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Varying degrees of silver scurf infection on potatoes.

Controlling Silver Scurf

This common disease that causes unsightly blemishes on potatoes can be a headache for fresh market producers. What options are available to growers for controlling silver scurf?

One of the more pervasive tuber diseases in North America, silver scurf has been increasing in incidence and severity in recent decades. This has resulted in economic losses that have been most clearly felt in the fresh sector, where unsightly potato skins can lower the market value of the crop.

Tracy Shinners-Carnelley is the director of research and quality enhancement for Manitoba’s Peak of the Market — a grower-owned not-for-profit vegetable marketing agency. She spoke about silver scurf and the fresh market experience at Manitoba Potato Production Days held in Brandon, Man. earlier this year, providing some valuable tips for growers looking for ways to battle the disease.

What is Silver Scurf?

Caused by a fungal pathogen known as Helminthosporium solani, silver scurf causes skin blemishes on potatoes that are tan to silver in appearance. These well-defined lesions can expand and join together as they spread across the tuber surface.

According to Shinners-Carnelley, silver scurf is one of those potato diseases that growers often don’t know is there until the blemished tubers come out of the storage. It’s also considered a superficial disease in that the fungus is usually confined to the epidermal layer of the tuber, although it can damage internal tissue that’s in direct contact with infected skins.

The skins of scurf-infected potatoes may also thicken and crack after some time in storage, causing internal tissues to lose water and shrivel.

“In those very severe situations, you can see it having a greater impact on quality,” says Shinners-Carnelley. Usually, though, silver scurf doesn’t affect a potato’s edibility much, provided the damaged portions are cut away before cooking.

Why then is silver scurf a cause for concern? Producers for the fresh market know that a potato’s appearance is a major selling feature, so anything that dampens consumer appetites obviously needs to be avoided.

These days, smooth and thinner-skinned yellow and red varieties of potato tend to dominate the produce aisles, and it’s on these kinds of cultivars that silver scurf tends to show up the most and inflict the most economic damage. For example, it’s hard not to detect silver scurf on a yellow-skinned potato after it’s infected, especially once it’s been washed.

According to Shinners-Carnelley, customer expectations are very high these days — and one of the assumptions is that potatoes they buy will be attractive and relatively blemish-free. “So silver scurf does have the risk of being a very important disease for the fresh industry,” she says.

Silver scurf does have the risk of being a very important disease for the fresh industry.

– Tracy Shinners-Carnelley

Control Measures

Prevention is the key to controlling silver scurf, since once a potato has been infected, it can’t be cured. With this in mind, a sensible approach is to start with the seed, since the primary source of this disease in potatoes is from other infected tubers.

“It’s important to use seed that is relatively free from silver scurf,” says Shinners-Carnelley. “When an infected seed tuber is planted, the spores will form on the surface of that seed. Then throughout the season, they’ll move down through the soil either by rain or irrigation water to get into that zone where the daughter tubers are developing, or they can also grow down the stolons to come in contact with the daughter tubers.

“The infection then takes place, and it isn’t necessarily that obvious at harvest time unless you look really, really carefully at the tubers,” she adds. “You don’t really tend to see the impact of silver scurf until [it’s] given some time in storage.”

It’s possible that seed from some sources may be contaminated with silver scurf. It’s useful, therefore, to keep an eye out for signs of disease when sorting seed pieces, throwing out tubers with significant lesions.

Applying a chemical seed piece treatment that prevents ungerminated scurf spores from becoming active is another important control option. Though seed treatments have been shown to be effective at reducing silver scurf, they will not completely prevent infection of daughter tubers, particularly if the seed infection is severe.

The silver scurf pathogen can also live in the soil for up to two years, so the optimal way to break up the life cycle of H. solani is to rotate crops. “If you are practicing a good three-year crop rotation with two years between potatoes, you shouldn’t see soilborne spores as a real source of inoculum for silver scurf,” says Shinners-Carnelley.

If you are practicing a good three-year crop rotation with two years between potatoes, you shouldn’t see soilborne spores as a real source of inoculum for silver scurf.

