WICHE: Fire blight in ornamental pears

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By Jeneen Wiche

This past winter we cut down a handful of old apple trees in the orchard because they were so diseased from fire blight. Two large White House ornamental pears and a few crabapples are slated for removal, as well.

I have noticed additional cases around town that brings us to the question, what is causing so much die-back in ornamental and orchard tress?

Every couple of years we see an increased incidence of a disease called fire blight on ornamental and fruit trees including apple, crabapple, serviceberry, pear, plum and cherry.

Fire blight is a disease caused by a bacterium that over winters in sunken cankers on the tree: once temperatures warm in the spring the bacterium oozes out of the canker, insects land on the sticky substance and spread it with efficiency as they travel from blossom to blossom.           

In a typical year fire blight is favored by warm weather: warm, cold, wet and warm again is the trend thus far. Though the fire blight incidence is predicted to be high this year this does not explain the existing decline of trees.

The damage has been building for several years, in fact.

In 2004 and 2005 we had two different 17-year cicada brood emergences; and in 2007 we had a nearly record freeze in late April.

That year, in fact, we had an early warm up so trees bloomed early; then the freeze hit, which caused frost, cracks within the blooms and along stems (the cicada egg-laying cycle created similar damage to twigs).

Freeze damage and cicada egg-laying scars opened up the tree which allowed the bacterium to gain access into the stems. Last year it rained all spring and rain and warm temperatures facilitate the spread further.

Get the picture?

University of Kentucky plant pathologist Dr. John Hartman warns the risk of fire blight is high this year, too, favored by our early warm, wet spring.

The symptoms will begin to show in June, and if you have trees that were previously infected there may not be much left by the end of the season!

This is why we removed the old apple trees, they literally had big black lesions on their trunks, far beyond the typical scorched branch tip scenario.

More than 50 percent of our White House pears’ canopy is black. That’s why they are on the chopping block.

Symptoms of fire blight include sudden wilting of blossoms, leaves and twigs that curl under and hang down. The most distinguishing symptom, however, is that the twigs turn black as if they were scorched by fire.

This is an important symptom to check because it will help you determine the difference between flagging (hanging dead stem tips) caused by cicada or frost damage, for example.

Frost damage will include blackened leaves and a straight fracture along the branch; fire blight appears scorched with a sunken, water-soak area (the canker).

If you notice minimal signs of infection, you can prune out the scorched branches as they appear but be very careful with your pruning technique.

Our tools and hands can also spread the disease so use rubbing alcohol to clean your pruners after each cut. Remove the infected branches about 12 inches above the damaged plant tissue (that part which is blackened) and destroy it.

Trees with a high rate of infection should not be pruned until fall, after the tree goes dormant.

These cuts can be made about 6 inches above the infected area. If significantly infected trees are pruned now, it will encourage new, succulent growth that requires higher sugar concentrations.

This will take energy away from the trees ability to fight the existing infection and you are more likely to spread the disease further through the pruning process.

If you see lesions on major branches or the trunk, or 50% of the tree is infected you should consider removal.