We were introduced to the majestic
Elm trees on Penn State's main campus at University Park some 30
years ago (1978). Part of our tree education involved "twig walks"
around campus for summer and winter tree identification classes.
There's a wide walkway area that travels straight through the
southwestern part of campus that is known as "The Mall." Large
American Elms line both sides of The Mall and create a unique tunnel
effect with their massive branches arching high overhead. Some of the Elm
trees are over 100 feet tall and more than 100 years old. This is
probably the only large group of centurion Elms left in the
Elms have historically been highly favored shade trees due to their
upright vase shape. Zelkova is usually the tree mentioned as being
the most similar tree to substitute, but officials at Penn State are
considering Sycamores and Burr Oaks as the tree varieties to replace
Dutch Elm Disease
Most people are familiar with
Dutch Elm Disease, caused by a fungus spread by the Elm
Bark Beetle, and infamous for its death march through American
streets, backyards and parks. Word of this second deadly
elm disease spreading, will be equally sobering news to those
Penn State Alums were
informed of this dire threat to University Park's 290 Elm trees in
the December 2008 issue of the alumni magazine. Penn State has
created a special Elm Yellows web site to keep everyone abreast of the latest developments.
Elm Yellows (Elm Phloem Necrosis) gets its name from the early
yellow coloration of leaves throughout an affected Elm's crown.
(Dutch Elm Disease usually affects individual branches in the
crown). Elm Yellows
was first discovered in State College about two years ago (2006) and
early monitoring of campus trees has indicated an alarmingly rapid
spread. Many of the affected Elms have already been removed. The
pathogen responsible is a mycoplasma-like organism, somewhere between a
bacterium and a virus. It is spread by the elm leafhopper, but also
travels between neighboring trees through underground root grafts.
Symptoms of Elm Yellows
Nymphal-stage of the Whitebanded
Leafhopper, vector of Elm Yellows
Early foliar symptom of Elm Yellows
over the entire tree crown
Photo credits -
Lester Paul Gibson, Research
Entomologist, NE Forest Experiment Station, Delaware, OH;
Arthur R. Hastings, Entomologist, NE Area, State &
Private Forestry, St. Paul, MN;
Leon A. LaMadeleine, Plant Pathologist, NE Area, State &
Private Forestry, Broomall, PA
Cultural control measures are aimed at removing diseased trees
quickly. And somewhat experimentally, American Elms are being injected with
tetracycline as a preventative/curative measure. The disease kills a
tree by clogging the Phloem, the vascular system known for bringing
the products of photosynthesis back down from the leaves to the roots.
Therefore, the first casualties of Elm Yellows are the root hairs,
the finer extensions of tree roots.
The disease isn't known for its fast spread, so other controls will
be focused on the movement of nursery stock, to further minimize the
Infested elms will exhibit discolored inner bark (butterscotch
color) that will also have a wintergreen odor. This disease should
not be confused with Elm wetwood caused by Slime Flux disease, where
smelly liquid weeps out of an Elm's trunk, wetting the outer bark.
Elm Yellows in the United States
Distribution of elm phloem necrosis in the United
Shaded areas show distribution in 1945; area within
line is 1975 distribution. Triangles indicate isolated
Map & text credit: USDA Forest Service