Devastating new potato disease headed north

The disease, which causes brown stripes in potatoes, 
has spread like wildfire since being found in Texas in 2000

Manitoba growers haven’t seen zebra chip disease yet — but it’s only a matter of time, says John Nordgaard.

The transplanted North Dakotan manages a potato operation for Black Gold Farms near Piersall, Texas, which he describes as one of the epicentres for the disease, which results in striped chips that look like a white and brown zebra.

The disease was first observed in Mexico in 1994, in Texas in 2000, and from there it spread like wildfire, he told growers at the annual Manitoba Potato Production Days.

“Within three years it was in New Mexico, Nevada, Arizona, California, Nebraska, Colorado, Kansas and Wyoming,” Nordgaard said.

In 2011, it was found in the biggest potato production states — Idaho, Oregon and Washington, which produce more potatoes annually than all the other states put together.

“It’s amazing to me how fast this went from an observation to making headlines,” Nordgaard said. “It became a national disease almost overnight, going from non-existent to being a national presence in just 10 years.”

The first signs of the disease are leaves that turn yellowish, with stunted plant tops and purple tips.

“Initially this disease was thought to be purple top,” Nord-gaard said.

Telltale signs

Another telltale sign is swelling at the plant nodes, with branching out of them. It also causes dead spots in fields with 100 per cent kill.

“I can say from experience that when you see that, a very large number of the tubers below are infected,” he said.

And it’s the tuber infections that are really going to hurt the grower. This is where the dark stripe-like symptoms appear in the tissue, likely a result of starch being converted to sugar. For fry and chip makers, it’s an unsightly cosmetic problem for an industry that’s all about first impressions, and even for baking potatoes it’s a big problem, Nordgaard said.

“There just isn’t enough sour cream to cover up the brown,” he said.

Researchers are still trying to discover the cause of the disease, but have found a clear correlation between the presence of the psyllid insect and outbreaks.

That’s not good news because those insects are found intermittently in all three Prairie provinces, blowing in on southerly winds. They’re also commonly found in the northern tier of the U.S. Great Plains, and appear to becoming established in the Pacific Northwest, all of which bolsters Nordgaard’s suggestion Manitoba will soon see the disease in its potato fields.

It’s suspected psyllid carries a pathogen called Candidatus Liberibacter solanacearum, a bacteria more commonly called LSO.

“LSO is a new pathogen, and it’s very difficult to study,” Nordgaard said. “Mainly because it can’t be cultured.”

The link to zebra chip was first identified in New Zealand, then confirmed in the U.S. and other countries. It’s made the New Zealand growers the front line in developing defences against the disease, and early reports aren’t good, said Nordgaard.

“They’re spending $500 to $600 an acre on insecticides and they’re still not stopping it,” he said.

Resistant varieties

Some varieties are proving to be more resistant than others, but again the news isn’t good for Manitoba growers, especially those with french fry contracts, Nordgaard said.

“Russet Burbank is very susceptible — we’ve shown that again and again,” he said. “Shepody is fairly susceptible too.”

The USDA is studying integrated pest management programs that might target psyllids, including bio-based products that don’t harm beneficial insect populations.

For example, one research team is doing lab and field tests on a clay-based powder product known as “kaolin particle film” which is mixed with water and sprayed on plant leaves. When it dries it forms a protective coating that disrupts feeding and egg laying by certain pests — and as an added benefit it appears to limit water losses, heat stress and sunburn damage.

Another study showing promise uses biopesticides based on ingredients from sources such as natural plant extracts, plant essential oils and mineral oil.

Beneficial fungi could also act as biopesticides and another research team is looking at two products in field trials in Texas. Both have significantly reduced the number of eggs and nymphs found on plants, and one product appears to outperform a commercially available chemical pesticide.

While all these tools will likely play a role in grappling with the zebra chip problem, the USDA still describes genetically resistant varieties as a “cornerstone defence” for this and other insect-related production issues. The most promising development so far is a potato clone that appears to have resistance to the LSO bacteria.

About the author

Editor

Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.

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