Warm winter weather combined with snow cover could mean a “severe epidemic” of stripe rust in some Northwest wheat fields, a USDA expert predicts.
Wheat varieties that are susceptible to the disease could have yield losses of up to 22%, said Xianming Chen, research plant pathologist at the USDA Agricultural Research Service in Pullman, Wash.
Chen and other researchers monitor varieties that are highly susceptible as a check. Yield losses in those varieties can range up to 44%.
Xerpha, ORCF-102 and Eltan are commercial winter wheat varieties that are susceptible, but still have some level of resistance, Chen said.
The most severe recent stripe rust years were 2010 and 2011, when susceptible check varieties showed yield losses of 60% and 90%, respectively.
This year won’t be nearly as bad, Chen said.
In a normal severe year, yield loss for highly susceptible varieties is 20-40%, Chen said.
Stripe rust has been severe in two out of five years, Chen said.
The years 2014 and 2019 were the lowest stripe rust years.
Stripe rust survives in infected leaf tissue. If the leaves die due to winterkill, the rust is also killed. Chen said temperatures need to drop to 5 degrees Fahrenheit to kill the rust.
However, snow cover can help wheat plants — and rust they harbor — survive, he said.
“If weather conditions continue to be normal for this region, not too cold and not too warm, then the rust situation will be very likely as we predicted right now,” he said.
If weather is warmer than normal, rust could increase, Chen said. If it’s cooler than normal, there could be less.
Wheat variety stripe rust susceptibility ranges from 1, least susceptible, to 9, most susceptible. Chen recommends growers plant a variety with a susceptibility of 1 to 3, with 4 considered marginal.
Chen recommends growers pay attention when planting Xerpha, Curiosity CL+, Eltan and UI Magic. They are the most vulnerable commercially grown varieties.
Given the expected rust severity, farmers should apply fungicide at the time of herbicide application if their variety has a susceptibility of 5 or above, Chen said.
Farmers planting varieties with a susceptibility of 1 to 4 should only apply fungicide if they’ve seen rust in their fields, particularly during early planting in August and September.
Chen will release an updated forecast in March.
Growers with questions should contact Chen at 509-335-8086 or firstname.lastname@example.org.