Fire blight running rampant on pear trees

By Marianne C. Ophardt, Special to the Tri-City Herald June 8, 2012 

KENNEWICK -- Plant diseases are rife in many local yards and gardens this spring.

Our usually dry local climate is not conducive to most fungal and bacterial plant diseases.

However, the abnormally wet spring and mild weather this year has led to some diseases running rampant. Of major concern is fire blight on flowering pear, crabapple and pears trees.

Fire blight is a bacterial disease that commonly attacks pear, flowering pear, apple and crabapple, but other members of the rose family also are susceptible. This includes cotoneaster, mountain ash, flowering quince, pyracantha, hawthorn and many others.

The first hint that something is wrong is the blackening of twigs and branches. Infected twig tips will droop and bend over to form a "shepherd's crook." Fruit that started to develop shrivel and turn black. A closer inspection reveals discolored and blackened bark that moves downward in the twig and branches as the disease progresses.

Fire blight is caused by the bacterium (Erwinia amylovora). It sits quietly during the winter in cankers on host plants just waiting for the right conditions: warm (65 to 80 degrees) moist weather.

As the weather warms, the bacteria multiply and form a gummy soup of bacteria that oozes out of cankers and infected tissues. This bacterial ooze spreads to uninfected tissues via insects and water.

Unlike fungi, which can make their own way into plant tissues, bacterial diseases such as fire blight must find an opening for entry. Frequently fire blight enters through the natural openings in blossoms, getting there on the bodies of insects such as bees, aphids and ants or via splashing rain or irrigation water. However, the entryway of infection isn't limited to just blossoms. The bacteria also enters through wounds caused by pruning, hail and insects and through other natural plant openings.

Severe infections occur when the conditions are just right and the host plants are in full bloom. This year's "perfect storm" for fire blight happened when flowering pears and crabapples were in bloom.

As soon as fire blight is noticed on a plant, the infected tissues should be pruned out, making pruning cuts well below the visibly infected tissues. How far below? Pruning cuts should be made 12 to 15 inches below any visible signs of the disease. Because the blades of pruners can pick up bacteria and spread it to other parts of the plant, pruning tools should be sterilized after every cut. This is done by soaking the tools in Lysol or a 10 per cent bleach solution.

(Note: Bleach is corrosive.)

If you're successful in saving your tree, you should protect it next year by applying a home garden copper fungicide spray when it's in full bloom.

(Note: Copper sprays will cause russeting on the fruit of Anjou, Comice and Forelle pears.)

There are also some cultural steps you can take to manage fire blight. The disease favors succulent plant tissues, so avoid stimulating excess shoot and sucker growth with heavy applications of fertilizer and water.

Heavy or severe pruning during the dormant season also stimulates vigorous growth, so limit major pruning cuts. Since rain and irrigation water can spread the disease, try to keep sprinklers from hitting the tree.

Some varieties of apple, pear, crabapple and flowering pear are more susceptible to fire blight than others. When purchasing these plants, check with your local nursery regarding a variety's susceptibility to the disease.

* Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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