Scurf in Sweet Potatoes
Scurf is a common disease of sweet potatoes that has been reported from most regions where the crop is grown, including Ontario. Formerly known as soilstain, scurf is caused by a soil borne fungus which attacks the skin and outer layers of sweet potato roots. This fungus, Monilochaetes infuscans, is a different pathogen than the one causing scurf of white potatoes and has a narrow host range, infecting only sweet potatoes and close relatives in the morning glory family.
Sweet potato roots with scurf have superficial, grayish- or purplish-brown to black lesions on the skin (Figure 1). These lesions slowly enlarge and can affect large portions of the root surface (Figure 2). Scurf lesions do not penetrate the inner portions of the root (Figure 3) and don't affect eating quality; however the cosmetic damage can affect marketability of affected sweet potatoes. Additionally, infected roots can lose water more quickly than healthy roots, leading to increased shrinkage in storage.
Scurf is spread primarily through infected planting material. If a scurf-infected root is used to produce slips, the fungus can spread from the root to the lower (below-ground) portion of the sprouts in the greenhouse or plant bed. The fungus can then spread from infected stems back down to the daughter roots after the slip is transplanted to the field. Disease development can occur at a range of temperatures, and is optimized at soil moistures that are also optimal for sweet potato growth (neither too high, nor too low). Lesions usually begin to develop on the roots in the field, and continue to enlarge in storage, particularly when relative humidity in the storage facility is high. While there is some evidence to suggest that the fungus can spread from infected roots to healthy ones in storage, most increase in scurf observed in storage is actually enlargement of existing lesions that formed in the field. The fungus can also persist in the soil for 1-2 years, with increased survival in heavier soils, or those with high levels of organic matter.
There are no fungicides registered in Canada for the control of scurf after it appears in the field or storage facility. Scurf is best managed by taking measures to prevent the disease before it appears. These include:
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