Development of a resistance screening method for sweetpotato foot rot caused by Diaporthe destruens

Foot rot in sweet potatoes, caused by Diaporthe destruens, has rapidly increased in East Asia and has become a major concern for production in Korea, highlighting the need for a reliable susceptibility test to assist the development of resistant cultivars. Diseased plants were collected from major production areas in Korea between 2015 and 2024, and seven isolates were identified as D. destruens based on multilocus sequence analysis. Mycelial growth was optimal at 25 to 30 °C and SPL24001 was selected as the representative isolate through pathogenicity tests at 27 °C. Screening conditions were determined by evaluating different inoculation methods (dipping, wounding, and soil drenching) and spore concentrations (1 × 10⁴ to 10⁶ spores/mL) in the three cultivars over 2 years. The soil drenching method, which involved applying 1 mL of a 1 × 10⁵ spores/mL suspension around the base of each transplanted cuttings, consistently differentiated between resistant and susceptible cultivars and was subsequently adopted as the standard screening condition. Using this method, eight cultivars were evaluated and classified into four groups ranging from resistant to highly susceptible. Lesion length showed a strong positive correlation with disease severity and field incidence, thus validating the reliability of the test. The soil drenching method provides a practical and reproducible approach for conducting resistance screening throughout the year, thereby facilitating the advancement of breeding and genetic research on sweet potatoes.