Symptoms of herbicide toxicity vary with the class of herbicide used. Among postemergent herbicides, glyphosate injury appears as distortion and reduction in the size of new leaves (Fig. 1). New leaflets may show some necrosis (Fig. 2). Paraquat or diquat injury appears as desiccation of the foliage contacted by the herbicide. Phenoxy herbicides such as 2,4-D cause distortion of the foliage and contortion of petioles. Metsulfuron causes leaflet puckering and necrosis of the central part of the rachis. The distal portion of the leaf will drop down and may become necrotic (Figs. 3 and 4). Although these postemergent herbicides can be fatal, palms often outgrow these symptoms and eventually resume normal growth.
Preemergent herbicides typically cause injury to newly emerging foliage. Symptoms include new leaf dieback, chlorosis, stunting, and varying patterns of necrosis (Figs. 5 and 6). Death of the meristem is common. In palms treated with metolachlor, symptoms often include the production of side shoots in addition to new leaf stunting, necrosis, and distortion (Fig. 7). Large palms growing in landscapes can also be affected by preemergent herbicides, often fatally.
Symptoms of systemic herbicide injury can be confused with boron deficiency. Indeed, excessive auxin levels are a characteristic of both B deficiency and phenoxy herbicide injury.
Herbicides applied to the foliage or soil can be absorbed by palms with variable results.
Injury due to preemergent herbicides is fairly common where these are applied to the soil of container-grown palms, but these products can also affect mature palms in the landscape that have been treated. Although some studies have shown certain preemergent products to be quite safe when used on palms at recommended rates, other studies have shown injury from these same products when applied to the same species at similar rates. High soil temperatures are believed to accentuate the toxicity of these products to palms. Postemergent herbicide damage is less common since these products are often used as directed sprays around the trunks of mature palms in field nurseries or landscapes and seldom contact the foliage. Most palms, Cocos nucifera in particular, are highly resistant to injury by glyphosate. Soil applications of metsulfuron can cause foliar injury.
Visual symptoms, plus a knowledge of treatment history, are needed to diagnose herbicide toxicity. Leaf analysis for suspected herbicides is sometimes used to confirm the presence of the herbicide in the foliage.