Consumers are discovering the joys of “specialty potatoes.” These potato varieties are available in a spectrum of skin and flesh colors and promise a visual and culinary adventure. With these potatoes, appearance is extremely important. Unfortunately, most of these new varieties are susceptible to skin blemishes due to infection by the silver scurf fungus.
Silver scurf is caused by a fungus called Helminthosporium solani that infects and spreads in the periderm (skin) of the potato. There it produces irregular silvery patches on the tuber’s surface that vary from pinhead size up to patches that cover most of the tuber surface. These lesions usually remain superficial with no internal damage. In most instances, the disease is not visible until the crop has been stored for several months. By the time you see it, spread of disease can be pretty extensive.
The scurf fungus moves from infected seed pieces to daughter tubers while potatoes are still in the soil. Exactly when these infections take place is unknown, but daughter tubers have shown evidence of the disease as early as six weeks after planting. Just prior to harvest, scurf spread becomes more extensive in the maturing periderm.
The stage of the disease that occurs on tubers while they are still in the ground is referred to as the primary infection, which results in primary lesions. These lesions are the fairly thick and prominent patches that are usually more severe on the stem end of the tuber. Field infections where there are no visible symptoms also occur regularly.
The primary lesions are one source of fungal spores for secondary spread of the fungus from infected to healthy potatoes within the storage facility. There are other sources. Research at the University of Idaho indicates that the silver scurf fungus can survive from the end of one storage season to the beginning of the next on materials such as wood, sheet metal, insulation (polyurethane), and even in soil from the cellar floor — all of which are present to some extent in every storage facility.
This “storage” cycle of infection leads to “secondary lesions,” which are usually less severe than the primary lesions but can still cause problems, especially on small tubers where any blemish is noticeable.
Recommended Control Measures
• Seed can be a significant source of silver scurf inoculum, so it is important to plant certified seed. Careful attention to seed production practices can help reduce the potential for scurf development in the seed crop. Storing early generation seed lots separately from older generations can reduce the amount of spread from older into younger generation materials. Planting seed that does not have obvious signs of silver scurf may turn out to be an important method of disease reduction, but remember that the disease may be present and not visible. Seed piece treatments can reduce silver scurf in the harvested crop.
• While this is not a common practice, do not plant potatoes into fields that had silver scurf the previous year. We do not know for certain how long the fungus can survive in soil, but recent reports indicate that it may be able to survive on the debris of cereals and alfalfa.
• The longer tubers remain in the soil after maturation, the more likely they are to become infected if the disease organism is present. Harvest tubers as soon after maturation as possible to limit the time the tubers may be exposed to the pathogen. If potatoes have visible signs of silver scurf, they may need to be marketed early before the disease has a chance to progress further.
• Before loading your storage, the facility should be cleaned and disinfected thoroughly. Provide adequate time for disinfecting, make sure to wet the storage surfaces well, and close the storage doors for a day or two. Then open and dry out the storage before loading potatoes. Once the potatoes are in storage, avoid conditions that may lead to condensation or free moisture inside the storage. This is especially important during the first three weeks or so in the storage.
• Monitor your stored crop frequently to see if a scurf problem is developing. Early marketing, while not the most desirable alternative, may at least allow the producer to get a decent price for his crop.
Specialty potatoes are important to American consumers. Careful attention to detail at all stages of production can help keep silver scurf to a minimum and allow you to keep these attractive new varieties as beautiful as they can be.