No one wants fire blight in their orchard this spring. For fire blight to take off in your orchard this year three things need to be there: the pathogen, the warm conditions and humidity that favor the disease, and a susceptible host. We can’t do much to change the weather, but we can remove the source of the disease in our orchards. I am sure you already pruned for fire blight in your orchard recently. However, it is worth the time to take another look through your blocks to remove additional cankers!
Where does Fire Blight Come from?
Erwinia amylovora, the fire blight pathogen, overwinters in cankers in the orchard. Bacteria overwinter in living tissue surrounding cankers formed at the base of spurs or shoots killed the previous season. Cankers will also form where cuts were made to remove infected shoots during the growing season.
Cells of the Erwinia pathogen survive primarily in the canker margins where diseased bark tissue meets healthy bark tissue. Frequently, the pathogen inside many of these cankers dies out over the course of the winter, but in 20% to 50% of cankers active cells of the pathogen survive until the next bloom period. In spring, during periods of high humidity, the pathogen oozes out of the canker margins. This ooze is attractive to insects (e.g., flies) as a food source who then move the infectious ooze to the flowers.
What do Fire Blight Cankers Look Like?
Cankers are areas of dead tissue. There are other types of cankers, but fire blight cankers are reasonably easy to identify. They are greyish, lavender-ish, and sometimes almost black. The tissue may be somewhat sunken and cracked. The cankers are associated with shoots that were killed last year. In the tissue of young shoots, the blight moves quickly through the tissue and down to a larger stem.
It is best to prune the cankers before the tree is shaped for structure, and remove the blighted prunings from the orchard as they can be a source of pathogen cells in spring. Compared to cuts made in summer, winter removal cuts can be made closer to the visible canker edge because the pathogen is confined to the cankered area. Cut at the next “horticulturally sensible” site below the canker. Focus your efforts in blocks where you had fire blight last year. But after a year like this one it is best to check all of your blocks.
WSU Extension Specialist, Tree Fruit
(509) 663-8181 ext 211