Frequently Asked Questions about Elm Yellows and the Penn State Elms

November 06, 2008

Frequently Asked Questions about Elm Yellows and the Penn State Elm Trees

1. What is elm yellows?
Elm yellows, also known as elm phloem necrosis, is a disease specific to North American elms. It is caused by a bacteria-like organism that infects the root cells and the phloem-the inner bark that carries nutrients to all parts of the tree. The disease essentially prevents the tree from receiving adequate nourishment.

2. How is elm yellows detected?
The outward signs-yellowing and wilting of the leaves in late summer or early fall-resemble symptoms of other diseases that can afflict elm trees. A visual examination of the inner bark and the presence of a wintergreen-like smell from the leaves of infected trees are more reliable ways of detecting elm yellows. However, cutting into the inner bark is invasive. Penn State scientists have developed a way to sample a tree's DNA to test for elm yellows without harming the tree. 

3. How did elm yellows spread to central Pennsylvania?
Elm yellows has devastated elm stands in isolated outbreaks in at least 22 states, mostly east of the Mississippi River. It is spread from tree to tree by a tiny insect, the elm leafhopper, whose only reported hosts are elm trees. There is some indication that once a tree becomes infected, the disease may spread to others via root contact, although research is not definitive on this question. It is impossible to say how the disease came to University Park, but storm winds depositing a cloud of leafhoppers on or near the campus is a likely possibility. University Park is a vulnerable target because it has the one of the largest stands of mature elms in the nation.

4. How many elm trees are there at University Park?
About 287 on campus, with many more in outlying Penn State property and in the surrounding community. As of Nov. 1, 2008, 47 elms tested postive for elm yellows.

5. How old are the elms?
The oldest elms were planted in the 1890s, stand as tall as 115 feet, and have spreads 100 feet across. Many of the elms have been designated "heritage trees." Visit to learn more about the heritage tree program. The most recent elm plantings date from 1996 and 2006.

6. How can a tree be cured of elm yellows?
There is no known way to cure a tree once it's infected. Based on limited observations, elm yellows is always fatal to the trees it infects, with most trees dying within a year or two from the time they are infected. By the time symptoms appear, the leafhopper has moved on to other trees.

7. Can the spread of the disease be controlled?
Control seems to depend on the ability to control the movement of the elm leafhopper or other vectors, and little success has been reported anywhere in that regard. Most commonly, once a tree is confirmed to have elm yellows, it is removed.

8. If only some of the University Park elms are infected, must they all be removed?
Penn State's goal is to slow down the spread of the disease and ultimately halt it. No tree will be removed unless repeated tests show that it has elm yellows. The University is firmly committed to protecting the elms.

9. What is involved in the removal process?
University tree crews remove the elm in pieces. The larger logs are held in storage pending a decision on whether and how to use the potential lumber. Smaller portions and roots are ground into mulch, which is safe to re-use.

10. Will all the elms become infected with elm yellows?
It is unknown at this time how extensive the infection ultimately will be, or how fast the disease will spread. Previous research on elm yellows is relatively sparse, and little is known with absolute certainty. It is hoped the Penn State experience will add significantly to the scientific knowledge about elm yellows. Experimentally, some diseased trees and some healthy trees will be injected with a substance that offers a possibility of at least slowing the spread of elm yellows. Observations will be made over a period of a year or more to determine the effectiveness of this treatment.

11. What kinds of trees will the University plant in place of the elms?
The University plans to replace diseased elms with several different species of trees that, when mature, will approximate the height and spread of the elms. Disease resistance will be a factor in the selection of replacement trees. Consideration also will be given to campus architecture and the size and kinds of buildings the new trees will complement.

12. Are there elm varieties that resist elm yellows?
All native North American elm species are susceptible. European and Asian species have shown resistance. Consideration may be given to planting some of those non-native species, although they do not generally match the height and spread of American elms.

13. How long will it take to have another canopy of mature trees where elms have been removed?
It depends on the growth rate of the trees planted in place of the elms, but in any case many years will pass before replacement trees reach full maturity. One tree under consideration, the bur oak, grows at the rate of about one foot per year and often reaches a mature height of 100 feet.

14. What will happen to the gifts made by the graduating classes of 1986 and 1996 in support of the elms?
The class of 1986 spearheaded the Elm Re-Leaf campaign, which raised funds to purchase new elm trees to replace those lost to Dutch Elm disease. Students from the class of 1996 created an endowment that provides annual income for supporting general maintenance and upkeep of the elms. The University is attempting to address the issue with leadership from these two classes to determine the appropriate next steps.

Last Updated March 19, 2009