The cool weather of 2010 highlighted a growing concern about little cherries showing up in some orchards. Follow-up testing by Washington State University confirmed that the trees were infected with a dreaded cherry disease that had not been found extensively in Washington before.
“The 2010 spring was very cool, and as a result, virus expression was at an all-time high in Washington State,” said Dr. Ken Eastwell, WSU virologist and director of the National Clean Plant Network center in Prosser. “As a rule, viruses generally express their symptoms much more strongly in cool weather.”
WSU Extension educator Tim Smith and Eastwell received a rash of calls last year from growers reporting problems of little cherries. “What we heard from growers in the Wenatchee and Yakima valleys was that they had seen the gradual expansion of little cherry disease in their trees the last three to five years, but, with the symptoms expressed this last spring, they couldn’t ignore it anymore.”
WSU tested 39 orchards for the virus last year. Nearly half (18) were infected with little cherry virus 2; 13 percent (5) were infected with little cherry virus 1; and three orchards had both little cherry viruses 1 and 2. More common viruses were found in the other 20 orchards that were sampled.
Little cherry disease is caused by three distinct pathogens—Western x, little cherry virus strain 1, and little cherry virus strain 2.
Western x was a major problem in Washington in the 1950s, but most of the infected trees were removed, and it wasn’t much of a problem until 2001 and 2003 when leafhopper populations were high. The phytoplasma bacteria of Western x are transmitted through phloem-feeding leafhoppers, and the disease is more problematic in warmer climates. Western x results in small, late-ripening fruit with poor color.
Little cherry 1 was responsible for an outbreak in Germany in the late 1980s, Eastwell said. Symptoms with little cherry 1 are less severe than other forms of little cherry disease, allowing the virus to slide in under the radar, Eastwell said. The disease is reported worldwide and has been found in Washington and British Columbia, Canada, though distribution is limited. No known vectors have been identified, and the disease spreads naturally.
Little cherry 2 was associated with a devastating outbreak in British Columbia in the 1930s. The crippling epidemic spread rapidly throughout the cherry industry there, requiring the removal of 60,000 trees. Little cherry 2, with the most severe symptoms of the three pathogens, is transmitted by the apple mealybug. However, Eastwell suspects grape mealybug could also serve as a vector. It was first confirmed in Washington in 2008.
Fruit from infected trees appear healthy until the straw-color stage, at which point they stop developing, he said. While there is varying sensitivity to and expression of the disease depending on rootstock and scion, regardless of whether a variety is severely affected or not, Eastwell noted that “… in every case, little cherry infection reduces the size of the fruit and also results in inferior flavor.”
Acids and sugars in the fruit are reduced, resulting in bland flavor. Fruit are poorly shaped, poorly colored, and small. The disease is hard to identify in the orchard because trees can harbor it without showing any obvious symptoms on yield or size in normal weather.
The Lambert cultivar is one of the most sensitive to the disease, making the disease in Lambert cherries easy to recognize. But symptoms on the widely planted Bing and Sweetheart are more difficult to recognize, he said.
Eastwell described a misleading “shock” symptom that can occur in Bings infected with little cherry 1 and 2: extreme symptoms appear the first year, but as the tree ages, symptoms lessen. “But even though fruit size improves after the first year of symptoms, the fruit still lack flavor.”
Growers can mistakenly think that the infected fruit on Bing that aren’t quite up to par have nutritional or irrigation problems. But he warns that though symptoms aren’t severe, fruit won’t have the flavor that they should.
What to do?
Eastwell believes that little cherry disease is an emerging problem in Washington. “The weather kicked off the concern this last year, but the disease has been slowly growing here for the last three to five years.”
Removing infected trees is the only remedy because there is no cure. It costs about $900 to remove, replant and bring a new tree into production, he noted. If the disease is limited to a few trees, growers can flag infected trees during the season when fruit symptoms are noted and remove them after fruit are harvested. A more widespread problem in the orchard requires an orderly tree removal plan.
He reminded growers during a talk at the Cherry Institute meeting last month in Yakima, Washington, that the disease can affect neighboring orchards.
When removing a tree, he recommended cutting the tree three feet above the ground and applying herbicide to the freshly cut wood. The little cherry virus can only survive in living tissue, and the herbicide application on the trunks will kill any root suckers and help identify nearby root-grafted trees. The disease is not mechanically transmitted.
“Be sure to replant with certified trees,” Eastwell stressed. “And that includes using certified stock for pollinators. I’ve seen too many growers invest in good quality, certified trees and then infect their orchard from diseased pollinators.”
Vigilant control of apple mealybug, as well as grape mealybug, is also recommended.
“Deal with diseases as soon as you see them,” he said.
The Washington Tree Fruit Research Commission, along with Oregon and California cherry groups, are funding a research project led by Eastwell to develop a grower management strategy for little cherry disease, including development of a rapid lab test that could be accessed by growers.