by Steve Turner

I have a large elm tree in my yard and on one branch the leaves are turning yellow and wilting. Does this mean it has Dutch elm disease? If so, is there anything I can do to save it and what can I do to help prevent my other elms from catching it?

From the symptoms you have described, it does sound like your tree has signs of Dutch elm disease. The yellowing and wilting is called flagging and is one of the first signs of infection. The water-conducting cells in that limb are becoming blocked and the tree can’t supply enough water to keep up with the transpiration (water evaporating from the leaves) in that part of the tree. So, the leaves turn yellow, wilt and then turn brown or drop.

A good way to explain Dutch elm disease, which is a fungus, is to compare it to cholesterol in humans. If your arteries become so blocked that blood cannot pass through them, the organ that is being supplied will fail. The same thing applies with a tree — if the water-conducting tissues become blocked by a fungus build-up, then that portion of the tree will die. All this can happen very quickly under the right conditions.

As far as saving your tree goes, that gets a little more complicated. Dutch elm disease is very hard to control, but it has been done. If the tree is flagging in 5 percent or less of the crown then there is some hope. The first step is to sterile prune out the infected limbs back to healthy wood and then do trunk injections with a systemic elm fungicide at a curative rate. There are several brands on the market. In my experience, I have found Arbortech and Alamo to work the best. Then water, water, water! Your mature tree needs a lot of water since its ability to transport water has diminished. This is especially true during the summer heat which causes the tree to transpire at a much higher rate than in cooler weather. It will need a constant supply to keep up with the increased demand. This is why your tree looked fine in the spring but is now showing the signs of infection. Chances are, your tree has been infected for a while but up until now has not faced a strong demand for water.

In regards to your other elm, there are three ways that elms become infected with Dutch elm disease: 1) by elm bark beetles feeding on diseased and then healthy elms, 2) through pruning with contaminated saws and loppers, and 3) through root grafts connecting elms together by their root systems below the ground. The first way is the main cause of the problem, but the second happens more often than people realize. Look at elms near power lines that have been pruned in the last couple of years; many of them are either dead or infected because the fungus was transferred from one to another. The third is common in neighborhoods with smaller yards or where the elms are planted closely. When one elm becomes infected, the others around it can start dropping like flies. In your case, this is probably the biggest threat to your other elm.

I recommend that owners of large, specimen elms have their tree injected every other year or every year if other trees in your neighborhood are infected. The cost of the injections is less expensive than the removal of a large, mature tree, which can cost up to several thousand dollars depending on the size and location of the tree. The bottom line is to keep your elm in good health through proper watering, fertilizing, pruning, and, on large trees, injecting. Also, to increase the chances for a prize elm’s survival, you may want to remove any small elms in the area, as they seem more prone to infection, especially if they are near power lines and subject to pruning. For those of you who would like to plant an elm, the American Elm Society (800-FOR-ELMS) has a disease resistant variety called the Liberty Elm. They are, however, hard to find in larger planting sizes, but free saplings can be obtained by joining the society, a small price to pay for the beauty of the great American elm.