Tuesday, June 10, 2008

Nursery and Landscape - Herbicide Injury Symptoms

The hot weather we are having brings another topic to the front: herbicide damage. There is more risk of herbicides moving off-target in hot weather, expecially growth regulator type herbicides that can volatilize. The following is information on types of symptoms you may see with herbicide injury.

Leaf chlorosis, or yellowing of foliage, is a common symptom of herbicide injury in plants. The application of many chemicals on the market today results in photosynthesis being disrupted and later ceased. Without photosynthesis, plants yellow, fade, and quickly display chlorosis. Often, soil-applied herbicides are responsible for this type of symptom. Unfortunately, chlorosis may also be caused by a number of other factors, with poor nutrition being a major contributor. Therefore, herbicides are often overlooked as the cause of yellowing. Nutrient deficiencies rarely result in rapid death of plants that many herbicides are capable of producing. Herbicide-induced chlorosis differs from nutritional chlorosis by a bright yellow to white interveinal space contrasted with sharply defined secondary bright green veins. Nutritional chlorosis, however, displays a shaded or gradual fading of green from the yellow interveinal space to the green midrib without secondary veins.

Foliar spotting may be a result of herbicide spray drift. Also, burn may occur where cupped foliage maintains direct contact between herbicide spray droplets or granules on plant foliage. It is essential that any herbicide residue be washed off the foliage of nontarget plants promptly after application. Overall necrosis, or death of tissue, is caused by a number of products and can occur in advanced stages of herbicide poisoning due to gross misapplication of chemicals. This symptom is often an extension of the chlorosis and/or foliar spotting mentioned earlier.

Epinastic growth is foliage or stems that are abnormally twisted, cupped, or otherwise distorted. These symptoms often indicate some type of phenoxy herbicide damage. Chemicals of this type are root and shoot absorbed and can cause injury in several different ways. Spray drift, root uptake, and volatilization may all occur in this category. Symptoms are like those seen on dandelions or other weeds soon after application of a broadleaf weed killer to the lawn. Commonly grown woody plants in Oklahoma, such as redbud and grape, are two of many species very sensitive to minute amounts of phenoxy herbicides.

Whole Plant Symptoms. Some trees will show a spiralling injury pattern. In these cases, abnormal or dead growth spirals from the base to the top of the plant. This has been reported as a very distinct and obvious pattern indicating a root absorbed chemical. In other species, as much as half of the plant can be markedly affected, while the remainder appears healthy. This could be a sign that spray drift has occurred and the side of the tree affected was in the path of the wind-carried chemical. When one half of a tree or shrub has been affected by a herbicide, the symptoms often closely resemble those of vascular wilt diseases. Many herbicide injuries mimic symptoms exhibited by a pathogen, insect, or abiotic stress. Proper diagnosis often requires a team approach that combines expertise from horticulturists, plant pathologists, and entomologists.

Information from Herbicide Injury in the Nursery and Landscape by Michael A. Schnelle
Professor, Extension Ornamental Floriculture Specialist and Janet C. Cole Professor, Nursery Management, Oklahoma State University.

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