Summer Management for Fire Blight

We've been getting calls this summer from gardeners finding fire blight damage in backyards and landscapes. Fire blight is usually associated with wet springs. Although spring 2014 wasn't particularly wet, rain occurring when apples, pears, quince, cotoneaster, and pyracantha are in bloom can induce the disease even in dry years.

The malady is called fire blight because terminals of affected branches suddenly blacken and die as if they've been scorched by fire (Figure 1). The disease is caused by the bacterium Erwinia amylovora, which commonly enters trees and shrubs through blossoms in rainy weather. Ideal conditions for disease development are rainy or humid weather with mild daytime temperatures (75° to 85°F), especially when night temperatures stay above 55°F.

In late spring or early summer after blossoms are gone, damage symptoms appear on new tree growth. By this time, it is too late to effectively use pesticide sprays. However, summertime is a good time to prune rapidly advancing infections out of trees.  Pruning is the only way to remove infections in old wood and is a critical part of a fire blight management program. If you choose to prune in summer, disinfect pruners with a 10% bleach solution to reduce chances of further spreading the disease. [In rainy years wait until all chance of rain has gone.]

All discolored tissue plus healthy wood at least 8 inches below the damage must be removed (Figure 2). It is important to remove infected wood because it can be a source of new infections in subsequent years (Figure 3). Infections can also be pruned out in the dormant season when less extensive damage can be more easily detected. Inspect trees for scars left by fire blight and also prune out infected wood 8 inches below darkened areas in wood.

When planting new trees, those who live in fire blight-prone areas may wish to consider apple or ornamental pear varieties that are less susceptible; most edible pear varieties are quite susceptible. Varieties of ornamental pear trees that are less susceptible to fire blight are Capitol and Red Spire, while

Aristocrat is highly susceptible. Among the more susceptible apple varieties are Fuji, Gala, Golden Delicious, Granny Smith, Gravenstein, Jonathan, Mutsu, Pink Lady, and Yellow Newtown. Also, too much nitrogen can increase fire blight, so be careful not to overfertilize.

For more information about managing fire blight and what to do in the spring, see Pest Notes: Fireblight at http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7414.html.

 

Modified from “Wet Weather Increases Outbreaks of Fire Blight in Apple and Pear Trees” from the Retail Nursery and Garden Center IPM Newsletter, Vol. 3, No. 1, March 2013.