– Tracy Shinners-Carnelley

Limiting Inoculum

While silver scurf can develop on potatoes below ground while the plants are growing, it’s at harvest time and afterwards that the real trouble usually starts.

Shinners-Carnelley stresses the importance of minimizing the amount of time that potatoes spend in the field after vine death and skin set, since silver scurf severity and damage increase the longer tubers are left in the ground.

“If tubers are sitting there and the tops have gone down, this is the window of time when the infection risk of silver scurf really increases. And if that happens in the field, it means you’re bringing a higher level of disease into the bin,” she says.

“Once inside that storage, you can have many disease cycles occur, and as more occur, of course the overall amount of silver scurf that you see will continue to increase,” Shinners-Carnelley adds.

“The reality is the conditions in the storage can be very conducive to having the disease develop, so really you need to focus on other management strategies that help limit the amount of inoculum that’s going into the bin.”

Shinners-Carnelley says one such strategy is reducing the amount of soil going into storage. “When you’re bringing potatoes [into] the bin, you’re also potentially bringing in a lot of soil, and in a scenario like that, there can be spores within that soil that can also be the source of inoculum.”

Jeanette Gaultier from Manitoba Agriculture, Food and Rural Development collaborated on the phosphorus acid storage study led by Tracy Shinners-Carnelley in 2009-10. Photo: Tracy Shinners-Carnelley.

Storage Conditions

When infected tubers are put into storage, the silver scurf lesions sporulate, producing inoculum for secondary infections that can, given the right conditions, spread quickly throughout the bin. According to Shinners-Carnelley, H. solani does best when relative humidity is above 90 per cent and temperatures are above 3° C.

“The same fundamental principals are in place whether it’s in the field or it’s in the bin. Your combination of temperature and moisture and inoculum is the recipe for disease, but we often have very little control over the amount of moisture and temperature,” she says, noting that growers often don’t have a great deal of latitude to be able to manipulate such factors in storage.

“We know that there are trade-offs when that happens. It’s impossible to really give specific parameters … [but] generally speaking, the cooler you can store your bin and the lower the humidity, this is going to help reduce disease development,” says Shinners-Carnelley, adding that this strategy for disease management must be balanced with maintaining the end use quality of the potatoes.

Shinners-Carnelley notes that once potatoes start getting shipped out of storage, this elevates the silver scurf risk even further through the movement and disruption of tuber piles, which causes spores to spread.

“You get these secondary infections that come with that,” she says, which can explain why people are so often surprised by the amount of silver scurf they see in the pile compared to what they observed going into storage.

“The amount of inoculum, the storage conditions and the time in storage are really going to determine the level of disease that can develop and what will impact the crop,” Shinners-Carnelley says.

Silver scurf spores can hide away in infested surfaces like concrete and wood within storage facilities. To help prevent the disease showing from one storage season to the next, a sensible precaution is making sure the bin is as clean and disease-free as possible before the next batch of potatoes goes in.

“It’s always best practice to be doing a good job of cleaning and disinfecting your storage bins between crops,” says Shinners-Carnelley.

Phosphorus Acid

Among the newer weapons against the disease emerging in recent years are phosphorous acid-based fungicides for control of late blight and pink rot that have also demonstrated to be useful in suppressing silver scurf. Research has shown that the post-harvest application of a phosphorous acid product can be an effective treatment measure for controlling the disease in storage, particularly when infection levels are low to moderate.

Shinners-Carnelley says she participated in storage trials that tested the efficacy of phosphorus acid products for silver scurf suppression back in 2009-10. “That was about the time there was a lot of interest in phosphorus acid, and what a great chemistry it could be to help with post-harvest issues,” she says.

The testing showed that a yellow potato variety treated with two phosphorous acid products, Confine (which is currently registered for silver scurf suppression in potatoes in Canada) and Phostrol (which is currently not registered for silver scurf in Canada), had significantly less silver scurf than untreated control potatoes.

According to Shinners-Carnelley, post-harvest phosphorus acid applications have become standard practice for many growers catering to the fresh market. “It’s had a significant impact on reducing the level of silver scurf that we see,” she says.

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