—story by Kate Prengaman
—photo by TJ Mullinax
A recent survey of Pacific Northwest cherry orchards uncovered the presence of a new dieback pathogen in the region — one better known in California orchards.
“I was really excited,” said Gary Grove, professor emeritus of plant pathology at Washington State University, when he spoke about the Calosphearia pulchella detection during a panel discussion on cherry dieback pathogens at the Cherry Institute, which was held in Yakima in January.
Leave it to pathologists to be excited about never-before-seen fungal fruiting bodies. But Grove stressed that the new pathogen is just one of a handful of wood decay fungi that cause cankers and limb dieback, similar to those that growers have been contending with for decades. There’s no reason to think this new arrival in the region is more virulent.
“None of these are things to panic about,” he said. “They are very weak and slow-moving pathogens, but if they go unmanaged, they take away a limb at a time, and over time, there can be a yield loss.”
In 2023, Grove and his collaborators conducted the first survey in more than 20 years for these dieback pathogens in cherry orchards, prompted by industry reports of unfamiliar canker sightings. The Washington Tree Fruit Research Commission paid for it.
Two collaborators, Ashley Thompson of the Oregon State University Extension Service and Garrett Bishop with G.S. Long Co., joined Grove to speak at the Cherry Institute, an annual convention organized by Northwest Cherry Growers and the Washington State Fruit Commission (which publishes Good Fruit Grower).
For the survey, researchers collected samples at 70 orchards in 2023 and 16 more in 2024 across cherry growing regions in Oregon and Washington. They found cankers and dieback, either from fungal pathogens or bacterial canker, at most locations, Grove said. That makes sense, since the survey targeted orchards that reported cankers.
While bacterial canker is distinctive — “for lack of a better word, it’s messy,” Grove said — identifying most fungal pathogens requires taking samples to culture in the lab until the fungi form fruiting bodies that can be used to distinguish different species.
The most common fungal suspects they found were: Leucostoma cinctum, which was formerly known as Cytospora and the most common canker pathogen in Grove’s surveys 25 years ago; Eutypa lata, a common decay pathogen in many woody species; Calosphaeria pulchella, the new detection for the region, found primarily in Oregon and southern Washington orchards; and Botryosphaeria spp, common in North America.
“They are all wound pathogens,” Grove said. “A fungus will follow just about any kind of wound. … They all need a wound to enter into the plant; it’s not like powdery mildew or fire blight.”
That means stresses, such as drought stress, winter injury or borer damage, can set the stage for decay, panelists said.
Grove and his team also found more than 30 other fungi, likely secondary pathogens, that would require further research to determine identity and pathogenicity.
In-field identification of the four common fungal pathogens poses challenges, Grove said, unless telltale fruiting bodies are present. The cankers present as decaying wood, often in a wedge of wood.
Of the common suspects, do growers need to know the specific species causing their cankers? On one hand, no.
“One of the best things you can do is remove the diseased wood from the orchard,” Bishop said. “Get Typhoid Mary out of there.”
The recommendation for pruning out the cankers remains consistent: Cut branches back about a foot below the canker, Thompson said.
“The tree can be salvaged, depending on where the infection occurs,” she said. “If it’s in the trunk, you might need to remove the tree.”
But better identification and more research into the pathogen-specific life cycles could help growers in the long term. For example, California research has shown that pruning during colder temperatures reduces the rate of infection for some pathogens, but not others. That would need to be studied under Washington conditions for the primary pathogens here, Grove told Good Fruit Grower, and that research was not part of this initial survey.
All four common cherry canker-causing fungi, Leucostoma/Cytospora, Eutypa, Calosphaeria and Botryosphaeria, need water to kickstart their spore formation and to spread their spores around, Grove said. That means irrigation plays a huge role.
During research in the 1990s, looking at cytospora cankers in particular, Grove found that under-tree impact sprinklers were a contributor. “If we aimed it right on the canker, it could splash the spores across the tree rows,” he said.
These fungal pathogens aren’t introduced to the orchard via irrigation water — to clarify, that’s more of a Phytophthora issue — but they can be prevalent on other woody species in the orchard environment.
Panelists recommended lowering sprinkler angles, or moving to microsprinklers, and pruning out any cankers when possible, especially in young orchards.
“If (your trees) are less than three years old, it’s important to be really aggressive,” Bishop said. “Older trees generally can handle it, but some varieties are more susceptible.”
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