Good Fruit Grower

February 15

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years, infected cherry trees will show foliar symptoms, but that's not the norm, he noted. The virus stunts tree development and shortens annual branch extensions. "Over time, little cherry virus 2 reduces the overall vigor of the tree and can be devastating," he said. Kootenay Valley growers in British Columbia, Canada, lost more than 30,000 trees to the disease in the 1940s and 1950s. Since 2010, the situation with little cherry virus 2 has been critical in Washington's Chelan, Doug- las, and Okanogan counties. In Grant and Yakima coun- ties, where infections have been verified, the disease is not as severe as in north central Washington. In addition to apple mealybug being a known vector, grape mealybug is also an effective disease carrier and transmitter, WSU researchers discovered last summer. Grape mealybug is a vector of grapevine leafroll associ- ated disease 3, so the recent finding was not surprising. "We suspect that any mealybug can transmit this disease," Eastwell said. Mealybugs are difficult to control because they over- winter in trunk crevices, inhabit the tops of trees, and have no natural enemies. As crawlers, they can travel long distances through dispersion from wind and hitch- hiking on equipment and debris. "They can quickly get out of control," he warned. If mealybug populations are found, growers should consult pest control advisors for specific recommendations. Little cherry virus 1 Little cherry virus 1 is in the same family as virus 2. It was the culprit in an outbreak of little cherry disease in northern Germany during the 1980s. "We know it is in British Columbia and Washington, though in relatively low incidence in the state," Eastwell said. It spreads more slowly than virus 2 and, in Europe, it's spread primarily by grafting. It has milder symptoms than virus 2. However, when both little cherry viruses are combined, symptoms can be brutal. "I've seen mixed infections of 1 and 2 in Wenatchee, and the symptoms were very severe," said Eastwell. Scientists have not identified a vector for little cherry virus 1. (Continued on page 18) 16 FEBRUARY 15, 2014 GOOD FRUIT GROWER W ith the dramatic rise of little cherry diseases in Pacific Northwest orchards and discovery of new vectors, it seems no orchard is safe from infection. Last year, 150 acres of cherries were removed in Washington's Chelan County because of little cherry disease. Forty acres were taken out during the previous two years. In more southern locations in the state, the uptick in disease incidence has not been as severe, with about 800 trees removed in 2013 in Yakima County. While both areas of the state are being affected by little cherry disease, the causal agents are completely different and require different control measures, says Dr. Ken Eastwell, plant pathologist for Washington State University and director of the Clean Plant Center Northwest in Prosser. "The landscape has significantly changed in the last few years," Eastwell said. Know your enemy So, what can you do to avoid little cherry disease? "First, you need to know whom you're fighting," he said. "When you have small fruit, it means that something is interfering with the vascular system of the tree." Small fruit can be caused from physiological condi- tions, like winter damage or mineral imbalance, and from pathogens, viruslike agents, and root or crown rots. Cherries are susceptible to a wide range of pathogens. According to Eastwell, 64 viruses affect cherries, of which 44 are known in the United States. If you see small fruit, contact your field representative to help diagnose the problem, Eastwell told growers attending the annual meeting of the Cherry Institute in Yakima, Washington. "If a virus is suspected, contact me at the WSU Clean Plant Center North- west." In Washington State, three diseases are known to impact cherry quality and cause small fruit—little cherry virus 1, little cherry virus 2, and Western X phytoplasma. Little cherry 1 and 2 are both members of the Closteroviridae family, but are different genera. "We know more about little cherry virus 2, the virus more commonly found in the Northwest and in north central Washington," Eastwell said. Little cherry virus 2 is closely related to grapevine leaf- roll virus. "We don't know a lot about little cherry virus 1." Little cherry virus 2 Little cherry virus 2 results in small fruit that is late to ripen, poor in color, and insipid in flavor. At yellow or straw color stage, infected fruit are similar in size to normal fruit, but by pink stage, there is significant difference in size. In some Know your ENEMY Different causal agents are involved in little cherry disease. by Melissa Hansen DETECTION kit A user-friendly kit to diagnose little cherry virus 2, the most common in Washington, will be available later this spring, allowing growers to quickly run a sample to detect virus using a modest laboratory (like those found in most packing houses). The technique was developed by Washington State University scientists through research funded by the Washington Tree Fruit Research Commission. The kit should be commercially available in May from Agdia, Inc. (, a leading provider of test kits for plant pathogens. This photo of a Chelan County cherry orchard being removed because of little cherry virus 2 was taken last fall. Some 150 acres were removed in the county in 2013. PHOTO BY LANDON MICHAELSON, STEMILT GROWERS

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