Rescue treatment for fireblight studied
Painting Actigard onto cut branches seems to boost the tree’s defenses against the disease.
Cutting out fireblight strikes and painting the branches with Actigard, a systemic acquired resistance (SAR) inducer, slows the spread of fireblight in pear trees, research suggests.
Dr. Ken Johnson, plant pathologist with Oregon State University, believes that the painting treatment has promise as a rescue tool to reduce the incidence or severity of reignited cankers.
In field experiments, trees were inoculated with fireblight, and, after running cankers were established in the trees, he cut the branches about two inches below the cankers and painted Actigard (acibenzolar-S-methyl) down the the limbs for 12 to 20 inches using a sponge brush. This treatment consistently slowed the run of the disease in the tree.
Johnson said this approach seems promising and will be the focus of his research in the future.
“We’re getting blight to settle down in the trees faster than if we didn’t put paint on those branches,” he reported to the Washington Tree Fruit Research Commission, which is helping to fund the research. “The bacteria are already in there in many cases when you cut—especially if you short cut—but at a low level, and they’re not causing disease at that point. If you can get the shields up inside the plant, you can perhaps fend off infection.”
He believes the Actigard doesn’t have a direct effect on the fireblight pathogen but, rather, encourages the plant to turn on its defense genes.
For his research, Johnson used only short cuts—cutting two inches below the margin of the canker—because he wanted to study how much fireblight came back. He’d like to see how it performs with more realistic cuts.
Actigard is not registered for that use, but the registrant Syngenta is interested in making it available for controlling summer shoot blight, he said. He hopes to obtain an Experimental Use Permit without a crop destruct requirement to do further research in large commercial orchards where the trees are properly pruned and the larger scale of the planting would increase the number of cuts that receive the painting treatment.
Johnson also tested Actigard as a soil drench. In greenhouse experiments, drenches slowed the expansion of fireblight cankers. For example, in control trees, the canker expanded from the shoot tip to halfway down the trunk, on average. Cankers on trees drenched either before or at the time of inoculation, expanded only a short distance into the woody trunk tissue, and the trees continued to produce new shoot growth.
But the soil drench treatment did not work in the field. Johnson said the confined root system in the pot might allow for more efficient uptake of the product than in the field, where it was placed only at the base of the tree.
Results with sprays of Actigard were inconsistent from year to year.