Development of a laboratory test using stem cuttings to measure resistance to foot rot disease caused by Diaporthe destruens in sweetpotato

Over the last several years, foot rot caused by Diaporthe destruens has become the most destructive sweet- potato disease in the southernmost region of Japan. Breeding of cultivars resistant to foot rot is required for effective and low-cost management. Field tests are often used to evaluate resistance of cultivars, but this ap- proach has several limitations, including a long test period of several months and the requirement of field isolation and labor-intensive procedures. To minimize these issues, we have developed an easier and faster laboratory method using stem cuttings for the resistance test by optimizing four parameters: the number of unfolded leaves per cuttings, the positions of stems from which a cutting was prepared, the adequate number of culture days after inoculation, and the density of conidia of D. destruens at inoculation. Significant correla- tion was detected between the resistance indices of the laboratory test and the field test, namely, the length of the rotted part of a stem and the proportion of the plants rotted at the basal part of a stem, respectively. These results indicated that the laboratory test could indirectly evaluate the foot rot resistance of sweetpotato stems in the field and will be helpful to breed resistant cultivars.