De Bary in Kenya

Late blight caused by Phytophthora infestans was monitored in field plots of potato genotypes selected from population A of the International Potato Center (CIP) germplasm collection. Disease severity was measured as percent blighted leaf area and used to compute area under disease progress curves (AUDPC), apparent infection rates (r) and severity at epidemic onset (Yo). AUDPCs revealed more distinct differences among the genotypes than any other disease assessment parameter. Percent disease severity measured 67–77 days after planting (D67-77)explained more variation in AUDPCs than measurements made on any other single day. Increase in percent diseased leaves fit the monomolecular model more closely than the Gompertz, logistic or exponential model. All disease assessment parameters varied among the genotypes and were significantly (p < 0.01) correlated with each other. Genotypes with larger AUDPCs generally had higher DS67-77 and faster rates of disease increase (r). Clones 386191.7 and381403.23 were more susceptible to late blight than all other entries tested. The lowest disease levels were observed on clone 382155.2. Frequency distribution of AUDPCs among genotypes appeared continuous and did not differ significantly (p <0.05)from normal distribution suggesting the observed resistance may be attributable to minor genes.