We have heard many inquiries at the Plant Clinic about elms that look prematurely yellow or just plain “funky” this August. I have seen this on elms, as well as on some maples, hackberries, and other tree species in Champaign. I have talked with several tree professionals about this and do not have a good explanation. On elm, there are two diseases you need to be able to eliminate as the cause of decline. These are Dutch elm disease and elm yellows (also called phloem necrosis). The widespread appearance of yellowing and early leaf drop over many species suggests that environmental stress is likely to be a factor in many cases.
There are still many elms in Illinois, and many of those are American elms. Each year at the Plant Clinic, we culture elm samples for the presence of the Dutch elm disease (DED) fungus. This year we have found 15 positive DED cases. In 2003, there were 9 positives; 2002, 13; and 2001, 22. It is very possible that DED could be involved in current cases of elm yellowing. Watch for yellowing of the leaves in the elm, followed by wilting and browning. Often, this happens so quickly that the problem is first noticed when branches with brown leaves appear in the canopy “overnight.” A single branch will usually show symptoms first (called flagging), with rather rapid spread to adjacent branches and the entire tree. Look for vascular discoloration to help with diagnosis of this disease. DED will cause a streaking of the sapwood. Peel back the bark of a symptomatic branch to reveal the brown streaks in the otherwise tan outer sapwood. We generally select branches of about thumb thickness with wilted leaves to culture and identify the DED fungus. Verticillium wilt and Dothiorella wilt can also cause this streaking in elm but appear infrequently at the Plant Clinic. Positive identification would require laboratory culturing of the fungus. Cut several 6- to 8-inch-long sections from wilting but living branches that show definite streaking in the sapwood. The fresh wood sections should be thumb thickness and can be sent in plastic or foil to the Plant Clinic for testing. Chilling the wood should not be necessary with Dutch elm suspect samples. Expect about 7 days of lab time for the fungus to grow to the point at which it can be positively identified. There is a $12.50 fee for this service. Remember, payment must accompany the sample, or it will not be processed. Call (217)333-0519 with questions. It is generally too late to save a tree once it is infected with DED, but an accurate diagnosis of the problem may help save nearby elm trees.
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 of this disease. Symptoms of elm yellows may appear any time during the summer but are most common in mid- to late summer. Symptoms 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 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 it in a closed container for a few minutes, and then 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 (http://www.agdia.com), can 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 cases of elm yellows have 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 in order to remove inoculum sources from the area. Siberian elm seems to be resistant to this disease problem. For additional information about this disease, consult Report on Plant Disease (RPD), no. 660, “Elm Yellows or Phloem Necrosis and Its Control,” or the book Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson. For more information on DED, including control procedures, consult RPD, no. 647, “Dutch Elm Disease and Its Control.” Both of the disease reports can be found on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm or in Illinois Extension offices.