Plant Pathology Infobytes
July 26, 1996
Where Have All The Elm Trees Gone?
Because of their graceful form and rapidity of growth, elm trees are highly valued as members of the Southern landscape. Unfortunately, native elm species are slowly disappearing from woodlands and residential areas across Mississippi and other states as a result of a disease known as elm yellows.
Elm yellows attacks all native elm species which occur in the state, but is most common on winged and American elms. Asiatic elms and hybrids between native and Asiatic elms are tolerant or immune to elm yellows.
Symptoms of elm yellows usually appear in mid to late summer and are marked by leaf yellowing on one or several branches. Leaf yellowing frequently appears on many branches over the entire canopy. This phase of the disease is followed by death of branches, rapid tree decline, and eventual tree death. Shriveled brown leaves may cling to dead trees for several weeks.
The cause of elm yellows is a microbe known as a MLO (technically referred to as a mycoplasmalike organisms). MLOs have many of the characteristics of bacteria but are smaller in size. Particles of MLOs are picked up from diseased elm trees and carried to healthy elms by any of several species of leafhopper insects. Leafhoppers feed on elms and introduce MLO particles into water and food carrying tissues where they are rapidly spread over the entire tree.
Once the tree has become extensively invaded by MLOs, the roots are unable to carry water to above ground parts and food materials manufactured by leaves can't be distributed to areas where they are needed.
Disruption of the tree's physiological processes leads to leaf yellowing and another characteristic symptom of elm yellows which shows up as a dark brown to black streaking of woody tissues just beneath the bark. The presence of this streaking can be used to help confirm elm yellows as the reason for tree decline.
Once the symptoms of elm yellows appear, it's only a matter of time before tree death occurs. This can happen in the same season that the disease is detected or during the following season.
Homeowners frequently ask what can be done to save elm trees which have been attacked by elm yellows. Unfortunately, there is no cure for this disease. About all that can be done is to remove diseased trees once elm yellows has been positively identified. This is advisable for several reasons. First, it's a good idea to remove sources of MLO which could be picked up by leaf hoppers and carried to other trees in the landscape. Secondly, the yellows MLO can move from infected to healthy trees by root contact.
If you need assistance in determining if elms in your landscape could be affected by elm yellows, contact us at your county Extension office for assistance.
Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.