Foliar and Postharvest Application of Phosphite Promotes Postharvest Resistance of Potato Tubers to Late Blight Caused by Phytophthora infestans

Postharvest losses due to late blight, caused by Phytophthora infestans, remain a significant challenge for sustainable potato production. This study evaluated the effectiveness of potassium phosphite (KPhi) applications during the growing season and postharvest in enhancing tuber resistance to late blight in three potato cultivars (Cv. Xinjia No. 2, Cv. Zhongshu No. 3, and Cv. Minshu No. 3). Whole tuber assays were conducted on four-month stored tubers inoculated with ASO and 1-12-25 isolates of P. infestans, revealing that foliar KPhi treatments significantly reduced the disease severity in tubers. Five foliar applications during the growth season provided more protection than three applications. Cv. Xinjia No. 2 displayed higher resistance, Cv. Zhongshu No. 3 has moderate resistance, and Cv. Minshu No. 3 is moderately susceptible. Half-tuber and slice assays confirmed these findings, demonstrating markedly reduced late blight development in KPhi-treated samples compared to the controls, particularly when challenged with the more aggressive ASO isolates versus the 1-12-25 isolates of P. infestans. The most pronounced protection against the 1-12-25 isolates was observed in Cv. Xinjia No. 2. While KPhi did not entirely inhibit rot development at all treatments, it consistently delayed symptom progression and reduced lesion size across all three cultivars. These results support the application of KPhi as an effective, cultivar-dependent strategy for managing late blight development in potato tubers, with the degree of protection positively correlated with the application frequency. Postharvest KPhi application is a promising strategy to reduce tuber late blight, prolong storage life, and limit disease spread. Keywords: cultivar resistance, Phytophthora infestans, postharvest disease control, potato tubers, potassium phosphite