Stone fruit pathogens on the rise in Washington

WENATCHEE — As COVID-19 continues to plague Washington state, there are two other pathogens that are on the rise among stone fruits in the state. The only solution for both of these pathogens is the removal of the tree.

X-disease phytoplasma and little cherry virus have been around for many years. But the past few years have seen a drastic increase in the number of trees with the pathogens. According to Tianna DuPont, of the Tree Fruit Research and Extension Center in Wenatchee, in 2019, 7,000 samples were submitted for testing. Of those, approximately 38 percent of the samples tested positive for one of the pathogens.

“That is three times what it was the year before,” DuPont said.

X-disease phytoplasma and little cherry virus cause small, pale, deformed, bland fruit that is unmarketable to consumers. The foliage of the tree is not affected by the pathogens. While the symptoms of the pathogens are similar, they differ in how they spread and what kind of trees they infect.

X-disease phytoplasma is spread by leafhoppers and can affect all types of stone fruit. Little cherry virus only affects cherry trees and is spread by mealybugs. Both pathogens are spread when the respective insects feed on the tree.

“When insects feed, they can pick up the pathogen and spread it to the next tree,” DuPont said. “One of the challenges with these pathogens is that it can take one to three years for the symptoms to become noticeable after the tree is infected. It is important to see symptoms as soon as they’re there.”

Once a tree becomes infected with either pathogen, it will always be infected. Removing the tree is the only way to get rid of the pathogens. DuPont said that according to surveys, over 28,000 trees were removed in 2019 to combat the spread of the pathogens.

“We know that is an underestimate,” DuPont said. “The survey was not well responded to. Hundreds of acres have been removed.”

When trees have been removed, it is recommended that growers wait from one to three years before replanting to prevent infection of the new tree.

“If growers see a few trees, they can keep it under control,” DuPont said. “If 20 percent or more is infected, we recommend that they remove the whole block. It will be 50 percent infected the next year.”

According to DuPont, this is not the first infestation to take place in Washington state or the country or continent. The Kootenai Valley in Canada experienced a major epidemic in 1938. By 1979, the industry in the area was shut down. Washington experienced a major outbreak and epidemic in the 1940s and ’50s. California has also had major infestations of the pathogens.

With harvest nearing, DuPont said that growers should be checking and marking possible infected trees. Samples should be taken from the suspected trees and sent to the lab for testing. A list of labs can be found at treefruit.wsu.edu.

DuPont noted that X-disease phytoplasma and little cherry virus aren’t the only things that can cause issues in stone fruit. She said that it is important to take a look at the whole tree to determine if it is a pathogen, lack of water or nutrients, too little sunlight or another issue.

“Does it just have small fruit or is something else going on?” DuPont said.

DuPont also noted that it doesn’t matter the size of the orchard or the number of trees present. All stone fruit trees are susceptible to pathogens.

Rachal Pinkerton may be reached via email at rpinkerton@basinbusinessjournal.com.