Elm Yellows or
Elm Phloem Necrosis

Elm trees face another deadly disease

TreeBoss 2008

The bad news landed with a THUD...
the landmark Elms on Penn State's main campus are on their way out!

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 United States.

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 dying Elms.

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 affected.


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 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
(Elm Phloem Necrosis)

Whitebanded leafhopper Elm tree exhibiting yellowing foliage from Elm Yellows or Elm Phloem Necrosis
Nymphal-stage of the Whitebanded Leafhopper, vector of Elm Yellows Early foliar symptom of Elm Yellows over the entire tree crown
Photo credits -  USDA Forest Service
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 spread. 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 Yellows on a map of the United States
Distribution of elm phloem necrosis in the United States. Shaded areas show distribution in 1945; area within heavy line is 1975 distribution. Triangles indicate isolated cases.
Map & text credit: USDA Forest Service


Where is the Phloem?
(Pronounced flow-em)

cross section of a tree trunk



Using leaf mulch around plants

HOW TO: Plant bare root trees

Horse Chestnut leaf blotch


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