Fire blight is a bacterial disease of apple, crabapple, pear, hawthorn and other related species like cotoneaster. It causes branch dieback and can lead to tree or shrub death.
We have seen an increase in fire blight in recent years. This is due to environmental conditions being conducive for infection and because weather extremes or hail damage stress trees and increase their susceptibility.
An important factor in reducing the spread of bacteria is pruning at the right time and making correct pruning cuts. Since we are in the pruning season, here are tips for preventing and managing fireblight.
The bacteria overwinters in branch cankers which appear as discolored, sunken or cracked areas on branches. Cankers produce a sticky exudate in spring. This is when bacteria is easily spread by insects, wind-blown rain, careless pruning or pruning at the wrong time.
When pruning susceptible trees or shrubs, especially any known to have fire blight, avoid pruning close to or during spring growth. This is when cankers are actively exuding bacteria. Pruning is best done while infected and susceptible trees are fully dormant.
Along with dormant pruning, make correct pruning cuts. These are cuts made where a branch attaches to another branch or the trunk, so a branch stub is not left after pruning.
While it’s important not to leave a stub, it’s equally important not to make a flush cut or one that is too close to the trunk or another branch. This creates a larger wound that will be slower in developing callus growth to seal the cut.
Where a branch attaches to the trunk or another branch, there is a branch bark ridge and a branch collar. The ridge is a rough area on top of the branch and the collar is a slightly swollen area beneath the branch. Pruning cuts on any tree should be made just outside the ridge and collar.
The ridge and collar are part of the trunk or main branch and it is beneficial not to remove these when pruning. If you make a correct cut, there will be an almost perfect donut of callus growth as wound wood begins to grow and seal the cut.
If a tree has fireblight, inspect it for cankers; the sunken, discolored or cracked areas on branches. If found, remove branches at least eight inches below any cankers; or ideally at the point where the branch attaches to another branch or the trunk. Do not prune into a canker.
To avoid spreading bacteria on pruning tools within a tree or between trees, dip or spray the tool between each cut with a 10 percent bleach solution, which is one part bleach to nine parts water.
Any pruned material should be removed from the area or the bacteria may still spread from cankers.
The most effective way to prevent fireblight is to purchase trees or shrubs not susceptible to the disease; or purchase fireblight resistant cultivars of apple, crabapple, pear and other species.
Reducing stress by not fertilizing trees too much, providing adequate water during dry periods, and using a four foot diameter ring of organic mulch around the base of the tree, will also reduce the risk of fireblight.
Kelly Feehan is a community environment educator for Nebraska Extension-Platte County.