Resistance to Phytophthora infestans Clonal Lineage US-23 in Potato Cultivars and Its Relationship with Early Blight Resistance and Tuber Yield

Resistance to late blight caused by Phytophthora infestans clonal lineage US-23 in 217 old and modern potato cultivars was evaluated in field trials in 2016 and 2017 in Pennsylvania. Significant differences in resistance were found among these cultivars (P < 0.0001). Significant interaction between cultivars and environments was found (P < 0.0001). The values of relative area under the disease progress curve ranged from 0 to 0.5841 in 2016 and from 0 to 0.5469 in 2017. Broad-sense heritability of late blight resistance was estimated to be 0.91, with a 95% confidence interval of 0.88 to 0.93. Cluster analysis classified the cultivars into five groups: resistant, moderately resistant, intermediate, moderately susceptible, and susceptible. Thirty cultivars showing resistance and 32 cultivars showing moderate resistance were identified. The 217 cultivars were also evaluated for foliar maturity, tuber yield, and resistance to early blight caused by Alternaria solani. A few cultivars with late blight resistance independent of late maturity were found. Late blight resistance and early blight resistance were positively correlated, and 17 cultivars possessed resistance to both diseases. Yield trade-off associated with late blight resistance was not observed among the cultivars in the absence of disease pressure.