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Fry Farmers Need Good Disease Control

“It can be healthy one day, you’ll come back a couple days later and it’s completely dead.”


Colour is quality in a french fry processor’s eyes.

Their customers want an attractive, golden-yellow fry without any mottling, and most importantly, no sugar ends.

A sugar end refers to a fry that has a higher concentration of sugar in one end that therefore fries up darker.

When a customer at a quick-serve restaurant gets these fries in their cardboard sleeve they stick out like a sore thumb and leave a terrible first impression. Likewise, strips that are entirely darker than the others also leave a bad impression.

Neil Gudmestad, a plant pathologist specializing in potatoes at North Dakota State University, says the issues affecting fry colour in Manitoba and North Dakota are largely disease related and in many cases manageable.

“There are other sectors of the marketplace and other parts of the world with a lot more issues affecting fry colour,” he told the recent Manitoba Potato Production Days meeting in Brandon.

But he also shared the words of another potato specialist that he’s worked with in the past: “He told me ‘There’s no limit to the multitude of diseases that can befall the potato plant’ – and that’s almost true.”

Gudmestad says the common diseases in the region share a couple of key traits. They can seem like minor ailments to the table market, for example, but translate into a much greater effect on fry colour in the processing plant. Many of them are also fungal infections that can spread in storage, damaging potatoes that went in healthy.


Gudmestad says growers should be aware of the major diseases in their area – and the management strategies to limit their damage.

Black dot is one of the toughest diseases for Manitoba growers.

It’s easy to mistake for other diseases, such as late blight when it attacks the foliage in the field, or the blemish disease silver scurf when it hits the tuber itself. It can also move extremely quickly.

“It can be healthy one day, you’ll come back a couple days later and it’s completely dead,” Gudmestad says.

Black dot infects the roots of the potato causing them to rot and it is also capable of invading the vascular tissue causing wilt. Black dot is most devastating when it infects potatoes in conjunction with another organism such verticillium or fusarium.

Black dots (black sclerotia) form on the stems injuring them and giving the disease its name.

“When black dot is present, the plant wilts and dies much more rapidly than with other diseases causing severe economic losses,” Gudmestad says.

Weather conditions are key to the development and survival of the disease. According to Gudmestad, soil saturation prior to row closure promotes high levels of the pathogen to germinate and infect.


This can occur from heavy precipitation or irrigation. Later in the season, higher air temperatures cause the plant to collapse more rapidly, cutting production.

Gudmestad says infections can occur very early in the season – as quickly as a week or two after emergence, which makes early fungicide applications very important.

“You want to be making that first fungicide application – especially the strobulurins – when the canopy is very small,” he told growers. “Controlling early infections is most important, especially in that seven-to 21-day period (after emergence).”

Applications should continue about every 14 days thereafter, and include alternating a fungicide with a different chemistry to prevent the development of fungicide-resistant infections.

Other management measures include avoiding heavily infected seed, maintaining crop residue on sandy soil to prevent injury to the plants from blowing soil that could facilitate infections, avoiding crop stress like low nitrogen or over -and underwatering.


Another class of diseases that Gudmestad hit upon were the various disorders and complexes such as pink eye, net necrosis, and early-dying syndrome.

“They can be anywhere from moderate to severe, and most are related to some sort of stress (to the crop) in season,” Gudmestad told the meeting.

One major headache for the processing industry is pink eye, which causes small infections around the eye of the potato that appears when the tuber is dug. It doesn’t affect the edibility of the potato, but that’s about all you can say for it.

“It gives (french fry) plants fits – it can be difficult to peel and process and provides a wonderful infection site for other things that want to infect the potato in the storage.”

There’s no clear-cut cause for pink eye, but most research points to a link between moisture and soil temperatures. This means a bit of a negative feedback loop can be created when various production issues hurt the crop in season.

“You’ll see a situation where dying spots are not using water, and the canopy is dying so the soil is warming up,” Gudmestad says.

Pink eye also seems to hit the industry standard Russet Burbank variety hard, and newer varieties that are now available in North Dakota and Minnesota are showing greater resistance.

“I’ve never seen pinkeye in any of these varieties,” Gudmestad says.


Plain old heat and water stress can also have a major influence on fry colour – and the effects can be impossible to deal with later.

“It’s the most important,” Gudmestad says. “This is the biggest cause of dark fry ends.”

There are a number of theories why it happens, but Gudmestad says the bottom line is that any heat and water stress early in the bulking process seems to equal lots of dark fry ends when the crop is harvested.

“The damage is fixed. It happens early in the season and it is irreversible,” he says.

Most of the research shows that it’s not just low moisture that will do it, but it takes the addition of heat to really cause a wreck and cause severe sugar end problems. That means keeping soils moist early in the season can help prevent it from happening.

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About the author


Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.



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