Cherry trees are susceptible to many diseases, and one that hit the Washington state cherry industry in the last few years has proven to be a challenge: little cherry disease (LCD). Trees with the disease produce small fruit with poor color and bitter taste.
What is perplexing to growers is some varieties show more exaggerated and pronounced symptoms than others. Plus, LCD symptoms become apparent only in the fruit a few weeks before or at harvest, making it difficult for researchers and growers to diagnose and study.
“Sometimes the disease can go undetected for several years because symptoms are not displayed throughout an entire tree, but rather in just one limb. The disease also can be confused with a nutrient deficiency or another disease. Also, after initial infection via insect vectors, symptoms may not appear for several years,” explains Andrea Bixby-Brosi, Post-Doctoral Research Associate with the Washington State University Tree Fruit Research and Extension Center.
3 Pathogens; 1 Disease
LCD is caused by three different pathogens — Little Cherry Virus 1 (LChV1), Little Cherry Virus 2 (LChV2), and Western X (WX).
Researchers do not know how LChV1 is transmitted. Currently, LChV2 is passed on to the trees by apple and grape mealybugs. Mealybugs make their way to cherry trees through wind disbursal.
A recent project determined Colladonus geminatus and C. reductus leafhopper species transmit Western X, which has been more prevalent in the Yakima and Prosser, WA, areas, Bixby-Brosi says. Chemical control of leafhoppers, however, is something yet to be looked in to.
“It’s important to know which pathogen you have because the management is going to be a little bit different for each one if you’re looking to control the insects that spread the disease,” Bixby-Brosi says.
Test Your Leaves and Scout
The most important line of defense against little cherry disease is to remove infected trees and to scout for new infections. However, given that the disease becomes apparent so close to cherry harvest, growers often delay leaf sampling until the off-season.
Bixby-Brosi suggests growers who suspect a tree has LCD should take a 10-leaf sample and send to the Clean Plant Center in Prosser, WA. Growers can send in dormant buds, too.
“One of the things that we’ve been stressing is that people should get their trees tested if they have any inkling that it might be little cherry disease,” she says. “Because it could be something else. There’s a lot of viruses in cherries.”
Once the disease is confirmed, trees must be removed. Otherwise healthy trees in the orchard are susceptible to the disease as well.
“If you leave a diseased tree in the field, it can potentially infect other trees through these insect vectors,” she says. “The other way the disease can spread is through root grafting, when the roots of two trees naturally grow together underground and share nutrients.”
Bixby-Brosi recommends removing a buffer area of two to three trees from the infected zone, in case root grafting occurs or there is exposure to the disease through the insect vector. Growers also need to scout the orchard to see if other trees show symptoms.
Because the disease can be spread through root grafting, there are still some unanswered questions as to the best strategy for replanting a cherry orchard. Bixby-Brosi says some growers opt to plant another commodity, however sometimes that is not the best economic decision. Currently, there is no best-practice a far as removing an orchard and replanting.
Growers often remove trees, rip the orchard, and fumigate before replanting. Unfortunately, some of that is a problem for organic growers.
“We’re not really sure if that’s the best method, or if you should leave that particular part of ground unplanted for a while,” she says.
For organic growers, Bixby-Brosi says there are some natural enemies of apple and grape mealybugs, which have the potential to help stop the spread of LChV2. But, the most important way to slow down and stop the spread of LCD is scouting and prompt action upon detection, she adds.
“With the knowledge that people have now of what the symptoms look like, and getting trees tested once they know the disease is present [in their orchard], I think that’s helping to solve the problem. Hopefully we can, at some point, have very few of these disease issues,” she says. “If everyone is looking for [Little cherry disease] and working on it, we can get it out of the system.”