Listen in the countryside and you’ll hear plenty of talk about herbicide damage to woody landscape plants, vegetable gardens, orchards, tree nurseries and plantations. But how can you tell the difference between herbicide damage and maladies caused by insects and pathogens? And what can you do to help plants recover?

When diagnosing plant problems, you need to pay attention to patterns, and ask a few questions:

• How widespread is the damage?
• How many plants are involved, and is it a single species or a wide range of plants?
• Is old or new plant growth affected?
• Is there a clear “pattern” to the damage?
• Did symptoms appear suddenly or over a long period of time?
• Which direction of the plant canopy is impacted?
• Are there common active pests and/or diseases present?

Generally, abiotic chemical damage (pesticide drift, pollutants, de-icing salts) will result in a fairly clear pattern. Usually only new growth is affected, plants downwind from prevailing winds are more susceptible, and damage tends to be observed on a wide variety of plants.

In contrast, plant damage caused by biotic factors such as insects and pathogens is usually random, affects a single species or a group of related plants, and varies with the season. Of course, there are always exceptions, like the Japanese beetle that feeds on over 200 different hosts.

Timing is another major factor. Insects and diseases usually build up gradually and usually occur around the same time from year to year. Herbicide damage can occur any time an application occurs and when environmental factors are favorable.

Growth regulator herbicides such as dicamba and 2,4-D are prone to drifting; injury symptoms show up as twisted or curled leaves, downward cupping on leaves, and narrow and straplike leaves on new growth. Root uptake of dicamba may cause leaves to cup upward. Preemergence grass herbicides are unlikely to cause drift problems, but postemergence herbicides may. Nonselective broad-spectrum herbicides are formulated to kill all kinds of vegetation. Affected nontarget plants may have yellow leaves, show dieback, and may even die.

Diagnose carefully
It is important not to jump to conclusions. There are many herbicide damage “look-alikes,” from things such as insects and mites, foliar diseases, adverse weather, soil compaction, leaf scorch, improper soil pH, inappropriate or misapplied fertilizers, genetic mutations, and de-icing salts. Proper diagnosis is important regardless of the reason. Know the symptoms produced by a particular herbicide, its mode of action and dosage rates.

Remember, regardless of the malady (i.e., insects, pathogens, chemical damage), any time a plant’s leaves are damaged or lost, this greatly reduces the ability of the plant to make food. (I like to think of it as “money.”) Food produced from photosynthesis is how the plant “pays it bills.” If the plant cannot pay its bills, then it becomes more vulnerable to other lethal agents such as environmental factors, wood-boring insects and canker-causing pathogens. In addition, herbicide damage can result in loss of mass production that affects wildlife, soil erosion and runoff, reduction in timber values, reduced recreational use, and premature tree death.

If the plant damage turns out to be due to a herbicide, help the plant recover by providing proper irrigation during dry spells, mulch the root zone, prune out dead and dying plant parts, and protect it from other stressors (i.e., defoliating insects and pathogens). Most healthy plants will recover from an isolated episode. More serious problems result from chronic exposure to herbicide drift.

For further information on diagnosing herbicide injury, sensitive plants and/or submitting samples, contact your local county Extension office or professional forester.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at [email protected].