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PROSSER, Wash. — Two of three strains of Little Cherry Disease “are well into epidemic levels” in cherry orchards throughout Central Washington and if they keep spreading eventually will impact the crop, a Washington State University plant virologist says.
“What I mean by epidemic levels is that it’s high enough that it’s not easily controllable, when it gets above 10 percent of an orchard,” said Scott Harper, virologist and director of Clean Plant Center Northwest at the WSU Irrigated Agriculture Research and Extension Center in Prosser.
The past year has been bad for the spread of LCD with most cherry orchards now having it to some degree or “if not they soon will,” Harper said.
Infestations range from a few scattered trees often on orchard edges, indicative of vector spread from the outside, to 100 percent of an orchard in heavily hit areas, he said.
The only way to control it appears to be tearing out infected trees and those around them and not replanting for more than a year. The main saving grace is that is spreads slowly.
B.J. Thurlby, president of Northwest Cherry Growers, the industry’s promotional arm in Yakima, said LCD is close behind labor and loss of the Chinese market as critical grower concerns.
“Some growers cut out the infected trees and some have simply pulled whole blocks (sections of orchards) and have started over,” Thurlby said.
No one is tracking acres of yanked trees but a couple years ago it was known to be over 1,000. There were 42,198 acres of cherries in the state in January 2017, according to USDA NASS. Planting has been expanding at 42 percent annually since 2000, Thurlby said.
LCD has no cure and robs trees of energy, reduces production and results in small fruit that’s unmarketable.
Little Cherry Virus 1 and 2 take away fruit flavor and Western X Phytoplasma is a bacterial-like strain that leaves bitter-tasting fruit. All three pathogens overwinter in roots and spreads tree-to-tree in roots. Apple and grape mealybugs spread virus 2, the dominate pathogen, and 2016 studies show leafhoppers carry Western X.
Bugs can be sprayed but the best control is tree removal, scientists say.
Virus 1 is rare perhaps due to the absence of an insect vector, Harper said.
Western X has been found in peaches and nectarines, he said.
In 1933, LCD all but wiped out the British Columbia cherry industry and 90 percent of trees were removed, Tim Smith, WSU Extension pathologist emeritus, has said. British Columbia experienced another serious episode in the 1970s. LCD was detected in Washington in the 1930s but was dormant until wet, cool springs may have contributed to an initial rise in the Wenatchee area around 2009.
From September 2017 through September 2018, the virus diagnostic laboratory at WSU-IAREC received more than 1,500 samples of trees leaves and wood to test, Harper said.
Of those, 24 percent had Western X, 15 percent had virus 2, less than 1 percent had virus 1 and 60 percent had no LCD, he said.
Western X was dominate in the southern counties of Yakima, Benton and Franklin and virus 2 dominated in the northern counties of Grant, Chelan and Okanogan, Harper said. This may be due to different vector compositions or geographic barriers, he said.
County percentages of Western X, V-2 and nothing found: Yakima, 58, 2, 40; Benton, 91, 0, 9; Franklin, 79, 0, 21; Grant, 25, 38, 37; Chelan, 8, 18, 74; Okanogan, 1, 56, 43.
Harper is in the final stages of testing a real time Polymerase Chain Reaction testing more sensitive to lower amounts of infection in leaves and tree tissue to detect disease before symptoms show at harvest so removal can occur earlier.
Western X has occurred in young trees with no known nearby infestations. Harper has studied weeds and other nearby plants looking for hosts but has found less than 1 percent with Western X.
“There are no easy or painless ways of dealing with this disease,” Harper said. “It will take a concerted approach of vector management, removal of infected trees and replacement from pathogen-free sources. It also takes close work with neighbors because LCD doesn’t respect boundary lines.”