CORVALLIS - A bacterial disease that plagues Oregon's $419 million nursery industry has recently developed resistance to both of the chemicals used to control it, according to a new study at Oregon State University.
These Pseudomonas syringae bacteria, which cause a disease commonly known as bacterial blight, are already responsible for more than $8 million in annual losses to the nursery industry.
As chemical resistance spreads it's reasonable to believe that problem will worsen - a survey of 44 Willamette Valley nurseries showed pathogens are now prevalent everywhere, with 467 strains isolated from 25 plant species.
However, OSU scientists have identified types of chemical treatments and plant management strategies that will provide some help to operators of Oregon nurseries, which are the state's most valuable agricultural sector.
Until more permanent solutions are developed, prices for many nursery plants may rise and homeowners may also face a more serious threat to some of their ornamental plants and fruit trees, said Heather Scheck, a doctoral candidate in the OSU Department of Botany and Plant Pathology.
"Pseudomonas thrives in wet, cool climates like we have in western Oregon," Scheck said. "It produces a toxin that kills flowers and new growth, and causes cankers on more than 160 green and woody plants, including many of those produced in Oregon's ornamental nursery industry."
"The bacteria can kill young plants or flowers," she said, "but more commonly it maims the plant, ruins its appearance and makes it unsalable."
Growers in recent years have complained that the chemicals they most often used to control bacterial blight - copper and streptomycin - seemed to be losing their effectiveness. And the disease often made plants appear at their worst during spring, the prime sales season.
The OSU study, supported by the Oregon Association of Nurserymen, the Oregon Department of Agriculture, and the U.S. Environmental Protection Agency, found the problem.
"We discovered that different phenotypes of Pseudomonas syringae have rapidly evolved in the past 10 years with resistance to either copper, streptomycin or both," said Scheck.
A collection of bacteria made by OSU in 1983 contained no strains of bacteria resistant to both these chemicals - now, 24 percent are.
"This problem pervades the Willamette Valley, which is the heart of the state's nursery industry," Scheck said. "You don't see it so much at retail nurseries, because the affected plants are often recognized and destroyed before they ever get there. But some plants infected with resistant bacteria may inadvertently be making it through to consumers."
However, OSU's research program has found some approaches that help address this problem.
Studies show that streptomycin resistance, if found, means the bacteria is virtually invulnerable to that chemical. But it appears that copper formulations with a higher concentration of "free ions," in a wettable power form, still provide some control.
Brand names of chemical products which meet that criteria and are available in most garden stores include Microcop, C-O-C-S, and Kocide 101 in its wettable powder form.
Other non-chemical measures that can help include more physical spacing of plants and good pruning to maintain air flow within the canopy. Cover from rain is also very helpful, and growth of more plants either under cover or in greenhouses may be one solution available to the nursery industry.
Among the plants that are by far the most vulnerable to bacterial blight, Scheck said, are lilacs and Japanese maples. The disease is getting sufficiently worse and lilacs are so prone to it, she said, that some Oregon nursery growers have abandoned efforts to cultivate them.
Continued research at OSU will work to find other types of treatments that can address this problem, Scheck said. It's not clear how many other areas of the nation may be facing similar concerns, although some spots in California and the Midwest may be affected, she said.
So far, there has been little success in finding genetic resistance among plant species that are susceptible to the bacteria, she said.