Issue 12, July 25, 2011

Got Wilt?

Hot, dry weather causes stress to plants as well as people. Wilting is a normal plant response to a lack of water, but severe wilting, partial-plant wilting, and flagging are all symptoms of Verticillium wilt. This disease is common and recurring in Illinois. The causal fungi are found throughout the state. Plants that are stressed are more likely to be susceptible to the pathogen, and more likely to show symptoms.

The wilt disease is caused by several species of related fungi, with both Verticillium dahlia and V. albo-atrum found in Illinois. Verticillium attacks a wide range of plants including trees, shrubs, groundcovers, vegetables, fruits, and herbaceous ornamentals. Due to their wide host range and the fact that the fungus can survive in the soil for decades, this is a problematic pathogen for homeowners, commercial growers and farmers. The most commonly affected plants in Illinois include maple, redbud and ash trees.

Verticillium may be seed- or soil-borne. Roots are infected first as the fungus enters through wounds and grows into the cortex. Fungal spores are produced and systemically transported upwards in the xylem. The spores lodge within the vascular system of the plant and germinate, eventually plugging the xylem and restricting water flow within the plant. Wilting foliage and branch or plant dieback are common symptoms. Half of the plant may be affected while the other half appears normal. Other indications of Verticillium wilt are flagging, where leaves turn red one branch at a time, and leaf scorch.

A maple tree with flagging branches, a common symptom of Verticillium wilt. Photo credit Bruce Paulsrud.

The development of Verticillium wilt is associated with plant stress. During periods of hot, dry weather plants experience high levels of stress and may become more susceptible to infection. Other frequent forms of stress include nearby construction, damage to the plant and, in trees, deep planting. Stress can also cause symptoms to develop or become more pronounced.

In woody plants, peeling back the bark of a wilting branch affected by Verticillium will usually show brown or green streaking (the exception is the ash tree, which can be affected without being discolored). In herbaceous plants, cutting open the stem will reveal dark brown or black streaked tissue. Verticillium can be tested for at the University of Illinois Plant Clinic. If you suspect you have an affected plant, collect a sample containing live, symptomatic tissue. Check for streaking; all woody plants other than ash trees must contain this streaking to effectively isolate the fungus. Ideally, branch samples have the same diameter as a thumb and are 8 to 10 inches long.

A sample of a maple branch, showing characteristic dark streaking of the vascular system. Photo credit University of Illinois Plant Clinic.

Unfortunately, control of this disease is very difficult once it has become established in a host. Most infected plants die, though reducing stress and actively increasing plant vitality by pruning out dead wood and applying appropriate fertilizers can extend an infected plant's lifespan. Several-year rotation is also recommended as a control strategy. Resistance to Verticillium has been developed in a few plant species, including strawberry, tomato and potato.

Why bother testing for Verticillium wilt if the prognosis is so poor? Often people wish to replace dead trees and shrubs. Verticillium fungi can persist in soil for several years, waiting for an appropriate host. Replacing a tree that died of Verticillium with another susceptible species is an exercise in futility; instead, a resistant variety should be selected. Apple, Ginkgo, Juniper, Oak, Pear, Pine, Spruce, Sycamore, Walnut and Willow trees are all considered to be non-susceptible to Verticillium wilt. (Diane Plewa, Suzanne Bissonnette)

Suzanne Bissonnette
Diane Plewa

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