A phytoplasms pathogen causes elm yellows. This disease used to be called elm phloem necrosis. American, red/slippery, three other elm species and some hybrid elms are susceptible. Leafhoppers are one of the known carriers of the pathogen. A number of sources indicate that Eurasian elm species and clones are immune or resistant to elm yellows, but recent studies have shown several of them to be less tolerant or susceptible. For example, U. parvifolia (Lacebark Elm, also called Chinese Elm) has been shown to vary from susceptible to moderately tolerant. For many Eurasian and American elm hybrids, the reaction to elm yellows remains unknown, but they are under investigation.
Foliar symptoms can occur any time during the summer. Symptoms start with wilting and yellowing of leaves followed by branch death. It may start with just one or two branches flagging or more likely - the entire tree declining/dying all in a short time period. The disease is sometimes confused with the Dutch elm disease. Generally by the time external symptoms occur, the internal damage is extensive. Discoloration of the inner bark (phloem) tissues of infected trees occurs as foliar symptoms develop. The brown discolored tissue is most easily observed in the trunk (upper branches usually do not show this symptom) by cutting away the outer bark. When exposed to air, the phloem (usually from the trunk of the tree) develops a butterscotch color. In addition, fresh, infected phloem gives off a faint wintergreen scent after having been enclosed in a jar for a few minutes. On red/slippery elms, it may take two years for death to occur and these infected trees may develop witches broom. The red/slippery elms disease smell is more maple like than the wintergreen smell.
The pathogen overwinters in the dead and dying trees. It is transmitted by leaf hoppers and possibly other peircing sucking insects and is believed to be transmitted primarily during the late summer and fall. The pathogen may also be transmitted via root grafts. It takes from three to ten months from inoculation to the first symptoms depending on tree size.
There are no chemical controls. Sanitation done correctly (immediate removal of the tree and stump) is one of the more effective means of reducing the spread of the disease.