A new discovery this summer could help explain the growing spread of little cherry disease in the Pacific Northwest, but it will make control more difficult, say Washington State University researchers.
In the last several years, the disease that results in small fruit with poor flavor has become a serious problem in Washington State.
The cool weather of 2010 triggered disease symptoms and brought the disease to the attention of the cherry industry. Dr. Ken Eastwell, WSU virologist, says the virus has been around for years, but prior to 2010, infections went unnoticed.
“But going past 2010, we’re still seeing a rise in the disease, in 2011, 2012, and 2013. That makes us believe that something else is going on.”
There is no cure for the disease—removing infected trees to prevent further spread is the only remedy.
Though there is no official count of infected trees removed, Eastwell, who is also director of the National Clean Plant Network Northwest in Prosser, estimated that more than 100 acres have been rogued just in the last two years. “That’s significant,” he said to Good Fruit Grower, adding that both young and old orchards are being hit.
WSU Extension educator Tim Smith predicts that growers will remove “hundreds and hundreds more acres” in the near future due to little cherry disease. “It’s going to get worse before it gets better,” he said, based on the wave of disease that he’s seen spread through orchards near Wenatchee and north in the last few years. “When you really start looking for the virus, you can find it everywhere.”
It’s difficult for growers to detect infection in the orchard because the trees look healthy, Smith said. He says the best time to look for the small, telltale fruit is about two weeks before harvest, a time when growers are busy doing other things.
He adds that growers often notice small fruit on a tree, but because the tree looks healthy, they think the tree will grow out of it by the following year. “Then three years later, they’re cutting down the whole orchard.”
Three pathogens are associated with little cherry disease: western X; little cherry virus strain 1; and little cherry virus strain 2. In Washington’s northern cherry region (Wenatchee and north), orchards tend to be infected with strain 2, while western X seems to be the predominant pathogen in the Tri Cities region, according to Eastwell.
A devastating outbreak of little cherry disease occurred in British Columbia, Canada, in the 1930s and ‘40s, resulting in the removal of 60,000 trees. “Back then, researchers didn’t have the tools to identify specific pathogens,” said Eastwell. “But now we know that the pathogen in the British Columbia outbreak was little cherry 2.”
But it’s the discovery last summer of apple and grape mealybug on cherry trees that will make disease management even more challenging.
It was just recently established by WSU scientists that grape mealybug could transmit little cherry 2 to healthy cherry trees. Before that, apple mealybug was the only known vector for little cherry disease 2. Leafhoppers are known vectors of western X disease.
Before the summer discovery, although apple and grape mealybug were known vectors, they weren’t thought to be a pest in cherries.
Apple mealybug is not a common pest in Washington, said Dr. Betsy Beers, WSU entomologist based in Wenatchee. She knows of five populations of apple mealybug in the state, and the populations are all in apple orchards. Grape mealybug historically has been a problem in pears, and occasionally in apples, but never in cherries.
That perspective changed last summer when a field representative brought a cherry limb into Beers’ laboratory for examination. The limb was from a tree that had just been cut down because of little cherry disease 2.
She found both apple and grape mealybugs on the limb. “And when we went into the orchard, we found both mealybug species coexisting.
“With the new mealybug revelation, we came to the realization that although we know a fair amount about little cherry disease, not much is known about the vectors themselves,” she added.
She noted that there’s a big difference in a pest being an occasional problem and a vector. “With a vector, there’s zero tolerance for the pest in the orchard.”
Eastwell said because mealybugs don’t usually cause an economic impact in cherries, few growers follow a spray program aimed at mealybug. Compounding the difficulty of mealybug control are the different life cycles of the two pests. Apple mealybug has one generation per year, while grape mealybug has two, requiring different spray timing for the two species.
Some things are known about the virus, said Smith, who works with tree fruit growers in north central Washington. It’s not spread through pollen, or mechanical means like pruning. It needs physical, tree-to-tree contact, which is why root grafting is one means of transmission. Vectors carrying the disease from tree to tree is the other means, he said.
“But we still don’t have a good handle on how rapidly the virus is spreading,” Eastwell said.
Beers agrees. “We need more information on how widespread grape and apple mealybug populations are, region by region,” she said. “We have little information following the disease in an orchard. We have way more questions than answers.”
Eastwell and Beers are teaming up in a transdisciplinary project to learn more about the virus and vectors. Baseline data was collected last summer from an infected orchard and the researchers are following its spread within the block. They also hope to work together on a research project designed to develop disease management strategies for growers.
Eastwell recommends that when removing a tree, growers cut it three feet above the ground and apply herbicide to the freshly cut tree. The herbicide application will kill any root suckers and help identify nearby root-grafted trees that will also need to be removed.
For growers who cannot burn but must chip trees, there is concern that mealybug could survive the chipping process, he warned. “So be sure to spread the chips in a nonhost location.
“And always replant with certified trees,” Eastwell stressed. •