Dutch elm disease is the first problem I consider when I think about elm in the lab. We still see plenty of it in Illinois, and it kills trees. That disease was discussed in issue 6 of this newsletter.
There are other elm disease problems in Illinois, some serious and some minor. We are seeing black spot disease now. This fungal leaf spot causes black leaf spots that range from pin-head size to about 1/4-inch in diameter. The foliage around the spots turns yellow, and leaves may drop prematurely. Most elm species show little growth effect from black spot infection. There are some other leaf-spotting fungi on elm, including Phyllosticta, Coniothryium, Cercospora, and Gloeosporium. All appear similar. If you feel the need to manage these leaf spotters, rake and remove leaves at the end of the season, remove dead wood in the tree, water small trees in drought stress, and fertilize in the fall or spring. The diseases occur when conditions are cool and wet as leaves emerge. Some elms have more resistance than others but heritability varies even within the species.
Elms, like most deciduous trees, are also susceptible to fungal cankers. I can list a few you may recognize, such as Botryosphaeria, Phomopsis, and Nectria. Still, these fungi invade only when the tree is under stress. You donít need to know the exact fungus causing the canker. Instead, you need to recognize what is stressing tree growth. As with most tree canker problems, identify and correct the stress, remove dead or badly cankered wood where possible, water in periods of drought, and practice sound horticultural practices.
Wetwood and slime flux are common on elm. That disease complex was discussed in issue 4 of this newsletter. There is no control for wetwood, but it is not fatal to a tree. The bacteria associated with the infection are common in our soil and probably infect from the soil. Most old elms in the state are infected with wetwood.
Elm yellows disease (aka phloem necrosis) is as serious as Dutch elm disease. The disease will kill mature elms. Symptoms of elm yellows, which may appear any time during the summer but are most common in mid- to late summer, 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 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 more commonly found in the trunk. A simple field test to help with diagnosis involves taking a few chips of the stained phloem tissue, placing it in a closed container for a few minutes, and then checking for a wintergreen odor.
Elm yellows disease is caused by a phytoplasma, an organism that cannot be isolated on agar in a lab. We cannot test for this phytoplasma at the Plant Clinic and must rely on symptoms for diagnosis. 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 but only at a high cost due to labor and equipment needs. Some specialty labs, such as AGDIA (http://www.agdia.com), offer the service. Generally, diagnosis is based on symptoms in the field and elimination of Dutch elm disease as a possibility. For this reason, no confirmed cases of elm yellows have been reported by the University of Illinois Plant Clinic, but confirmation has come from several knowledgable tree specialists in the state. For additional information, consult Report on Plant Diseases No. 660, Elm Yellows or Phloem Necrosis and Its Control, or the book Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson. The disease report can be found on the web at http://www.ag.uiuc.edu/%7Evista/horticul.htm or in Illinois Extension offices.