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Managing Fusarium Dry Rot in Potatoes

From planting to storage, fusarium can affect tubers at every stage of the production cycle. Here are some tips for protecting your potatoes.

Fusarium is a major problem for potato growers. The fungi, says Alberta Agriculture plant pathologist Michael Harding, can affect tubers all the way through the production cycle, causing seed piece decay, wilt during the growing season, and dry rot in storage. There are ways, however, to mitigate risk, and should that fail, ways to manage the disease if it does occur.

“Fusarium is a really important fungal genus for potato diseases,” says Harding. “It will affect potatoes all the way through, and all of those steps are quite connected. If you only try to manage fusarium at harvest, you could end up having a real struggle because of all of the problems that come up if you have fusarium on your seed and you don’t manage it in-season.”

Avoiding fusarium starts with good crop rotation and sourcing clean seed, says Harding. Be sure to clean and disinfect cutting equipment between handling lots, he warns, and promote rapid
healing of cut seed. Finally, he advises growers not to store seed longer than necessary, and to apply a registered seed piece treatment. “Then they’re much more likely to not exacerbate a fusarium problem,” Harding says.

During the growing season, Harding says growers should avoid over-irrigation. Vine kill is important, too, he says, as allowing vines to die prior to harvest will help the skins to set, a natural defense against infection.

Potato Skin a Natural Barrier

“We tend to talk a lot about chemical products or chemistries that can help manage fusarium dry rot in storage, but really the potato is very capable of fending off the disease on its own with its skin,” says Harding. “Fusarium species don’t have the capability to directly penetrate a healthy potato skin.”

Since the skin acts as a barrier, anything the grower can do to avoid opening up that barrier will go a long way in protecting the tuber from disease in storage, Harding says. He adds harvest should only be done under the right conditions, ideally when it’s not too hot, too cold or too wet.

“If the tuber core temperatures are higher than 10 C, the skins can be very delicate,” he says. “And if the tubers are too cold, then they’re very prone to bruising and shatter-cracking.”

Harding also advises growers to wait to harvest potatoes until vine kill occurs. The reason this is important, he says, is so that tubers don’t get wounded as they are yanked off the plant stolons.

During harvest, Harding continues, if the harvester is moving too quickly or equipment isn’t properly set up to avoid large drops, excessive bouncing or shaking, tubers can get bruised and their skins can get damaged.

“That’s really one of the primary ways that potatoes get primed for fusarium dry rot — is during the harvest time where there’s all the handling of the tuber,” says Harding. “The more that can be done to minimize damage to the skin at harvest, that’s really going to protect the potatoes in storage from dry rot.”

Potatoes exposed to sunlight over long periods of time can experience wear that can lead to fusarium issues as well, says Harding. If that happens, growers will be looking to post-harvest fungicides to knock back a potential fusarium problem in storage.

Rick Peters is a vegetable pathology research scientist with Agriculture and Agri-Food Canada. He seconds Harding’s comments on how important it is to manage fusarium dry rot right from the beginning. “Things really do get going right at planting,” he says, “and what you do at that time will dictate what happens at harvest as well.

“In the case of fusarium, a lot of the inoculum is from contaminated or infected seed,” he continues. “Those spores then contaminate the soil around the developing daughter tubers, so that at harvest the inoculum is there to infect those daughter tubers as they’re harvested and enter storage. It really does start right at planting.”

Even when growers heed the advice of plant pathologists and manage for disease throughout the plant’s growth cycle, fusarium is still a possibility. “You do the best you can, but obviously you can’t control everything,” says Peters.

Increasing Fungicide Resistance

Growers do have access to products that help control fusarium, although in recent years they’ve seen increased resistance to some. According to Peters, fusarium strains have been shown as of late to be resistant to thiabendazolem fungicide.

Growers now have access to a new post-harvest product, Stadium, which uses a combination of three active ingredients: azoxystrobin, fludioxonil, and difenoconazole. According to Peters, difenoconazole has been shown to be very effective against resistant strains of fusarium. “Stadium will offer a new option for growers to combat some of these resistant strains,” he says.

As always, there are cautions that come with using products that work, says Peters. To avoid fungicide resistance, do not apply back-to-back treatments with site-specific fungicides or with a medium- to high-risk resistance. When disease pressure is high, avoid using site-specific fungicides altogether. Finally, rotate applications between different mode-of-action groups and use formulated mixtures where both mixing partners are active against the targeted pathogen.

“That’s one of the reasons that a three-product mixture can be effective,” Peters says. “One way of delaying resistance development is by combating a pathogen on a number of different fronts, so having product mixtures can be one way of helping with resistance management issues.”

“There is a supposition there that if you’re going to be using it at planting, then you shouldn’t use it at harvest time,” Peters continues. “The more times that it’s in the mix, the more risk there is of resistance developing.”

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