Tip rot, or restricted end rot, is a sweetpotato disease/disorder with unknown etiology that has caused recent problems in Mississippi and possibly other production areas. Symptoms appear after 2 to 4 weeks in storage resulting in significant losses. In general, postharvest rots reduce pack-out efficiency, which affects profitability, but the internal necrosis associated with tip rot can go unnoticed until consumer use, which affects quality perception, and therefore, diminishes demand. Pathogen isolations have been inconsistent with this disorder: different pathogens and nonpathogens were isolated while in some cases nothing was isolated from lesions (da Silva and Clark, 2012; Stokes et al., 2012). Therefore, a relationship with stress has been suggested. Ethylene is produced in plants under stress/injury and a previous study has indicated that preharvest foliar applications of ethephon enhance the incidence of a similar disorder in North Carolina (Dittmar et al., 2010). This suggests that ethephon-induced stress may be influencing the incidence of this disorder.
Sweetpotato is one of the leading vegetable crops grown in Mississippi with over 24,000 acres planted in 2011 (U.S. Department of Agriculture, 2012). However, profitability can be affected significantly with 20% to 25% postharvest losses because of diseases, shrinkage, and loss of quality (Edmunds et al., 2008). Postharvest deterioration is usually the most economically costly stage for growers to lose marketable product because all the expenses of production, harvesting, and storage have already been incurred. In Mississippi, pack-out losses attributed to tip/end rots have increased dramatically during the last 6 years contributing to significant reductions in production efficiency and profitability (B. Graves, personal communication). In a survey of 15 packinghouses, losses due to tip/end rots were estimated at 6% to 59% (W. Burdine, unpublished data). Therefore, understanding the factors influencing tip/end rot incidence is necessary to develop proper management strategies.
Tip rot is manifested at or close to the proximal end of the storage root as a small sunken and irregular lesion delimited by a sharp border (Fig. 1). The lesion is usually shallow and progresses onto storage roots asymmetrically, but in some cases necrosis expands internally with little or no external symptoms. Tip rot is distinct from other nonrestricted or moderately aggressive fungal end rots caused by known pathogens (Clark, 1992; Clark et al., 2013) that are referred to as end rots in this study (Fig. 1). The margin of the lesion of nonrestricted end rots are not sharply defined, and the lesion is usually deep (spans the storage root) and progresses onto the storage root symmetrically.
Tip rot incidence in Mississippi has been observed more frequently when dry conditions prevail before harvest. There is also evidence that preharvest applications of ethephon and other defoliants used in potato (Solanum tuberosum) induce physiological changes in sweetpotato storage roots resulting in skin set and reduced skinning at harvest (Schultheis et al., 2000). Since ethylene plays an important role in stress responses and induces senescence in plants (Abeles et al., 1992), we hypothesize that preharvest applications of ethephon triggers physiological responses that may result in the tip rot disorder. Therefore, the objective of this study is to determine whether there is an association of tip rot incidence in sweetpotato with ethephon-induced stress before harvest.
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