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Bacterial canker is the leading killer of young sweet cherry trees in Oregon, and an Oregon State University researcher is exploring the role winter damage and acclimation play in establishing infections, as well as whether plant growth regulators and defoliants can help to prevent them.

A Sweetheart cherry tree on Mazzard rootstock shows signs of bacterial canker infection in The Dalles, Oregon. <b>(Courtesy Drew Hubbard)</b>

A Sweetheart cherry tree on Mazzard rootstock shows signs of bacterial canker infection in The Dalles, Oregon. (Courtesy Drew Hubbard)

The bacteria (Pseudomonas syringae) live everywhere, including on the surfaces of weeds and grasses commonly found in orchards, and have adapted to hide on the plant.

Young trees show symptoms when they are under stress, such as drought, low temperatures, pruning cuts, heading wounds or root injuries.

Understanding how stresses affect the survival of bacteria is key because control is so challenging: The bacteria hide well in the crevices of a tree, and any that are missed in a bactericide application emerge again on the next warm day to replace those bacteria just wiped out.

OSU doctoral student Drew Hubbard, who also works as a crop consultant for growers in Oregon’s Mid-Columbia region, aims to meet a few objectives:

—Examine the role of acclimation and induced early winter damage on infection by the bacteria and subsequent bacterial canker formation.

—Evaluate plant growth regulators for their ability to induce defoliation and increased cold hardiness.

—Evaluate the effects of defoliating compounds on nutrient remobilization and tissue content during dormancy and early spring development.

Hubbard hopes to have final results from his three-year study, funded with $44,500 from the Oregon Sweet Cherry Commission, sometime next year. In the meantime, he shared his initial findings with Good Fruit Grower.

Acclimation and dormancy

Overall, bacterial canker is not a primary pest; it’s an opportunistic pathogen. Any injury or damage to the tree serves as an open door, if you will, and removing entry points can reduce infections.

“Bacterial canker is a ubiquitous bacterium in the environment,” Hubbard said. And when growers east of the Cascades in Oregon and Washington have warm, damp fall weather, the population surges. “You have bacteria growing everywhere.”

Winter damage can serve as a key entry point; the benefits of acclimating the trees and sending them into dormancy earlier could be twofold, preventing both freeze damage and potential for infection.

Hubbard received Gisela 6 cherry rootstocks for the project in fall 2015, separating them into three groups for different acclimation treatments: naturally acclimated under ambient, outdoor conditions; non-acclimated in a greenhouse; and artificially induced to acclimate by exposing plants to low nighttime temperatures within a cold storage unit and moving them outdoors during the day.

After sufficient cooling was achieved, Hubbard then divided the plants equally into inoculated and non-inoculated treatments, which were carried out prior to exposure to freezing.

Overall, he said he’s found that clean plants, meaning those that have not been inoculated, grow back healthier regardless of acclimation type, “which shouldn’t come as a big surprise.”

He received Mazzard rootstocks in 2016 to repeat the experiments but is still awaiting final results.
PGRs and defoliants

In a grower collaborator orchard, Hubbard also treated trees with plant growth regulators and defoliants to examine defoliation and bud hardiness under different treatments.

By initiating the removal of foliage or making the plants go into dormancy more quickly, they could be “sealed up” for protection; removing foliage also removes a good source of inoculum that infects the tree.

Overall, Hubbard found that all of the defoliants he worked with worked well — ABA (abscisic acid), ACC (1-aminocycloproponae-1-carboxylic acid) and lime sulfur — but there is a trigger that will help the products work better: a light frost. “The first frost we get is enough to start the cascade into dormancy. It accelerates the movement into dormancy.”

There are two components at work, he said. One, defoliation removes habitat for the bacteria. Two, trees that are fully dormant are less likely to be devastated by a hard freeze because they weren’t acclimated to it yet.

“If we could increase cold hardiness early, then the damage of a freeze like that would be greatly subsided,” he said. “The better you set your tree up to survive hard freeze damage, the better prepared it is to ward off canker.”

Defoliating trees is nothing new, but one problem is that if growers just knock all the leaves off, it cuts off the natural process of the tree bringing nutrients in through the leaves.

For that reason, Hubbard also is looking at the role these compounds play in the remobilization of key nutrients, zinc, boron and nitrogen.

He is measuring nutrients being brought from the leaves back into the tree, and in early results, found that treatments that included ABA seemed to be accelerating remobilization, “which is a good thing.”

Other treatments, while they worked as defoliants, did not aid remobilization because they simply defoliated the tree too quickly to stimulate remobilization.

Hubbard is continuing his research for another year. •

– by Shannon Dininny