Homegrown Stripe rust spores normally blow in from the U.S., but this infestation may have overwintered
It’s very early to find such a severe infection,” he says. “And, from the strength of the pathogen so early in the season, we think it overwintered here.
“We had heavy infestations last fall because of the stripe rust epidemic last year. This year, we’ve had cool, wet weather that’s favoured the pathogen, along with winds from the southwest that could have carried the stripe rust spores from the Pacific Northwest — that’s the usual source of rust in southern Alberta and the western Prairies,” said Gaudet.
Last year, the Pacific Northwest had epidemic levels of stripe rust. According to some of Gaudet’s Lethbridge colleagues who recently visited Pullman, Washington, the area has high levels of stripe rust again this year, so southwest winds may carry disease spores. There are also heavy infestations in Oklahoma and the Midwest — the usual source of stripe rust for the eastern Prairies.
Traditionally, stripe rust blew into the Prairies too late to affect wheat yields. Last year was different. The fungus seems to have overwintered in winter wheat in southern Alberta, so spores were produced early in the year and spread on wind currents causing potentially damaging disease as far away as Melfort, Saskatchewan and in pockets across the Prairies. It seems this pattern may be repeated in the future.
Gaudet does not think that’s a big risk this year. He and other specialists have been scouting for the disease in southern Alberta. They’ve found only trace levels of the disease in a few other fields. He advises winter wheat growers to check their crops carefully for any sign of stripe rust, especially in late-seeded, or late-germinating crops. He also recommends keeping an eye on susceptible varieties of spring wheat.
Winter wheat is particularly susceptible to attack by stripe rust. The fungus has evolved to overcome the resistance in AC Radiant, which was one of only a few resistant winter wheat varieties. McClintock, Moats, Peregrine and Sunrise winter wheats are still considered resistant to stripe rust.
Several varieties of spring wheat have genetic resistance to stripe rust, but it’s often “adult plant resistance.” The resistance is only expressed after the crop tillers, when the crop grows out of the disease. In seedlings, the rust may form blotches on the leaves, rather than the distinct stripes it forms in susceptible varieties. This type of resistance is not race specific, so it is not so easily overcome by the evolution of the rust pathogen. “If you see signs of stripe rust in spring wheat early in the season, check first to find if it’s a resistant variety,” says Gaudet. “The resistance in spring wheat is still effective against the disease.”
If you see signs of stripe rust — orange pustules on the flag leaf — in a susceptible wheat variety, spray it with a full rate of fungicide as soon as you can, say plant pathologists. The new races of stripe rust are more aggressive than older forms of the fungus, growing more rapidly and causing more damage to plants. Also, in the U.S. last year, these strains remained active and virulent even when nighttime temperatures are high, when rust infection usually drops. Spraying to protect the flag leaf can boost yields of infected grain considerably. Some farmers have seen yield benefits of 40 per cent or more.
If you use a fungicide to control stripe rust, it’s important to check the chemical’s pre-harvest interval before you apply it. That’s the time it takes for the fungicide to be dissipated out of the grain so it can be safely harvested. Seed treatment can give crops some protection against stripe rust early in the season. Stripe rust spores can move from spring wheat that’s infected late in the year to newly emerged winter wheat and from maturing winter wheat to spring wheat especially when the crops are in neighbouring fields.
Agriculture Canada plant pathologist Kelly Turkington, advises avoiding situations where winter and spring wheat are growing in adjacent fields. He also recommends removing volunteer cereals and grasses, especially foxtail barley and rotating out of cereals to limit the “green bridge” that allows stripe rust spores to infect susceptible grains, especially winter wheat.