Released: June 05, 2001

Late Spring Cool Fosters Verticillium Wilt in Trees

MANHATTAN, Kan. – When the leaves on branches or entire trees wilt in the central United States, that can herald a life-threatening plant disease.

Unfortunately, the disease thrives in the cool weather that’s been typical this spring, warned Ned Tisserat, plant pathologist at Kansas State University.

Verticillium wilt is most likely to appear in barberry, catalpa, maple, Russian olive, golden raintree, redbud, smoke tree and the stone fruit-producing trees (including cherry and peach). In Kansas, it also has attacked some ashes and elms, he said.

The Verticillium fungus that causes the disease can survive in soil for years.

"It usually enters healthy trees through wounds in the roots or root collar. Insects can cause that kind of injury. As often as not, however, trees have small wounds as a result of lawn mowers and weed whips," Tisserat warned. "And, once inside, the fungus soon blocks the tree’s water system, creating a kind of internal drought."

The affected leaves look off-color. Other than that, however, Verticillium wilt symptoms can vary, depending on the tree’s species and health, the time of year, and a range of other environmental factors.

"Sometimes the disease develops slowly over months or years, resulting in a gradual dieback and decline," he said. "In other cases the foliage drops within days or weeks and the branch or tree dies rapidly.

"The disease also can affect just one part of the tree, but then develop in another part of the crown the following year. A vigorous tree may be able to restrict the fungus to certain parts of its vascular system and produce new tissue to keep the water moving."

Girdling roots and gas line leaks can create similar symptoms. So, Tisserat suggests tree owners cut off a section of recently wilted branch and carefully slice away some bark..

Sapwood streaking or discoloration is typical in most trees that get the disease – particularly if the branch is more than one-half inch wide and the infection isn’t too recent, he said. The streaking may occur where the bark and sapwood connect or show up close to the branch center. It is brown to greenish-black in many species, but dark green in maple trees.

Verticillium wilt isn’t the only cause of such streaking, but it’s a sure sign tree owners should be sending a 4- to 6-inch sample to a diagnostic lab, Tisserat said. In Kansas, each county’s Extension office can send local samples to K-State Research and Extension’s plant pathology lab in Manhattan.

"There are no satisfactory chemical controls for Verticillium wilt," he added. "But if you know it’s the problem, you can take steps to improve an affected tree’s appearance and vigor."

Tisserat recommends:

* Water the tree once a week, to ease its stress.

* Remove the wilted branches, pruning at the junction of the main stem and the diseased branch. Disinfect pruning tools between cuts with a 10 percent bleach solution.

* Avoid inflicting new wounds on roots or root collar.

* Apply ammonium sulfate or a 10-10-10 fertilizer, to help suppress the disease’s development. Do NOT exceed label directions or apply a high nitrogen fertilizer, as both may increase the wilting symptoms.

"None of this will cure Verticillium wilt. But it may improve the tree’s chances for handling the disease on its own," he said.

The Verticillium wilt fungus becomes less active during hot summer weather, the plant pathologist said. When fall brings cooler weather, however, it can continue an invasion or start a new one.

So, if a replacement planting becomes necessary, homeowners should choose an ornamental with excellent Verticillium wilt resistance, Tisserat said. The possibilities include all types of pine, spruce, yew, juniper, arborbitae, apple, crabapple, hackberry, sweetgum, sycamore, walnut, hickory and linden.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Kathleen Ward, Communications Specialist
kward@oznet.ksu.edu
K-State Research& Extension News