Introduction:

Sweet potatoes are not only a staple crop but also a beloved addition to diverse cuisines worldwide. However, like any crop, they are susceptible to various diseases that can significantly impact yield and quality. Effective disease management strategies are crucial to safeguarding sweet potato crops and ensuring sustainable production. In this comprehensive guide, we explore common diseases affecting sweet potatoes, their symptoms, preventive measures, and proactive management strategies. Whether you're a commercial grower or a home gardener, understanding these aspects can help you protect your sweet potato harvest and promote healthy, thriving plants.

Black Rot: Ceratocystis fimbriata

Symptoms:

Small, circular, slightly sunken, dark brown spots are the initial symptoms of black rot. Spots enlarge and appear greenish black to black when wet and greyish black when dry. Within the spots are small, black fungal structures (perithecia) with long necks which appear to the naked eye as dark bristles. The rot usually remains firm and shallow.

If secondary fungi or bacteria invade the tissue however, the flesh beneath the spot turns black, and this blackened area may extend to the centre of the root.

Tissue near the discoloured area may have a bitter taste. Eventually, the entire root may rot. Roots may appear healthy at harvest but rot in storage, during transit, or in the market.

Management:

Control black rot with crop rotation, since most crops are unaffected by the disease. Disinfect seedbeds if a clean site is unavailable. Propagate plants from healthy stem cuttings.

Cure roots immediately after harvest. (Cure roots at 85 to 95 degrees F and 85 to 90 percent relative humidity for 5 to 10 days.) Apply a postharvest fungicide. Do not wash and package roots showing symptoms of black rot. Decontaminate equipment that meets an infected crop. Spray empty washing machines and crates with a fungicide. Fumigate storage structures. 

Rhizopus soft rot: Rhizopus stolonifer

Symptoms:

Infection and decay commonly occur at one or both ends of the root, although infection occasionally begins elsewhere.

Rotting may be inhibited under dry conditions, but under humid conditions the affected sweet potatoes become soft and watery, and the entire root rots within a few days.

If the humidity is high, the sweet potatoes become heavily "whiskered" with a greyish black fungal growth. This feature distinguishes Rhizopus soft rot from other storage rots.

The colour of the root is not significantly altered, but an Odor is produced that attracts fruit flies to the area.

Infection is especially likely if the relative humidity is between 75 and 85 percent during storage or transport. Also, the longer roots are stored, the more susceptible they become.

Chilling and heat damage also predispose sweet potatoes to infection. Soft rot is very destructive when sweet potatoes are washed, packed, or shipped to market during cold weather.

Management:

Carefully handle sweet potatoes during harvest to prevent unnecessary wounding. This is the most important control method for soft rot.

Properly cure roots immediately after harvest.

Store roots at 55 to 60 degrees F.

Avoid handling stored roots because handling can create new wounds. Recuring is one possible solution to this problem.

Apply a recommended fungicide after harvest.

Do not allow sweet potatoes to be exposed to sunlight for extended periods (to prevent heat damage) or to be chilled in the field.

Summary

Sweet potato growers face two major storage diseases: Black Rot and Rhizopus Soft Rot. Both wreak havoc during storage and transport. To combat Black Rot, focus on preventive measures like crop rotation, using disease-free plant material, and proper curing. Fungicides can offer additional protection after harvest, and vigilant sanitation of storage areas is crucial. Rhizopus Soft Rot thrives on wounded and stressed roots. Careful handling during harvest is paramount, followed by proper curing and maintaining cool storage temperatures. Minimizing root disturbance and applying a wax coating can further protect the crop. By implementing these strategies, growers can significantly reduce storage losses from these destructive diseases.