Dr. Andrea Bixby Brosi, Dr. Elizabeth Beers WSU Tree Fruit Research and Extension Center; Tim Smith, WSU Extension Specialist Emeritus; Tianna DuPont, WSU Extension Specialist
Little Cherry Disease is a critical concern to sweet cherry producers. Diseased trees produce cherries of small size and poor color and flavor making the fruit unmarketable. Unlike other diseases that can be managed by removing infected limbs, the entire infected tree must be removed. As part of a three-part newsletter article series Dr. Andrea Bixby-Brosi and Extension Specialist Emeritus Tim Smith discuss Little Cherry Disease Basics. Stay tuned for diagnosis and control.
What is Little Cherry Disease?
Little cherry disease is caused by one or more of three pathogens and all are known to occur in Washington State: Little cherry virus 1 (LChV1), Little cherry virus 2 (LChV2), or Western X phytoplasma (WX). Disease symptoms include small size, poor color and bitter tasting cherries, while foliage is not affected.
Where does Little Cherry Disease Come from?
Little Cherry Disease is a common problem in ornamental cherries. Since the disease does not cause dieback in the tree and no one is concerned about the quality of the fruit on an ornamental variety, Little Cherry Disease generally goes undiagnosed in ornamental trees which then serve as a source for disease. Pollinator varieties can also be a source from which the disease spreads.
When did Little Cherry Disease become a Problem?
Little Cherry Disease is not a new problem. In 1938 it was found in the Kootenay Valley of Canada where by 1943 it had infected 30,000 trees. By 1979 the disease had caused so much damage that the last packing line in the Kootenay Valley closed down. In Washington, Little Cherry Disease became a statewide problem in 2010 and has since resulted in unpicked limbs/trees, tree removal, and even orchard removal. While the exact amount of infected acreage in Washington is unknown, Little Cherry Disease has been verified in commercial sweet cherry orchards in Grant, Chelan, Douglas, Yakima, Benton, and Okanogan counties.
How does Little Cherry Disease Spread?
Little Cherry Virus 2 is spread from tree to tree by the apple mealy bug (Phenacoccus aceris (Signoret)) and the grape mealy bug (Pseudococcus maritimus (Ehrhorn)). Both mealybug species can be found in sweet cherry orchards in Washington and their presence has been verified in LChV2 infected orchards. During 2014 and 2015, researchers surveyed mealybugs in LChV2-infected orchards to determine if their presence is an indicator for the disease, however, only half of the orchards had active mealybug populations. Mealybug presence can be a red flag, but it doesn’t mean an infected orchard has to have an active mealybug population. Western X phytoplasma is primarily spread by leafhopper (several species). Studies are underway to identify the specific leafhopper species present in WA sweet cherry orchards. Both LChV2 and WX are also readily transmitted by all types of grafting. LChV1 can also be spread by grafting; other means of spread are not known.
For more information please visit the WSU Tree Fruit Little Cherry Disease Page.