Little cherry disease made a dramatic resurgence in Washington in 2010 and has since spread rapidly. These Sweetheart cherries show symptoms of small and puny fruit.
PHOTO BY KEN EASTWELL
With no cure for cherry trees infected with most viruses—short of cutting them out of the orchard—Washington State University researchers are focused on developing effective management strategies for growers to use in controlling the spread of viruses.
Dr. Ken Eastwell, Washington State University plant virologist and director of the Clean Plant Center of the Northwest that’s based in Prosser, Washington, has worked with the Pacific Northwest cherry industry for many years to help growers deal with viruses and diseases that cause economic loss through decline in tree health, fruit quality and yield. Cherry leaf roll virus, little cherry disease, and western X are among the diseases of late that have posed problems for Northwest growers.
Infection of cherry trees in Washington State by cherry leaf roll disease was first detected in 1998, according to Eastwell. “The virus is now found with increasing frequency in all cherry production areas,” he stated, noting that two areas have extremely high virus pressure—Yakima Valley and the lower Columbia Valley.
In developing management strategies for growers, WSU scientists are trying to learn more about leaf roll disease phenology and how the virus spreads. Moreover, how do new infections appear in isolated orchard blocks?
Eastwell’s research, supported by industry through the Washington Tree Fruit Research Commission, is beginning to unravel the leaf roll virus transmission mystery. During the Research Commission’s Cherry Research Review held last fall in Yakima, Washington, he updated cherry growers, shippers, and research committee members with the most recent results of his studies.
Before leaf roll virus was detected in the state in cherries, it was found in golden elderberry, Eastwell said, adding that there’s been concern that host elderberry plants could be aiding in the disease’s spread. But his research found different virus isolates in cherry trees and elderberry plants that harbor the disease, dismissing elderberry plants in spread of the disease.
Spread by pollen?
“Aerial transmission through pollen has long been a suspect,” Eastwell said. But researchers came up short there, too.
In controlled pollination of some 10,000 blossoms with virus-infected pollen, scientists found infection in seed, embryos, seedlings, and fruit stems, but not the tree. “Our work failed to result in transmission of virus to a single tree exposed to infected pollen,” he said. “If this is a route for transmission, it’s a very inefficient way of spreading disease.”
Thus far, root grafting appears to be the major means of leaf roll virus spread within an orchard, based on grower experiences and research trials. Eastwell believes that if transmission by root grafting can be eliminated, transmission and spread within a single orchard after a new infection could be substantially reduced.
“To prevent spread through root grafting in an orchard, it means that one of the first steps in a management plan is to cut out the infected tree as soon as the first infection is detected in an orchard,” Eastwell said, adding that rapid identification and elimination is the key in controlling spread of the disease.
Another potential strategy in managing leaf roll in new plantings of high-pressure areas may be to select scion and rootstock based on disease expression or sensitivity. Appropriate selection of scion and rootstock could be a tool, he said, especially if growers are located in a concentrated area of leaf roll infection.
The rootstock part of Eastwell’s research involved eight different rootstocks. Colt and Gisela 6 rootstocks responded very aggressively to infection from the inoculated Bing scion wood, exhibiting hypersensitive reactions and tree deathin the second year.
“Colt and Gisela 6’s rapid response quickly eliminates the potential for secondary spread in the orchard,” he stated in his research report. “Alternatively, growth of trees on Gisela 5 rootstock does not seem to be impacted by infection.” Though more long-term research is needed, Eastwell can envision growers in areas with high disease pressure using certain rootstocks to serve as an indicator of disease presence.
Little cherry disease
Little cherry virus made a dramatic resurgence in Washington State in 2010, hitting Chelan and Grant counties particularly hard. Laboratory analysis showed that the outbreak was associated primarily with little cherry virus-2.
More troubling has been the rapid increase in little cherry disease incidence in the state since 2010. based on grower samples submitted for diagnostic testing. In the first year of the disease’s comeback, 44 percent of the samples submitted by growers for diagnostic testing were positive, according to Dr. Tefera Mekuria, WSU postdoctorate researcher. In 2011, the number grew to 55 percent, and last year, 65 percent were positive. Mekuria says that nearly 275 grower samples were analyzed for little cherry virus during the last three years.
A key focus of WSU’s research involving little cherry disease is to develop industrywide strategies to prevent the continued intrusion into sweet cherry production regions.
Little cherry virus has a big negative effect on fruit quality. As the name implies, trees with the disease produce small fruit, with reduced sugars and off flavors. The puny, poor-tasting cherries are unmarketable.
Some varieties, like Bing, are fairly tolerant. When tolerant trees are infected, they exhibit small fruit for one or two seasons (called the shock phase of the disease) and then return to near-normal size, though fruit flavor never recovers.
It’s believed that the cool spring of 2010 imitated the return of already infected trees to the shock phase of disease. The cool weather again in 2011 and 2012 also provided ideal conditions for symptoms.
Mekuria believes that the rapid spread could be due to increasing populations in the state of grape mealybug. The only known vector to little cherry virus was apple mealybug, a pest found in low numbers in the state. However, WSU transmission studies (exposing grape mealybug to an infected orchard and then putting the insects on healthy potted trees) confirmed that grape mealybug is also a vector for the disease. In the study, 80 percent of the healthy trees exposed to grape mealybug became infected with the virus.
The disease can also be transmitted via grafts using noncertified budwood or rootstocks. Root grafting also can pass the virus from infected to healthy trees.
Rapid and accurate virus detection is the foundation of any management program, Eastwell said, noting that other horticultural issues can mimic little cherry virus, including zinc deficiency and winter damage. He’s working to develop cost-effective diagnostic tests that can be used by industry to detect a variety of diseases, including little cherry virus. One method under study is RPA (recombinase polymerase amplification).