Scientists at Washington State University have teamed up with cherry growers to find improved defenses against a disease that devastated orchards 70 years ago and has resurfaced in the Pacific Northwest.
Infected trees, tiny cherries
Named for its unpalatable symptoms—small, insipid, colorless fruit—Little Cherry Disease encompasses several pathogens that infect sweet cherries and other stone fruit trees.
In Washington and Oregon, the main culprit is a bacterial pathogen called X-disease phytoplasma, which is spread in orchards by small insects called leafhoppers. Symptoms are usually noticed only a few weeks before harvest.
“You get no warning,” said Scott Harper, WSU virologist and director of the Clean Plant Center Northwest. “Everything looks fine, and suddenly things are not fine.”
Once infected, there is no remedy other than speedy removal of sick trees to slow transmission.
In the late 1940s and early 1950s, Washington cherry growers experienced a major outbreak of Little Cherry Disease. They managed to quell it through aggressive destruction of infected trees. A smaller outbreak followed in the 1980s.
Now, the cycle is repeating. Reports of Little Cherry began rising in 2017, and today it is again a significant concern for Northwest orchards.
Scientists at WSU and Oregon State University, cherry growers and processors, and other partners formed the Little Cherry Disease Task Force in 2018 to coordinate research in response to the syndrome. Tobin Northfield, an entomologist based at the WSU Tree Fruit Research and Extension Center in Wenatchee, chairs the task force.
“If we do nothing, this disease will have a huge impact on our ability to grow cherries,” Northfield said. “We’re trying to find effective controls as quickly as we can.”
This summer, Northfield and fellow task force members launched several experiments aimed at halting the disease’s spread. Funded by the Washington Tree Fruit Research Commission, task force members are working closely with growers on research.
Pathogen hides in plant hosts
Building on discoveries made during past outbreaks, scientists are seeking to identify important plants in the environment that harbor the Little Cherry pathogen.
While leafhoppers and annual plants die off every winter, perennial plants don’t.
“That’s why removing infected trees is so critical—they link one year to the next,” Northfield said. “But perennial weeds may also be doing the same thing.”
Task Force scientists are performing food choice tests on captive leafhoppers to learn which plants they prefer. Results could help orchardists remove plants and weeds that harbor the pathogen.
Baffling and blocking ‘hoppers
In the field, Northfield is testing sprays of kaolin clay as a leafhopper deterrent. Sprayed on cherries, clay repels insects and protects against heat.
“When you spray plants with clay, some insects don’t see them as food,” Northfield said. “Leafhoppers are very visual insects, and the clay works as a camouflage. We may be able to convince them to feed on something else.”
Some growers use clay during summer months to protect trees from heat, which can cause cherries to be misshapen the following year. Phytoplasma levels rise after harvest, so clay application after harvest could also prevent disease transmission at a critical time. It also remains on trees longer than conventional insecticide, so it’s more affordable for growers in the long run.
Northfield is also working with growers to test white plastic ground cover as a leafhopper-stopping barrier.
The plastic cover may eliminate potential habitat for insects and pathogens, while retaining moisture and reflecting sunlight that can help lighter-colored cherry varieties, such as Rainier, develop an attractive blush color. Because insects rely on ultraviolet light to orient themselves, the reflection may also make it harder for pesky leafhoppers to fly over it.
“By masking that ground cover, we’re hoping to block the avenues they use to infest orchards,” Northfield said.
His team is trapping and counting leafhoppers at the ground and tree level to gauge the techniques’ impact.
“If the clay works, we may have similar numbers in ground cover, but not in the trees,” he explained.
Studying varieties, resistance
At Prosser, Harper and WSU cherry breeder Per McCord are analyzing the severity of the disease and its impacts on commercial orchards, while studying how Little Cherry pathogens cause the disease at the genetic level. They are also watching to see how a range of cherry varieties hold up against the disease.
“Coming off a 40-year research drought, we don’t know a lot about how the disease affects different cherry cultivars,” Harper said.
The Prosser researchers are currently preparing to plant an acre-sized test orchard with varied cultivars to study genetic resistance to Little Cherry. Plant pathologists plan to inoculate different trees with both the phytoplasma and the viral forms of the Little Cherry pathogen, and study how they create symptoms. Their work could eventually help breed sweet cherries that resist the disease.
The varied projects draw together growers, scientists, industry experts, and WSU Extension outreach specialists.
“Little Cherry Disease is a complicated problem,” Northfield said. “An entomologist or a plant pathologist can’t handle it on their own.
“That’s the advantage of bringing the task force together,” he added. “We can beat this disease, but it’s going to take a lot of effort by everyone involved to protect Northwest cherries and keep them on our plates.”