Interaction between Chemical-induced Stress and Biological Agents on the Incidence of Sweetpotato Tip Rot in Mississippi

Sweetpotato tip rot or restricted end rot is a disease/disorder with unknown etiology that has been observed in recent years.  Tip rot is manifested as a small sunken and irregular lesion at or close to the proximal end of the storage root that appears after 2-4 weeks in storage.  The lesion is usually shallow and progresses onto the root asymmetrically, but sometimes extends into the interior of the root.  Tip rot is distinct from unrestricted end rot since the latter is usually deep (spans the storage root) and progresses onto the storage root symmetrically. Several pathogenic (Fusarium spp., and Macrophomina) and non-pathogenic microorganisms have been isolated from symptomatic storage roots in Mississippi, however, stress-inducing factors such as ethephon appear to be involved to some extent in predisposing and/or enhancing the syndrome.  Field studies were conducted to determine the interaction of biological agents with preharvest application of defoliants (ethephon and paraquat) on tip rot incidence.  Plots were planted with sweetpotato slips that were dipped in biological agents and water as control. A week before harvest, plots were sprayed with ethephon or paraquat for defoliation prior to harvest. Control plots were mechanically defoliated. Storage roots were harvested and after 2 months in storage (60^oF) were evaluated for rot incidence.  Tip rot incidence was less than 1% in all treatments that were mechanically defoliated prior to harvest.  In contrast, tip rot incidence increased for all biological agents and water control when ethephon was applied prior to harvest. Tip rot incidence also increased with paraquat applications except for the untreated water control. Results indicate that biological agents were unable to suppress this disorder and support previous studies suggesting that induced stress is a factor in predisposing and/or enhancing tip rot incidence.