In a recent poll, potato people from across the United States were asked to rank what they thought the greatest challenges to the industry in the near future will be. One of the most pressing problems the group identified was the blemish diseases of potato, with silver scurf ranking among the highest. And no wonder, as the U.S. industry continues to expand the availability of "speciality varieties" with various colored skin and tuber flesh combinations, the importance of blemish diseases is rapidly becoming more evident. This article will discuss what we can do to manage silver scurf in specialty potatoes. As with any management strategy, the first order of business is to gain a better understanding of the problem.
Rise of Silver Scurf
First off, silver scurf (caused by the fungus Helminthosporium solani) is considered to be a seed-borne disease. There are some efforts underway currently to further investigate the potential for soil survival of the fungus but
The prevailing view as of right now is that this disease organism doesn't survive more than a single season in soil. This means that the seed you plant will very likely be the source of the silver scurf you see in your harvested crop.
That's only part of the story, however. While seed is probably the source, there is a far more important component of the silver scurf disease cycle that occurs within the storage facility after the crop has been harvested and put to bed.
Daughter tubers become infected in the field from fungal spores produced on the periderm of the seed piece. The actual mechanism for how this process occurs remains somewhat of a mystery, but the end result is that the daughter tubers have become infected before harvest-usually on the stem end. These stem-end patches of silver scurf are called "primary lesions." While you may or may not see them on freshly-harvested tubers, they usually become visible and/or more prominent after a month or so in storage. Primary lesions can be responsible for some cosmetic damage in their own right, but are much more important as a source of spores for a far more destructive phase of the disease: secondary spread in storage.
Spores produced in the primary lesions are released into the air stream of the storage facility and can then spread throughout the pile. Actual droplets of condensation on tubers may not be absolutely required for these spores to initiate new infection sites, but research has shown that the presence of moisture certainly can greatly exacerbate the problem. These new infection sites are referred to as "secondary lesions" and can be found anywhere on the tuber. Secondary lesions are usually smaller than primary lesions, but they cause a much greater problem due to their sheer numbers.
The keys to managing silver scurf involve a series of methods starting with the use of seed potatoes free of silver scurf. There are also seed piece treatments and in-furrow fungicides that can help reduce or eliminate spread from the seed piece to the daughter tubers. To avoid conditions that lead to condensation, careful attention to storage management can help reduce spread in storage.
At the present time, there are no post-harvest treatments that will reliably manage silver scurf over the long term in storage. Some suppression of the disease has been obtained with phosphorus acid applications, but not all formulations of this product are labeled for this use. Be sure to check the product label. There may be help coming just over the horizon, effective post harvest treatments are currently under development and should be available within the next year or so.