Warmer temperatures stoke damage done by fire blight

Until recently, fire blight, a deadly disease in pear and apple trees, was only considered a problem in southern areas of the country.

Now, however, it threatens northern orchards and is causing millions of dollars in agricultural losses in the United States.

Erwinia amylovora, the bacteria that causes fire blight, is native to North America.

The bacteria is harmless if the temperature remains under 75 degrees when susceptible trees are in flower.

But in warmer weather, the spores germinate and use the blossoms to enter the tree’s vascular system.

Unless the infected branches are removed, the blight spreads through the tree.

Once it reaches the roots, the tree dies.

Damp conditions during flowering accelerate the spread of the disease. Trees can die in a single year.

Pears are the most vulnerable to fire blight, but today alarms are sounding about fire blight in apple orchards.

In recent years, the blight has been found in states like Massachusetts and Maine, which historically have been fire blight-free. With more erratic weather patterns, however, apple blossoming times are seeing temperatures in the 70s and even 80s.

The results have been catastrophic as pollinators and high winds during rain storms spread the bacteria.

New cultivation techniques are also increasing fire blight’s impact. Instead of growing old-fashioned, tall apple trees that produce hundreds of apples of various grades, today orchards use the “trellis system,” which makes orchards resemble vineyards.

Trees are pruned to make them an easy-to-harvest at 8 feet tall but very wide.

They’re planted about 2 feet apart and may only produce 50 apples per tree, but all of the harvest is premium grade.

The high number of trees that can be crammed on an acre produce many more apples of higher quality than the old methods.

But the density of the orchards makes it easy for fire blight to spread.

In the past, fire blight has been treated with the human antibiotic streptomycin, but the disease has become increasingly resistant to it.

Several heirloom orchards have been lost to the blight.

Tower Hill Botanic Garden in Boylston, Mass., which has the oldest apple varieties in the country, took scion stock from their collection and grafted new trees before destroying the original collection.

They will be used to replant the orchard once the blight is gone.

Other apple collections are being preserved solely as DNA in liquid nitrogen in Fort Collins, Colo.

The U.S. Department of Agriculture also maintains a collection of 6,000 wild and cultivated apples and is researching varieties that may have natural resistance to fire blight.

With even warmer temperatures predicted, commercial growers and home gardeners can expect increasing numbers of insect, fungal and bacterial pests to expand their ranges.

We’re already seeing that with diseases in beech and oak trees usually found only in southern populations appearing in northern states like Wisconsin.

Insects like gypsy moths and Japanese beetles are also more common as the sub-zero temperatures needed to kill their larvae disappear.