Dem “Yellows” Give Me the Blues
ANNAPOLIS, MD (October 5, 2009) - Elm yellows is a systemic disease of elms caused by a bacteria-like organism called a phytoplasma. The disease has been reported sporadically in Maryland for many years. Outbreaks are usually localized but can be severe and subside after many of the native elms have succumbed. Elm yellows, which occurs in the eastern half of the USA and in central and southern Europe, is spread by the white banded leafhopper, Scaphoideus luteolus, and possibly by other leafhoppers. In North American elms the disease is lethal, but is generally considered of little importance in Europe. Two former names for this disease are elm phloem necrosis in the U.S. and witches’ broom in Europe. Now, when most of the yellow fall leaves you see are NOT caused by disease, MISC has chosen elm yellows as the October Invader of the Month.
No one is certain how the elm yellows phytoplasma came to the U.S., but the first published report of a disease now recognized to be elm yellows dates from 1882. Elm yellows causes disease in American elm (Ulmus americana) and all of its hybrids and cultivars tested so far. In addition, winged elm (U. alata), cedar elm (U. crassifolia), Chinese elm (U. parvifolia), September elm (U. serotina), and red or slippery elm (U. rubra), and the elm hybrid, U. rubra x serotina have all been found to be susceptible. Susceptible trees usually die within two years after symptoms are visible.
Symptoms usually appear by late summer and include rapid yellowing of the foliage on a few branches or the entire tree. Other symptoms include downward bending of the leaf stems (petioles), early defoliation and scattered branch die-back. Leaves may also wilt rapidly, shrivel, brown and remain attached for several weeks. On red elm and Eurasian species such as Chinese elm, a symptom called witches broom will develop on small branches. Witches broom looks like an upside-down broom head, with a dense cluster of twigs all growing from the same point on the tree.
In diseased elms, the inner surface of the peeled back bark (phloem tissue) will show a yellow to butterscotch color with darker flecks sometimes visible. This differs from branches infected with the Dutch elm disease, a fungal disease, which have brown streaks in the wood (xylem). Although not always discernable, diseased twigs or sapwood from winged elm, American elm, cedar elm and September elm will often give off a characteristic “wintergreen smell” when sealed in a small container. Twigs from red or slippery elm smell like maple syrup. As trees die, the inner surface of their bark becomes chocolate brown.
Leafhoppers transmit elm yellows phytoplasmas to trees throughout the growing season when they feed by sucking the sap of elm leaves. Symptom development takes at least 3 to 10 months to appear. Disease outbreaks may kill nearly all native elms in an area, but the epidemic spreads outwards slowly because dead trees no longer serve as a source of the phytoplasma or a host to the leafhoppers. Although an old publication suggests that the disease can also spread between closely spaced trees by root grafts, there is no evidence to support that notion. Additional outbreaks beyond localized epidemics and single tree deaths are thought to be from long-distance wind dissemination of leafhoppers carrying the phytoplasma.
Currently, there are no proven chemical controls or prevention techniques for elm yellows. Experiments with antibiotic injections to protect trees that are still healthy are being evaluated. It is not likely that antibiotics can cure a tree already infected with elm yellows. Insecticides that are used to manage the bark beetles that spread Dutch elm disease may also suppress leafhopper populations and thus help protect trees from the disease-spreading insects.
For more information about this and other Invasive Species of Concern, visit www.mdinvasives.orgphotos available electronically on request.