You know it’s summer when bags of sweet cherries appear in the grocery store. Cherry production occurs globally, but the clear leader in the U.S. is Washington state. Little cherry disease has ridden fields in Washington state since the ’40s, according to Dr. Scott J. Harper, Assistant Professor for the department of plant pathology at Washington State University.
“There’s an old state monograph I’ve got from 1947 that mentions the disease, though they didn’t know what caused it at the time,” Harper says.
The disease was first found in Canada in 1938. By 1943, it had infected 30,000 trees. By 1979, the disease had caused so much damage that the last packing line closed down. Canada spent decades rebuilding its sweet cherry industry.
But in Washington state, the epidemic peaked in the ’50s and then leveled off to maintain a slight presence from then on.
“No one was too concerned with it or thought of it as a major problem,” Harper says. “But in the last five to 10 years, it’s been increasing in frequency. We’re seeing more and more trees coming down with this disease. It’s starting to reach peak again and becoming a serious problem.”
Little cherry disease became a statewide problem in Washington in 2010, resulting in unpicked limbs/trees, tree removal, and even orchard removal. The exact number of infected acreage in the state is unknown, but little cherry disease has been verified in commercial sweet cherry orchards in Grant, Chelan, Douglas, Yakima, Benton, and Okanogan counties.
Little cherry disease is caused by three different pathogens: Little cherry virus 1, little cherry virus 2, and Western X phytoplasma. All pathogens result in one disease with similar symptoms, and only one is needed for infection. However, sometimes more than one pathogen is present in a plant.
While the first two pathogens are viruses, Western X is a phytoplasma, a bacteria without a cell wall, and is the most commonly found in Southern Washington. Central and Northern Washington see more of little cherry virus 2, while little cherry virus 1 is more commonly found in Europe — all for unknown reasons.
Whatever the pathogen, the disease doesn’t kill the tree but instead hinders fruit development. Therefore, symptoms of little cherry disease tend to be visible only in the fruit, which develop to be small, colorless, and bitter tasting due to low sugar content.
“Even though the disease itself doesn’t kill the tree, economically it’s the same thing for growers since you can’t sell the fruit,” Harper says.
Unfortunately, growers’ ability to control little cherry disease is quite limited.
“You can’t treat a virus or phytoplasma by spraying, as you would a fungus or bacteria that’s just on the plant,” Harper says. “They’re in the plant.”
While sprays can help protect unaffected trees from insects carrying the disease, such as the mealybugs or leafhoppers, once trees are affected, the only option is to cut them down and replant. This leaves growers in a bad position. Harper says some areas have been so heavily hit that growers are removing entire blocks.
Harper thinks the rapidly increasing rate of little cherry disease in recent years is the result of a cumulative effect.
“The pathogens have been here for a long time, but now it’s built up enough that it’s snowballing,” he says. “The more you have, the more you’re going to get.”
Harper encourages growers to look out for signs of tree infection. It’s important that little cherry disease isn’t mistaken for small cherries affected by winter or cold damage, inadequate tree nutrition, and other factors. Generally, in these cases, the entire tree will be affected, such as having small leaf size or gumming. Symptoms of little cherry disease are only visible in the fruit. However, because fruit ripeness only appears a couple of weeks before harvest, Harper also recommends growers submit samples for lab testing throughout the year. Testing is especially important before tearing out a block of trees thought to be infected. Additionally, though the effects of spray programs are limited, he suggests growers incorporate them.
“The bugs are active through much of the year and don’t discriminate,” he says.
Little cherry virus 1 and 2 aren’t known to affect other stone fruits, but Western X might. The phytopasma, among others, has been found to affect fruit size in peaches, nectarines, and possibly plums.
Taking a Closer Look
To find out more, Harper is working on a three-year research project looking at phytoplasma-caused diseases in stone fruits other than cherries. The research is funded with $92,000 from the Washington Tree Fruit Research Commission.
“It’s an active parallel research program consisting of several large projects that flow into one because in some areas around here you’ll find a block of cherries next to a block of peaches,” he says.
Part of the research consists of figuring out what pathogens are present where, how they’re spreading, and through what hosts in addition to their effects.
Harper says he’s also trying to understand how the Western X pathogen causes little cherry disease because doing so may help with future breeding programs to identify tolerant or resistant cultivars. These cultivars may be the long-term solution to the problem.
It’s not an easy task, but it’s an important one to Washington’s sweet cherry growers. Harper is up to the job.
“It’ll keep us busy,” he says.