Rhizophus 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 grayish black fungal growth. This feature distinguishes Rhizopus soft rot from other storage rots.
• The color 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.