You thought you had Dutch elm disease in your tree, but lab results say you don’t? The Dutch elm disease fungus is the easiest of the vascular pathogens to isolate. It survives in heated twigs and can be isolated from even small-diameter wood. If you are certain you sampled a live section of the tree exhibiting vascular discoloration, then maybe your tree has a similar-appearing disease called elm yellows, or phloem necrosis.
Refer to issue no. 7 to review Dutch elm disease. Dutch elm disease is still with us, but another problem elm disease is elm yellows, also known as phloem necrosis. It is caused by a phytoplasma (type of pathogen) that is found only in phloem tissue. This fact, along with the infected phloem's turning brown, gives the disease the alternative name of phloem necrosis. Because elm yellows and Dutch elm disease can be similar in apperance, it is important to know the differences.
Symptoms of elm yellows may appear anytime during the summer but are most common in mid- to late summer. Look for symptoms to start soon—including 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 only 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 infected with the Dutch elm disease fungus usually show vascular discoloration in symptomatic branches, discoloration from elm yellows is not usually in the branches: It is more commonly found in the trunk. A simple field test to help diagnose 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 mycoplasmalike organism). Phytoplasmas are bacterialike 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 our 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 on elms.
What is the fate of infected trees? Some may live for several years, but most infected elms die within 1 or 2 years of developing symptoms. There is no cure. The good news is that elm yellows does not move into new areas as quickly as Dutch elm disease. Removing infected trees is advised to eliminate inoculum sources from the area. Siberian elm seems resistant to this disease. Watch for the development and release of resistant Asiatic or European elms. For more information, consult Report on Plant Disease RPD no. 660 or the book Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson, as well as many Web sites discussing elm yellows. There is a publication available from the US 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