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Influence of Wound Type and Storage Duration on Susceptibility of Sweetpotatoes to Rhizopus Soft Rot

Susceptibility of stored sweetpotato roots (cvs. Beauregard and Hernandez) to Rhizopus soft rot caused by Rhizopus stolonifer was tested at 4- to 6-week intervals over a storage period of 335 days in 1998-1999 (year 1) and 1999-2000 (year 2). In each experiment, roots were wounded by four methods (puncture, bruise, broken, and scrape), inoculated with freshly harvested spores from 4- to 10-day-old cultures, and compared with a nonwounded but inoculated control for their susceptibility to decay. Roots were totally resistant to infection after harvest for 60 days in year 1 and 30 days in year 2. The bruise wound type was most commonly associated with infection, with disease incidence peaking 100 and 175 days after harvest in years 1 and 2, respectively. Inoculation by the puncture method followed a similar pattern in Hernandez but was ineffective in Beauregard. Following the peak in disease incidence at 100 or 175 days, susceptibility of roots declined to levels comparable to that of freshly harvested roots. This period of heightened susceptibility was longer in Hernandez than in Beauregard. The effects of injury types broken and scrape were more variable and did not show the same trend in both years.