Verticillium Wilt of Woody Ornamentals: A Comprehensive Guide

Understanding Verticillium Wilt

Verticillium wilt is a serious fungal disease affecting a wide range of deciduous trees and shrubs. Conifers are not susceptible. The disease is caused by the soil-borne fungus Verticillium, which enters through root wounds and blocks water-conducting vessels.

Symptoms

Early signs include heavy seed production, smaller-than-normal leaves, and browning leaf margins. Often, foliage on only one side of the tree wilts. Wood under the bark of wilting branches shows streaked discoloration: green to black in maples, brown in elms, and brown to black in black locust and other trees. The smallest branches may not exhibit discoloration.

Disease Cycle

The fungus survives dormant in soil. It enters wounds in roots or the tree buttress and remains primarily in the current year's growth. The fungus plugs water-conducting vessels, restricting flow to branches and leaves. The tree responds by plugging additional vessels with gums, further restricting water flow. Large trees may survive years with minor symptoms, but infected trees often die within 2–3 years.

Management Strategies

• Avoid replanting susceptible species where a tree was killed by Verticillium.
• Prune affected limbs when symptoms are mild, and water and fertilize to maintain vigor.
• Soil fumigation by a licensed applicator can greatly reduce Verticillium populations, though not eliminate them entirely.
• Never use wood chips from infected trees as mulch or potting medium, even after composting, as the fungus may survive.
• Fungicide injection with benzimidazoles is unpredictable; fungal populations vary greatly in sensitivity, and protection cannot be guaranteed.

Resistant Plants (Pennsylvania Conditions)

Yews and conifers, mountain ash (Sorbus), birch (Betula), honey locust (Gleditsia), katsura tree (Cercidophyllum), sweetgum (Liquidambar), hornbeam (Carpinus), crabapple (Malus), dogwood (Cornus), sycamore (Platanus), hawthorn (Crataegus), firethorn (Pyracantha), ginkgo (Ginkgo), willow (Salix), mulberry (Morus).

Additional Resistant Plants (Ohio Reports)

Beech (Fagus), white and burr oaks (Quercus), boxwood (Buxus), pear (Pyrus), hickory (Carya), walnut (Juglans), holly (Ilex), juniper (Juniperus), larch (Larix).

Wilting maple.