Sweetpotatoes and Scurf

Each year some sweetpotato scurf or “soil-stain” is reported.  However, this year it would appear that the disease has been more prevalent than normal in some fields.  Scurf is caused by a soil-dwelling fungus, Monilochaetes infuscans, and results in darkened, discolored skin (Figure 1).  The affected portion of the root can range from a small patch to nearly the entire storage root surface.  The injury is considered by most to be superficial because the sweetpotato flesh below the stained skin surface is normal (Figure 2), and the sweetpotato maintains the same eating quality.  However, affected roots are considered unacceptable for the fresh market.

Figure 1. Sweetpotato storage roots with varying degrees of scurf.

Figure 2. Scurf injury is not evident below the skin surface or in the flesh of the storage root.

Here is what you need to know:

• The fungus associated with the disease has a narrow host range- namely sweetpotatoes and morningglories.
• The fungus can typically survive 1-2 years in the soil.
• The fungus is primarily spread on plant material- infected roots that are bedded for seed, for example.
• The pathogen can spread from infected roots to adjacent healthy roots when stored under conditions of high humidity.
• In storage, scurf-infected sweetpotatoes lose water faster than healthy sweetpotatoes.

How to manage scurf:

• Use scurf-free sweetpotatoes for seed.
• Treat seed roots with Mertect and/or Botran before or at plant bed establishment.
• Establish plant beds in fields with a scurf-free history.
• Cut slips at least 1” above the soil surface.  Pulling slips is a good way to move this and other pathogens into the production field and should be avoided.
• In problematic fields, avoid planting sweetpotato for at least 2 years.  Rotating out of sweetpotato for up to 4 years may be necessary if the pathogen is well-established.
• Control morningglory weeds, especially in rotational crops, to avoid providing the pathogen with a host in non-sweetpotato production years.
• When possible, avoid storing scurf-invested sweetpotatoes in bins of healthy sweetpotatoes.