Elms have two major wilt disease problems in Illinois. Most of us know Dutch elm disease. Its causal fungus can be isolated in most labs in about a week. That dis-ease was discussed in issue 12. This season, we have received many positive cases.
The lesser known disease is elm yellows, or phloem necrosis. It is caused by a phytoplasma (type of pathogen) found only in the phloem tissue. This fact, along with the fact that the infected phloem turns brown, gives 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 anytime during summer but are most common in mid- to late summer. Look for symptoms now: yellowing and drooping of foliage, followed by leaf drop and branch death. This pattern may occur on one or a few branches or 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 branch tips (witches'-brooms) or on the trunk. Inner bark tissues of infected trees often exhibit a butterscotch or light brown discoloration in small streaks or flecks. Although trees infected with Dutch elm disease usually show vascular discoloration in symptomatic branches, discoloration from elm yellows is not usually in the branches--it is more often found in the trunk. A simple field test involves taking a few chips of stained phloem tissue, placing them in a closed container for a few minutes, and checking for a wintergreen odor.
Phytoplasmas are bacteria-like organisms with no cell wall, too small to be seen with a compound microscope; and they cannot be cultured in diagnostic labs. Confirmation usually involves extracting DNA from a diseased plant, amplifying a DNA fragment by polymerase chain reaction (PCR), and identifying it. Such procedures are available at a high cost (due to labor and equipment). Some specialty labs, such as AGDIA, Inc., offer this service. Generally, diagnosis is based on symptoms in the field and eliminating Dutch elm disease as a possibility. For this reason, no confirmed cases of elm yellows have been reported by the U of I Plant Clinic, but confirmation has come from several knowledgeable tree specialists in the state. Phloem-feeding insects such as leafhoppers are thought to vector this phytoplasma, which overwinters in infected tree roots and witches'-brooms on elms.
There is no cure for elm yellows; infected trees usually die within a year or two. The good news is that elm yellows does not move into new areas as quickly as Dutch elm disease. Remove infected trees to remove inoculum sources from the area. Siberian elm seems to be resistant to this disease. For more information, see Report on Plant Disease, no. 660, or the book Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson.