Last week, we discussed Dutch elm disease, the fungal vascular disease that killed many American elms in the United States starting in the 1950s. Dutch elm disease is still with us, but a more current concern about elms is a disease called elm yellows, also known as phloem necrosis. It is caused by a phytoplasma (type of pathogen) that is only found in the phloem tissue. This fact, along with the fact that the infected phloem turns brown, gives the disease the alternate name of phloem necrosis. Because elm yellows and Dutch elm disease can appear similar, it is important to know the differences.
Symptoms of elm yellows may appear any time during the summer but are most common in mid- to late summer. Symptoms of elm yellows include 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 matter of a few weeks. Tolerant trees become stunted and may develop bunchy, prolific growth at the tips of branches (called 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 infected with the Dutch elm disease fungus 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 taking a few chips of the stained phloem tissue, placing them in a closed container for a few minutes, and then checking for a wintergreen odor.
Elm yellows is caused by a phytoplasma (formerly called a mycoplasma-like organism). Phytoplasmas 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. Generally, diagnosis is based on symptoms in the field. For this reason, no confirmed cases have been reported by the University of Illinois Plant Clinic, but confirmation has come from several knowledgeable tree specialists in the state. The disease is believed to be spread by such phloem-feeding insects as leafhoppers. The phytoplasma overwinters in infected tree roots and witches’ brooms.
What is the fate of infected trees? Some may live for several years, but most infected elms die within 1 or 2 years of symptom development. No cure exists. The good news is that elm yellows does not move into new areas as quickly as Dutch elm disease. Removal of infected trees is advised in order to remove inoculum sources from the area. Siberian elm seems to be resistant to this disease problem. Watch for the development and release of resistant Asiatic or European elms. For additional information about this disease, consult Report on Plant Diseases No. 660 or the book Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson, as well as many Web sites discussing elm yellows. A publication is available from the U.S. Forest Service called How to Differentiate Dutch Elm Disease from Elm Phloem Necrosis. Check this out on the Web at http://willow.ncfes.umn.edu/ht_ dednecr/ht_ dednecrosis.htm.