2018 ASHS Annual Conference

The North Carolina sweetpotato industry has been concerned with a physiological disorder internal necrosis (IN) that can occur at high levels in its predominantly grown cultivar Covington. IN symptoms of brown and black areas in the root cortex can only be discovered when the root is thinly sliced at the proximal end of the root. Certain cultivars like Covington are more prone to IN, but various factors that cause this problem continue to be investigated. The goal of our research was to evaluate several preharvest and postharvest conditions and how it affected occurrence of IN. Three replicated field studies in 2016 and two in 2017 were conducted. Application of a high chlorine versus minimal chlorine fertilizers and mowing versus not mowing prior to harvest were the four preharvest treatment combinations evaluated. For the postharvest treatments, 30 roots were obtained from each preharvest plot and placed in 75ºF and 82ºF rooms in 2016, with the addition of a 95ºF room in 2017. Fresh harvested sweetpotatoes were stored in these rooms for duration after harvest of ½, 1, 2, 3 & 5 weeks in 2016, and for up to 2 weeks in 2017, then placed in a 58ºF storage room. A control treatment was included where roots were not cured and placed immediately after harvest in a 58ºF storage room. Samples were cut approximately 90 days after harvest and IN incidence and severity recorded. Data were statistically analyzed with SAS Mixed procedure, comparing main effects (preharvest & postharvest) and its interactions. Preharvest treatments showed no differences in the main effects but were significant when combined with postharvest treatments. No mow treatments showed higher incidence than Mowed treatments as temperatures and duration increased in one of the three 2016 studies, increasing from 4% to 6% and 11% when cured for ½, 1 and 2 weeks, respectively, then leveling off at 3 & 5 weeks. The no mow treatment showed higher incidence when cured at 85ºF (11%) vs 75ºF (6%) versus the mow treatment (85ºF (6%) vs 75ºF (5%), which has similar IN incidence. The same trend was observed in 2017 studies in both locations. The no mow treatments showed higher incidence when curing duration increased from ½ to 2 weeks (6% to 18% in location 1 and 3% to 32% in location 2) while the mowed treatment increased less (3% to 11% in location 1 and 3% to 14% in location 2. When no mow treatment was combined with temperature in 2017 studies, incidence was higher at 85ºF (21% and 22%, for locations 1 and 2, respectively) vs 75ºF (11% and 10%,for locations 1 and 2, respectively) than the interaction for mow treatment at 85ºF (11% and 10%, for locations 1 and 2, respectively), versus 75ºF (3% and 8%, respectively) for locations 1 and 2. Postharvest main effects were significant both years at all locations. Curing at 85ºF had higher incidence than the 75ºF, while the control (58ºF) treatment was as low as 0% the second year. Duration of curing treatment from ½ to 2 weeks is a key factor, increasing IN significantly at all locations both years.