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Flowers, Fruits, and Frass

Fire Blight by Travis Cleveland

Fire blight symptoms were observed on several Callery pears this past week,” says Travis Cleveland, University of Illinois extension specialist.

“The symptoms were more severe than those observed during the 2013 growing season.”

Fire blight is a bacterial disease that affects rosaceous plants. Apples, pears, crabapples, and ornamental pears are the most seriously affected species. Other rosaceous hosts include: cotoneaster, hawthorn, quince, firethorn, and mountain-ash.

On Callery pear, look for water-soaked or wilted new growth that quickly turns black and remains attached to the stem. Affected foliage is usually at the branch tip.  Symptoms will look similar to frost injury. The affected foliage on other host species usually turns brown.  Dark cankers also develop in the wood of infected stems and branches.

Fire blight is caused by a bacterium (Erwinia amylovora). The pathogen overwinters in living tissue at the margins of trunk and branch cankers that were formed by infections initiated in previous years. The disease can cause numerous cankers on a single tree.

Not all cankers survive the winter, but the few that persist produce millions of bacteria capable of causing new infections. Rain or insects may move the bacterium from cankers to open blossoms, vigorous shoot tips, and leaves. Fire blight outbreaks sometimes occur following severe storms. Gusty winds and hail wound the trees creating an entry point for the pathogen. The bulk of infections occur during flowering when temperatures are warm (optimal 76°F) and conditions are wet. These conditions also encourage rapid disease development.

An important step to controlling fire blight is the selection of resistant cultivars such as Cleveland Select or Bradford. Avoid highly susceptible cultivars such as Aristocrat, Autumn Blaze, and Red Spire.

There is no effective management option for infected trees. Prune out infected wood in the dormant season, if you can wait. If not, prune in an extended dry period and disinfect pruning tools after every cut. The bacterium may have extended down the stem ahead of the canker. Unfortunately this means wood should be removed 8-10 inches below the edge of the visible canker.

Chemical options are limited, especially for home growers and the timing of sprays are also critical. Commercial growers apply copper products in the dormant season and streptomycin at 4-5 day intervals throughout bloom. Fertilization and watering are not recommended. Such practices will promote lush growth, which is more susceptible to infection by the fire blight bacterium.

Source: Travis Cleveland, Visiting Extension Specialist, PSEP,