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44 MAY 15, 2015 GOOD FRUIT GROWER www.goodfruit.com T here's much still to learn about little cherry virus, but one thing is certain. Infected trees need to be removed as soon as possible, says Tim Smith, Washington State University Extension specialist. Little cherry virus is a serious disease that spreads quickly. Some years ago, growers thought the main concern was small fruit. But Smith said there's a big difference between nat- urally small cherries and virus-infected cherries. Symptoms are visible only during the lead up to harvest when fruit on infected trees will be smaller and paler than fruit on healthy trees. Sometimes the cherries are pointed, with flat sides. On Rainier cherry trees, the fruit will be white, making it easy to diagnose. Trees infected with Western X disease will have fruit of various colors, whereas trees with little cherry virus will have fruit that are more uniformly lacking in color. The cherries also have a bitter taste, which Smith said is the way he diagnoses little-cher- ry-virus affected trees. Fruit quality will not improve once the tree has the virus. The rest of the year, the trees will look quite healthy, with large, glossy, well-formed leaves, which is why it's important to look for symp- toms around harvest. The virus might affect only certain limbs, or the whole tree. The virus typically starts on an outside row of the orchard and spreads quickly, moving more rapidly across rows than down rows. The pollinizer variety Black Republican seems particularly susceptible, Smith said. "I've seen it spread across an orchard in three to four years where they neglected to remove the trees." Hot spot It used to be thought that spread of the dis- ease was linked to the weather in the spring, but Smith said that in recent years it's continued to spread regardless of whether the spring was hot or cool. Many more orchards were found to be infected in 2014. The Wenatchee area seems to be the hot spot for the disease, although it has also been seen as far north as Tonasket and in southern parts of the state. Hundreds of acres of cherry orchards have been removed, but still more need to come out, Smith said. "This cannot be cured by any product or treatment. You have to pull the tree. "There are still people who think this is not happening and are not pulling out their trees. It's going to get worse in this industry before it gets better. There are hundreds of acres left to come out." He suggested that a coordinated effort to remove diseased trees might be needed. The virus can spread across highways, and if a grower removes an orchard and replants it, there's the risk of reinfection from a neighboring orchard. "Taking out an orchard and putting it back into a cherry orchard is a risky situation," he said. "We need to find out more about this dis- ease because it has the ability to spread to a new orchard from an existing orchard—or maybe it's in the soil. We can spend the next 10 years trading the virus back and forth or we can clean up the community." • Remove infected trees quickly However, when tested in 2014, the kit did not give true results, Bixby said. Retesting of the same leaf sam- ples with RT-PCR showed that the kit tended to give false negatives, particularly when used in early summer. When used later in the year, in August or September, it gave more accurate results. Eastwell hopes to address the problems this year. He is also working to develop quick tests that growers can use to test for Western X and little cherry virus 1. Samples analyzed last year showed that most infec- tions were little cherry virus 2, though a surprising num- ber turned out to be Western X disease, which seems to be making a resurgence in the northern half of the state. Little cherry virus 1 was never found alone, but only in combination with one of the two other viruses, Brosi reported. The kit, designed to pick up little cherry virus 2, cannot detect when Western X is present. Vectors Little cherry virus 2 is spread by mealybugs, as well as by grafting. There is no evidence the virus is spread by pruning tools. Mountain leafhoppers are vectors of Western X, but vectors of little cherry virus 1 have not been identified. Initially, it was thought that the apple mealybug (Phenacoccus aceris) was the culprit, but two years ago scientists were surprised to find that the grape mealybug (Pseudococcus maritimus) is also a vector. Researchers are looking at how to control mealybugs in cherry orchards in order to stop the spread of the disease. Grape mealybug, a common pear pest in Washington, is not normally controlled in cherry orchards because it doesn't affect the crop. Little is known about the apple mealybug in Washington State. The pest is thought to have originated in Europe and has a wide host range including nut and ornamental trees as well as many types of fruit. Brosi said she is using an apple mealybug infestation at a WSU research orchard to learn more about the insect. The apple and grape mealybugs have different life- cycles. Apple mealybug produces only one generation per year, whereas grape mealybug has at least two. The There is no cure for little cherry virus. Removing trees will help stop it spreading. by Geraldine Warner "It's going to get worse in this industry before it gets better." —Tim Smith