Winter wheat in southern Alberta has been hit with stripe rust and the disease will likely spread to other wheat types.
The stripe rust fungus cannot survive temperatures below -10 C so it generally winters in the Pacific Northwest and blows into southern Alberta and the rest of the Prairies in mid-to late summer. It rarely causes serious problems as it doesn’t arrive until the wheat has finished flowering.
“Until this year I’d never seen stripe rust before the end of June,” said Agriculture Canada cereal pathologist, Denis Gaudet.
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“This year we found it on winter wheat on April 28. The pathogen overwintered here. That’s something new, something I’ve never heard of in 30 years of studying cereal disease at Lethbridge.”
By mid-June, Gaudet had reports of stripe rust south of a line from Lethbridge to Oyen and extending to Melfort, Regina and Indian Head. Gaudet said he believes the fungus was able to survive over the winter because even though it was consistently cold, early snowfall provided insulation, at least in some patches, for the fungus. There were no chi-nooks long enough to clear the snow completely.
The other way the fungus may have overwintered was via a “green bridge” last fall. Last year’s cool, wet summer delayed crops, and spring wheat was still green when newly seeded winter wheat emerged. The stripe rust fungus could have spread from late-maturing spring crops to the winter wheat and overwintered there. Last summer’s cool, wet weather was also perfect for plant disease organisms, so they built up huge populations and increased infection rates making overwinter survival of some organisms more likely.
Stripe rust only affects the leaves of wheat, not other cereals, and it’s a different species from stem rust. Each infection causes a stripe of brown, dead tissue along the leaf, and orange pustules form on the leaf surface. The orange spores collect on your shoes and pant legs as you walk through an infected crop, giving them an orange tinge. The disease ruptures the leaf epidermis and causes excessive moisture loss. In hot, dry weather, the leaf becomes desiccated and browns off. That leaves the plant without foliage causing shrivelled kernels, lowering protein, and cutting yields by 20 to 40 per cent.
Spray timing critical
Gaudet advises monitoring crops from tillering through the boot stage until after flowering. If you see infection building up at those stages, he said, consider spraying, but that’s not a simple decision.
“The flag leaf and flag-minus one are the leaves that determine yield in wheat,” he said. “So if these are relatively clean, the disease will cause very little yield loss. And, if you spray before the flag leaf is fully exposed, remember you need a systemic fungicide to protect it. Once flowering is complete and the heads are starting to fill, it’s too late for a fungicide to affect yield.”
Stripe rust is not necessarily a problem on seedlings, according to Gaudet. Many varieties have “adult resistance” to stripe rust and will grow out of the problem. Most winter wheat is quite susceptible and many spring types are quite, too. Durum and some hard red spring types have good resistance, but white wheats and CPS types are susceptible. AC Lillian, Harvest CDC Go, Kane and durum are quite resistant, but Barrie, Superb and McKenzie are susceptible. In the past, most problems were on soft white wheat because of its long season. Triticale has good resistance, and barley and other cereals are not affected by the stripe rust species that affects wheat.
Gaudet said he is concerned that this outbreak may not be an isolated or short-term, weather-related incident.
“Stripe rust is a crop disease that’s on the move and emerging as a concern around the world,” he said.
In Alberta, the pathogen has overcome a single resistance gene that is incorporated into Radiant winter wheat that was effective until 2009. The fungus mutates and changes very rapidly to break down resistance, Gaudet said. Although we have effective resistance in many of our spring wheats, a significant proportion of our wheat is vulnerable to the disease.
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“Striperustisacrop diseasethat’sonthe moveandemergingas aconcernaroundthe world.”
DENIS GAUDET
