Ken Johnson

Ken Johnson

Scientists in western pear- and apple-producing regions are testing a product that has the potential to enhance control of fireblight and slow the run of the disease in infected trees.

Actigard has a totally different mode of action from the antibiotics generally used to fight fireblight and is nontoxic, according to Dr. Ken Johnson, plant pathologist with Oregon State University in Corvallis. It works by turning on a tree’s defense mechanisms so that it can better fight the fireblight pathogen, a process called ­systemic acquired resistance.

SAR happens naturally in plants when they are attacked by a pathogen. For example, cells around the site of a leaf infection produce salicylic acid, which signals to the plant to turn on its defenses so that the leaves anywhere on the tree are more resistant to secondary attack. The resistance is general and not specific to any type of bacterium, fungus, or virus. Actigard contains the active ingredient acibenzolar-S-methyl, an analog of salicylic acid, and can be applied to the entire tree. (Aspirin is closely related to salicylic acid.)

Actigard, manufactured by Syngenta, is available under an experimental use permit and expected to be registered in 2015. However, it will not be a stand-alone product, Johnson explained during the Washington State Horticultural Association’s annual meeting. It will be most cost-effective when combined with antibiotic applications during bloom. The application rate for Actigard is two ounces per acre.


In trials in 2011 with Gala apples that were inoculated with fireblight, the untreated control (water application) resulted in 78 strikes per tree, versus 13 for trees treated with streptomycin, 8 strikes per tree for those treated with streptomycin and one application of Actigard, and only 5 strikes per tree when streptomycin and two applications of Actigard were made.

The following year, Johnson did a similar study using oxytetracycline. Untreated trees had 215 strikes per tree, compared with 58 when oxytetracycline was used, and 30 or less when Actigard was used along with the antibiotic.

In trials with Bartlett pears last year, Actigard once more had an additive effect. “Compared to antibiotics alone, we’re getting about half as much control again,” Johnson said.
Trials in commercial orchards in Washington, Oregon, and California last season also showed benefits. Overall, two applications of Actigard on top of the growers’ regular fireblight program resulted in 37 percent fewer strikes than the grower program alone.

Clean up

Actigard also has potential to be applied as a paint to help clean up an infected orchard and reduce the run of the pathogen in the tree limbs.

Johnson has been testing the idea of painting Actigard onto branches after fireblight-infected wood has been pruned out. The product is applied about 12 to 18 inches below the cut in order to prevent reignition of fireblight and slow canker expansion.

For painting, the product is used at a rate of one ounce per quart to form a concentrated slurry and applied with a silicone surfactant, such as Pentabark. Johnson said that in tests over the past three years, the paint reduced the amount of diseased wood that developed in the trees and resulted in fewer reignited cuts.  He hopes to do more tests to determine the best rates and timings.

Johnson stressed that Actigard only works when fireblight symptoms are not apparent—either before infection or after infected wood with symptoms has been cut out. “It’s not going to work if the trees already have blight,” he said.
When used as a spray, the product has a residual life of perhaps 10 to 14 days, but the concentrated paint is probably effective for several weeks, he estimated.

Asked if the product could be applied through an irrigation system, Johnson said he has done some research using the product as a drench. When applied in the spring to wet soils it had little effect, but he felt it might have potential as a summer treatment. •