Postharvest storage rots in Ontario sweet potatoes

Brian Collins, MSc Student, University of Guelph; Melanie Filotas, Specialty Crops IPM Specialist/OMAF and MRA

From ONvegetables in The Grower, June 2013

Sweet potatoes are susceptible to a variety of postharvest storage rots.  While a number of different bacterial and fungal pathogens can cause storage rots in sweet potatoes, over the last several years fungal pathogens in the genus Rhizopus and Fusarium have been the most common causes of postharvest storage losses in Ontario sweet potatoes.

Rhizopus soft rot

Rhizopus soft rot, caused by the fungus Rhizopus stolonifer, is one of the most common postharvest diseases of sweet potatoes in North America.  R. stolinfer is extremely common in air and soils, and enters sweet potato tissues through wounds, such as those occurring during harvest and packing.  Sweet potatoes with Rhizopus soft rot develop a white, hairy fungal growth (Figure 1) which produces very large quantities of dusty black spores (Figure 2).  A ring rot can also occur when the fungus infects the middle portion of the root.  Infected tissue often has a pronounced, sweet odour, which often attracts fruit flies. This fungus can spread rapidly, and a soft wet decay can spread over the entire root within three days under storage conditions (13⁰C).  Wet, cool soil at harvest can make sweet potatoes more susceptible to this disease. Sweet potato cultivars can vary considerably in susceptibility to Rhizopus soft rots, with the white fleshed cultivars like O’Henry often being more susceptible than orange-fleshed cultivars. However, even resistant cultivars like Beauregard can suffer losses to Rhizopus under the right conditions (e.g. cool, wet weather at harvest and injury to roots during harvest or packing).

Fusarium root and surface rots

Sweet potatoes are susceptible to several different diseases caused by Fusarium species, including Fusarium root rot, surface rot and stem canker.  While Fusarium stem canker affects plants in the field, root rot and surface rot are predominately post-harvest diseases and are significant storage diseases of sweet potatoes in many sweet potato growing regions.

Fusarium root rot, caused by the fungus Fusarium solani, causes light and dark brown circular concentric lesions on the skin of sweet potatoes, which may coalesce and overlap over time.  Fusarium root rot is generally dry and infected roots remain firm.  Symptoms often originate from the end of the root and thus this disease has also been referred to as Fusarium end rot.  Upon cross-section of severely infected roots, discolouration varies from light to dark brown (Figure 3) with lens-shaped cavities that often have white fungal mycelium growing inside.  If infected sweet potatoes are stored in a humid environment, a white fungal growth can develop on the exterior of the roots near the infection site (Figure 4).

Fusarium surface rot is caused by the fungus Fusarium oxysporum and by some strains of F. solani. Symptoms appear as circular light brown, firm dry lesions.  Lesions are generally superficial and often occur at points of injury to the root.  Early symptoms of root rot can be easily confused with surface rot, as external symptoms are very similar.  The differentiating feature is that surface rot does not colonize the inner tissue of the root and symptoms are restricted to the outer layers of tissue.

Both species of Fusarium are persistent soilborne pathogens, and require wounds in the fleshy roots of sweet potatoes for infection to occur.  These wounds are most often caused by mechanical harvesting, leading to the more common storage rots.  However rots can occasionally occur in the field through wounds caused by insects, nematodes or rodents, and through growth cracks.  This pathogen will not spread between roots in storage unless new wounds occur.  However, F. solani can spread from seed roots to sprouts, leading to Fusarium stem canker in the resulting slips.  Sweet potatoes with minor F. solani infection can appear healthy but if used as parent material the pathogen can be transferred to the sprouts.

Management:

All of the fungi discussed here are widespread in the environment, and are likely present in all sweet potato fields.  However since they can only enter sweet potatoes through wounds in the skin, the best way to manage these diseases is through sanitation and proper handling of roots to minimize entry points for these pathogens.  Specific management techniques include:

• Careful handling of roots at harvest to minimize injuries to the skin.
• Completing harvest early to avoid chilling temperatures (prolonged exposure to 10-12⁰C or less), which can predispose roots to storage rot pathogens.
• If possible, avoid harvesting from wet soil. Extremely dry conditions can also affect disease incidence by increasing the rate of skinning, thus creating more sites for infection.
• Properly cure roots as soon as possible after harvest to heal any wounds that do occur during harvest.
• Ensure the entire storage facility has adequate air flow to maintain appropriate temperatures (13-16⁰C) and humidity levels throughout.  In some cases, the storage rots have been reported to be more severe at the bottom of solid containers or along outer walls, where there is inadequate heating or air flow.
• Remember that wounding of roots at the packing stage can also lead to development of Rhizopus soft rot after sweet potatoes have left the storage facility.  Gentle handling during packing and minimizing/cushioning any drops along the packing line can help reduce incidence of the disease.
• The fungicides Scholar (fludioxonil) and Bio-Save (the biofungicide Pseudomonas syringae) are registered for the control of Rhizopus soft rot on sweet potatoes in Ontario.  There are no fungicides registered for control of postharvest Fusarium diseases in sweet potatoes in Canada.