By MATTHEW WEAVER
The common barberry bush remains as the key link in the spread of stem rust in Pacific Northwest wheat and barley crops, an expert says.
Stem rust fungus can overwinter in the stubble of infected grain fields, Diana Roberts, regional extension specialist for Washington State University Extension, said. Then the black, powdery fungus spores can blow into nearby barberry bushes in the spring.
The common barberry plant is necessary to complete the stem rust disease cycle in the Pacific Northwest along with late summer moisture and warm nights.
The fungus spores form pustules on the barberry bushes, creating new races or rust varieties that can overcome resistance in wheat varieties.
In the summer, the stem rust can blow from the barberry back to the wheat, where it reproduces quickly and can blow great distances.
Stem rust causes 20 to 100 percent yield reductions in wheat fields. Spores remain on the stubble of the plant, where it overwinters.
The common barberry plant must be close to the wheat or barley field it infects, Roberts said.
Japanese barberry grown for landscaping are not a host for stem rust, Roberts said.
"Our big concern is Washington has the potential to develop new and virulent races, which can affect other states," Roberts said.
Those new races could potentially blow over the Rocky Mountains to the Midwest and cause "havoc" on their durum wheats, she said.
In 2007 and 2008, isolated but severe infections occurred in Stevens and Whitman counties in Washington, indicating barberry was growing back, she said.
The year 2012 was the closest to a summer rainfall season in Roberts' 20 years with WSU Extension. Stem rust was common in Stevens, Spokane and Whitman counties and beyond. Researchers found 20 to 100 percent of plants to be infected.
Stem rust pustules typically begin on the stem of the wheat or barley plant and can spread to leaves or the head of the plant. Early in the season, pustules are reddish-brown, but then become black around harvest time, causing black clouds to appear behind combines, Roberts said.
Roberts said stem rust can infect barley and spring and winter wheat. Late-seeded grains are most susceptible. Crop monitoring should begin about mid-June.
It will become the landowner's responsibility to eradicate barberry, Roberts said. WSU Extension will publish a bulletin with further details.