The Blight Stuff

By Dennis Hinkamp

It’s time for evaluations. Looking back on the growing season one problem that was worse than in the past was coryneum blight on peaches and apricots.

The common name for this disease is shot-hole disease. Its name comes from the appearance of infected leaves of some hosts, says Jerry Goodspeed, Utah State University Extension horticulturist. Holes left in the leaves make them look as if they have been shot with a shotgun.

Besides the leaves, coryneum blight also infects the twigs, branches, buds and fruit, Goodspeed explains. The holes in the leaves are the most noticeable, but the infections on the wood and buds inflict the most damage. Infected twigs and buds turn purple or brown, appear somewhat sunken and can be gummy. As the infection spreads it can girdle the twig, killing the wood beyond the infection. Fruit infected with this disease normally has small brown spots about the size of a freckle. Although the infected fruit can still be used, it looks bad and is often discarded. Most of the infections on the fruit are only skin deep and can be cut out and removed so the fruit can still be eaten.

So, why worry now about a disease that has been around all year, and about fruit that has been harvested months ago?

“Control for next year is dependent on treatments made this fall,” he says. “A couple of things can be done now that will help if we have another wet spring and early summer.

The first thing to do is to clean up all fallen fruit and leaves, Goodspeed says. Sanitation is always a good idea, regardless of the disease you are treating. Many diseases over-winter in piled debris and other materials left on the ground.

The most important activity is to spray the tree with a registered fungicide right after leaf drop, he says. This protects the buds against infection. A registered fungicide will list the tree you are spraying on the label and claim to control coryneum blight. Daconil is one fungicide that is registered for use on peach and apricot trees. When using any pesticide, be sure to read and follow all label directions.

There are other diseases that require preventative measures now, in order to reduce spring infections, he adds. Go to your trees and landscape and look for limbs or twigs that are infested or are dead. They can and should be removed from the landscape.

One sign that a limb or twig is diseased or dead, is that particular section of leaves remain on the branch after the rest of the leaves have fallen off the tree, Goodspeed says. Leaf drop is an energy using process for the tree. When a branch is sick or dead this process does not take place and the leaves hang onto the tree well into the winter.

“Be careful not to cut plants and branches that normally hold their leaves long into the winter,” he says. “Roses and beech trees are examples of plants that keep them after all the other plants have dropped their leaves. But, don't cut the tree down or remove the rose.

“Pick up and remove diseased leaves. Aspen and oak leaves especially need to be removed if they had leaf spots this year. Maples are another tree that has a couple of fungal diseases, so the leaves should be raked up and thrown away or burned.”

Accomplishing these preventative measures this fall will not guarantee that the trees won't have problems next year, but it will reduce the likelihood, he says.

For more information, contact your local USU County Extension office.
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Issued in furtherance of Cooperative Extension work, Acts of May 9 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert L. Gilliland, Vice-President and Director, Cooperative Extension Service, Utah State University, Logan, Utah. (EP/11-99/DF)