Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 27, 2001
Publication Date: April 1, 2002
Interpretive Summary: Consumers expect year-around availability of all produce. For sweetpotatoes, this is accomplished by digging the roots in Autumn and storing them for up to 10 months in pallet-sized field boxes. Prior to storage, roots are cured for several days at warm temperatures and high humidity. This prevents the most serious disease of sweetpotatoes, Rhizopus soft rot, by promoting the healing of injuries which occurred during harvest. The fungus invades only through fresh injuries. Roots are washed, "tailed" (snapping off the ends of roots), sorted, and packed just prior to shipment to market. Unfortunately numerous types of mechanical injuries occur when the stored roots are prepared for market, and the roots are again susceptible to Rhizopus soft rot. The incidence of disease observed at market is highly erratic, frustrating both growers and researchers alike. To provide some insight into this problem, we stored two cultivars of sweetpotatoes for 48 weeks in 1999 and 2000. Periodically some roots were removed and evaluated for susceptibility to Rhizopus soft rot. Uninjured (gently handled) roots were compared to roots injured by puncturing, bruising, scraping, or tailing. For bruised roots, there was a period of heightened susceptibility at 14 to 25 weeks in storage. This was also true for puncture-wounded roots of cultivar Hernandez. Decay of tailed roots was greatest in 1999 and Hernandez was more susceptible than Beauregard in both years. Scraping resulted in the least decay for injured roots. The interaction between time in storage and type of injury merits further study. For packers, the importance of gentle handling is reemphasized.
Susceptibility of stored sweetpotato roots (cvs. Beauregard and Hernandez) to Rhizopus soft rot 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 inoculated using four wound types (puncture, bruise, broken and scrape) and compared to a non- wounded, but inoculated control for their ability to cause decay. Roots were totally resistant to infection after harvest for 60 days in year one and 30 days in year two. The bruise wound type was the most successful for establishing infection, with disease incidence peaking at 100 and 175 days after harvest in years 1 and 2, respectively. Inoculation by the puncture method followed a similar trend in Hernandez, but was ineffective in Beauregard. Following the peak in disease incidence, susceptibility of roots declined to levels comparable to freshly harvested roots. This period of heightened susceptibility was longer in Hernanadez than in Beauregard. The effects of injury types "broken" and "scrape" were more variable and did not show the same trend in both years. Freshly harvested spores from 4- to 10-day-old cultures were effective at inciting disease. Characterization of changes in root physiology during storage are needed to elucidate the mechanism behind observed changes in susceptibility and to help predict when roots would be prone to decay.