Dutch elm disease is still fairly common in Illinois. It is not unusual for the Plant Clinic staff to isolate this fungus from elm wood on a weekly basis (see issue no. 7). There are still cases of elms that appear to have Dutch elm disease but do not yield the causal fungus. Another look-alike disease to consider is elm yellows, also known as elm phloem necrosis. The Plant Clinic cannot test for the presence of this disease. Read through the symptoms that follow. If you need laboratory confirmation, some “how to” details are included.
Symptoms of elm yellows may appear anytime during the summer but are most common in mid- to late summer. Look for symptoms now: yellowing and drooping of foliage, followed by leaf drop and death of branches. This pattern may occur on one or a few branches or may quickly involve the entire tree. Susceptible trees may show symptoms over the entire tree in a few weeks. Tolerant trees become stunted and may develop bunchy, prolific growth at the tips of branches (another example of witches’-brooms) or on the trunk. The inner bark tissues of infected trees often exhibit a butterscotch or light brown discoloration in small streaks or flecks. Although trees with Dutch elm disease usually show vascular discoloration in symptomatic branches, the discoloration from elm yellows is not usually in the branches--it is more commonly found in the trunk. A simple field test to help with diagnosis of this disease involves placing a few chips of the stained phloem tissue in a closed container for a few minutes and checking for a wintergreen odor.
Elm yellows disease is caused by a phytoplasma. These pathogens are bacteria-like organisms that have no cell wall, are too small to be seen with a compound microscope, and cannot be cultured in plant diagnostic labs. Confirmation usually involves extraction of DNA from a diseased plant, amplification of a DNA fragment by polymerase chain reaction (PCR), and identification of the fragment. Such procedures are available at a high cost due to labor and equipment needs. Some specialty labs, such as AGDIA, Inc., offer this service. Generally, diagnosis is based on symptoms in the field and elimination of Dutch elm disease as a possibility. For this reason, no confirmed case of elm yellows has been reported by the University of Illinois Plant Clinic; but confirmation has come from several knowledgeable tree specialists in the state.
There is no cure for elm yellows; and infected trees usually die within a year or two. The good news is that elm yellows disease does not move into new areas as quickly as Dutch elm disease. Removal of infected trees is advised to remove inoculum sources from the area. Siberian elm seems to be resistant to this disease problem. For more information about this disease, consult Report on Plant Disease, no. 660, or the book Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson.