IDENTIFICATION OF RESISTANCE IN SWEET-POTATO TO RHIZOPUS SOFT-ROT USING 2 INOCULATION METHODS

Two inoculation methods were developed and compared to evaluate cured sweetpotato (Ipomoea batatas) storage roots for resistance to Rhizopus soft rot caused by Rhizopus stolonifer and R. arrhizus. Roots that had been cured and stored for at least 3 wk were first washed in a commercial washer. In the impact/dip method, roots were wounded by allowing them to drop approximately 1 m from the end of the washer into crates, after which they were dipped in a suspension of sporangiospores. In the puncture inoculation method, deep-threaded wood screws were dipped in a spore suspension and then hammered about 5-10 mm deep into the median of the root. Most genotypes were more susceptible to R. stolonifer than to R. arrhizus; thus, subsequent evaluations were made using only R. stolonifer. Ranking of genotypes was similar with both methods, and there was an overall correlation in soft rot incidence (R2 = 0.22) between the methods. However, overall incidence of soft rot and differences among genotypes were greater by the puncture method. Genotypes with white-fleshed storage roots were uniformly susceptible, while those with orange flesh varied. Linear relationships were observed between inoculum concentrations and soft rot incidence on four different genotypes by the puncture method. By the impact/dip method, differences in soft rot incidence were not influenced as strongly by inoculum concentration. Of the commercial cultivars evaluated, Beauregard was the most resistant, Jewel and Hernandez varied from intermediate to susceptible, and the white-fleshed cultivars HiDry and Sumor were among the most susceptible.