Impacts of Herbicides on Young Citrus Trees

By Ramdas Kanissery, Nirmal Timilsina and Mongi Zekri

Weed control is crucial for the growth and establishment of young citrus trees. Chemical weed control with herbicides is an efficient and cost-effective method for managing weeds in newly planted groves.

However, before applying herbicide products, care must be taken to avoid damaging young trees and newly planted resets. Every so often, incorrect herbicide application can lead to injury or even dieback of young trees (Figure 1). Also, herbicide injury in young trees can reduce their vigor, delay successful establishment and lead to other problems like increased disease and pest infestations. Herbicide damage can cause a growth setback in newly planted trees, resulting in lost money and effort for growers.

INJURY SOURCES
Understanding how trees absorb herbicides is essential to avoiding damage. There are two primary routes for herbicides to enter trees.

The first is foliar or bark absorption, where herbicide sprays come in contact with leaves or stems. Trunks transplanted less than one year ago may still have bark that is immature and green. Spray contact of herbicides with tissues other than matured brown bark can result in severe damage.

The second route for herbicide entry is soil uptake, where the tree roots absorb the herbicide residues from the soil. This type of uptake from the soil is more prevalent while applying pre-emergent or residual herbicides.

Pre-emergent herbicides provide weed control through soil residual activity and can remain active in the soil for a period of time depending on the product’s half-life (time for herbicide breakdown). These herbicides tend to be more active in the soil than post-emergent products and can potentially be absorbed by the tree roots from the sprayed area.

Applying higher rates of pre-emergent herbicides to suppress weeds in the tree rows can damage the trees. This is a fairly common occurrence. Young trees will absorb the herbicide, resulting in severe injury. Observable symptoms typically appear slower in trees with roots that have absorbed soil-active herbicides than trees with damage from spray contact causing foliar or bark absorption.

DAMAGE SYMPTOMS
The effect of a herbicide on citrus trees varies with the type, extent of exposure, tree age and areas contacted with the sprays. Due to their relatively low tolerance to herbicides, young trees will have severe symptoms compared to mature trees. While some herbicides produce distinct symptoms, others result in injuries that resemble damage from different causes, like pests and diseases. Some of the common injury symptoms caused by herbicides in young citrus trees are described below.

Leaf Discoloration
Some herbicides discolor (chlorosis) tree foliage. This is more or less typical of root uptake of residual herbicides. Some examples include diuron (Karmex) and bromacil (Hyvar X). The leaves will turn slightly yellow after diuron root uptake, followed by the chlorosis of veins (Figure 2a). Contact with the foliage or stem from certain residual herbicides can harm the trees. For instance, spray contact of diuron can cause a bleached appearance on the leaves of young trees (Figure 2b).

Similarly, although a residual herbicide, foliar contact with indaziflam (Alion) will cause white or yellow spots on the leaves (Figure 2c). The root uptake of norflurazon (Solicam) causes midrib chlorosis of the leaf, followed by vein chlorosis as the exposure increases. White spots on the leaves and, in some cases, leaf distortion are typical symptoms of damage from direct spray contact from this herbicide.

Brown Spots
Certain herbicides damage only the parts of the trees that come in contact with the spray. Paraquat (Gramoxone) and carfentrazone (Aim) are two examples. A minor spray drift causes brown necrotic spots or speckles on the foliage (Figure 3a) and green bark tissues with no distinctive pattern. However, a significant spray coverage can severely damage the affected tree. Similarly, injury symptoms of glufosinate (Rely, Scout, etc.) include sudden onset of leaf yellowing, wilting and tissue death in the contacted areas.

Distortion
Typical examples of herbicides that cause distortion of immature leaves and growing points are products containing glyphosate, 2,4-D, etc. Glyphosate spray injury on trees starts with yellowing (chlorosis) in the growing points and needle-shaped immature leaves (Figure 3b). Direct spray contact with glyphosate on the green bark will cause burning, lesions and tissue damage in the stem. Spray contact with products containing 2,4-D (e.g., Landmaster) results in the cupping of leaves (Figure 3c). Similarly, sprays of fluazifop (Fusilade) and sethoxydim (Poast Plus), the post-emergent herbicides used to control annual and perennial grasses, cause dead growing points in young trees.

TIPS FOR SAFE USE
“Prevention is better than cure” holds up in cases of herbicide-related injuries to young citrus trees. The product labels have to be thoroughly followed before selecting and applying herbicides in young groves and new plantings. Labels typically provide application rates and a list of safety considerations for use in young and non-established trees.

Growers can access the labels at www.cdms.net for most herbicide products used in citrus. If there are not rate suggestions for young trees specified on the label, consider using a low range of labeled rates during the initial years of tree establishment.

While applying herbicides, avoid spraying stems and foliage. Using non-porous wraps or tubes around the trunks of young trees and resets with green stems helps protect them from direct spray hits.

Newly planted trees and resets also can be affected during establishment by residual herbicides in the soil from previous weed-control applications in the grove. It is worthwhile to refer to the product labels for such plant-back intervals on all the herbicides used over the last two to three years before planting new trees. See edis.ifas.ufl.edu/publication/HS316 for more information about these restrictions.

Once herbicide has been absorbed into the tree, the options are somewhat limited. It is then a waiting game to see how the young tree reacts to the herbicide. This waiting period can range from a few days to several weeks, depending on the herbicide type and exposure dosage.

If root uptake of the herbicide is suspected, activated charcoal may be incorporated into the soil to help chemically bind and remove the residues. The activated charcoal should be incorporated into the soil immediately after herbicide application. However, the activated charcoal application may not effectively prevent the injury once extensive root uptake occurs.

Ramdas Kanissery is an assistant professor and Nirmal Timilsina is a graduate research assistant, both at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center in Immokalee. Mongi Zekri is a UF/IFAS multicounty citrus Extension agent in Labelle.

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