By Ramdas Kanissery, Nimal Timilsina and Ruby Tiwari
Chemical weed control is an essential component of the toolbox to keep weeds in check in citrus groves. Citrus growers primarily rely on herbicide to manage weeds due to their efficacy and cost-effectiveness.
Despite their benefits, herbicides may have a wide range of unintended effects on citrus. For instance, an herbicide-related injury may occur due to improper spray practices, unfavorable weather conditions, product misapplication, tank contamination or herbicide carryover from a past application. Moreover, the movement of herbicides as a result of drift, volatilization or runoff — either on-site or from applications on adjoining areas — can result in tree injury. This article discusses the diagnosis of such injuries from commonly used herbicides in citrus production.
Determining if herbicide injury is the cause of phytotoxic symptoms is very challenging in citrus; it often involves assessing multiple types of scenarios. Damage caused by diseases, pests and nutrient deficiency is often misinterpreted as herbicide injury symptoms. Also, the injury symptoms vary based on the specific herbicide used and the concentration at which the tree was exposed. The presence of physiological and environmental stresses with the ability to produce similar injury is always a possibility as well. Following are some strategies to consider when diagnosing herbicide injury in citrus.
UNDERSTAND MODE OF ACTION
Herbicides injure the non-target plants, or crops, by interrupting/preventing their normal biochemical and physiological processes. Herbicides are grouped based on their mode of action (MOA), which determines how the herbicide interrupts the normal plant mechanism. Herbicides within the same MOA group target the same biological process or enzymatic mechanism in the plant and produce similar injury symptoms.
A group number, also known as MOA code, is typically on the manufacturer’s herbicide label. The code is intended to help users identify the MOA of a particular product and use the information to rotate herbicides from different groups to minimize the possibility of herbicide resistance and tolerance issues. This grouping also helps applicators understand what injury symptoms can be expected in the non-target crops.
Apart from the product labels, several other sources provide information on the MOA grouping of herbicides. For instance, an online database (hracglobal.com) from the Herbicide Resistance Action Committee allows users to look up the MOA classification code of common herbicides used in citrus. The general injury symptoms caused by some of the herbicide MOA groups commonly used in citrus weed control are described below.
Lipid Synthesis Inhibitors (Group 1)
Herbicides in this group are primarily used for managing grasses in citrus. Fluazifop-P-butyl (Fusilade) and sethoxydim (Poast) are the most common citrus herbicides in this group. These herbicides mostly affect monocot plants by inhibiting an essential enzyme for membrane synthesis.
These herbicides rarely result in damage to citrus when applied per label instruction. However, in rare instances, direct foliar contact with these herbicide sprays sometimes damages citrus. This damage is often from the surfactant used rather than the herbicide itself. Primary injury symptoms include chlorosis (chlorophyll loss in the leaves and subsequent yellowing) and wilting of newly formed leaves. Necrosis (tissue death) and dieback may also occur in instances of severe exposure.
Growth Regulators (Group 4)
Herbicides commonly present in this group include products containing 2,4-D (Embed Extra, Landmaster, etc.), triclopyr, aminopyralid, clopyralid and dicamba, among others. The extent of damage from these herbicides to citrus depends on the concentration at which the plant has been exposed. Common symptoms include bending of shoots, distortion and cupping of leaves (Figure 1). These symptoms are prominent in new shoots and leaves (flush).
Photosynthesis Inhibitors (Group 5)
This herbicide group impedes photosynthesis in susceptible plants. The herbicides in this group are mostly used for pre-emergent suppression of weeds in citrus. Simazine (Princep) and diuron (Karmex and Direx) are some examples. Primary symptoms of direct spray contact with these herbicides include yellowing foliage (interveinal and whole leaf). The symptoms of root uptake injury may appear as yellowing of the prominent leaf veins (Figure 2). Shoot and twig dieback follow after heavy exposure.
Amino Acid Synthesis Inhibitors (Group 9)
These herbicides inhibit the production of an enzyme vital to producing amino acids in plants. Glyphosate (Roundup) is a widely used herbicide in citrus from this group. The herbicide is foliar absorbed, and damage usually occurs through direct application or drift contact.
The typical progression of glyphosate damage is gradual in citrus and may appear several weeks after the application. It usually starts with the foliage yellowing, resulting in defoliation over time. After the herbicide exposure, new leaf growth may appear small, narrow and needle-shaped on the affected shoots (Figure 3). Similarly, fruit that comes in contact with glyphosate sprays may exhibit rind damage, which causes fruit drop if it is severe.
Nitrogen Metabolism Inhibitors (Group 10)
Glufosinate-ammonium (Scout and Rely), a broad-spectrum post-emergent herbicide used in citrus, is included in this group. It inhibits the activity of an enzyme that is necessary for the plant to convert ammonia into other nitrogen compounds. The herbicide is mainly absorbed through foliage but has limited translocation in the plant. Initial symptoms of spray injury include wilting in the contacted areas within days of application, followed by tissue death in a few weeks.
Pigment Inhibitors (Group 12)
This group includes herbicides that interfere with pigment production and the protection of chlorophyll in plants. Norflurazon (Solicam), applied for pre-emergent weed suppression in citrus, is in this group. When exposed to the herbicide, the affected parts in citrus become white to translucent and appear bleached.
Cell Membrane Disruptors (Groups 14 and 22)
Herbicides in this class are used for both pre- and post-emergent, broad-spectrum weed control. Common post-emergent citrus herbicides in this group are carfentrazone (Aim) and paraquat (Gramoxone). Flumioxazin (Chateau) is a pre-emergent herbicide in this group. These herbicides work by interfering with photosynthesis, primarily through cell membrane destruction. They are known as contact herbicides because translocation within the plant does not occur. The resulting damage is limited to the point of contact with the herbicide spray. When sprays contact the tree, the injury will appear as spots of dead tissues on the leaves or other parts, including fruit (Figure 4).
Cellulose Synthesis Inhibitors (Group 29)
Herbicides in this group restrict cell division in the developing roots of germinating seeds, consequently suppressing germination. These herbicides are typically applied as pre-emergent weed control. Indaziflam (Alion) is an herbicide in this group that is commonly used in citrus. Damage to citrus occurs from herbicide contact with the shoots or foliage during spraying. Typical symptoms in scenarios of heavy drift include yellowing of foliage (Figure 5), especially at the tips of low-hanging branches.
LOOK FOR INJURY PATTERNS
After identifying concerning symptoms in trees, evaluate the weed species in the affected area for similar symptoms. If weeds are also symptomatic, then the injury is likely herbicide related. Also, consider comparing the tree with injury symptoms to trees from different areas in the same grove and trees in the neighboring groves. If similar symptoms occur in nearby areas with different management and production practices, the issue could be environmental or biotic rather than herbicide induced.
It is essential to understand how injured trees are distributed in the grove. Usually, there are distinctive spatial patterns to herbicide injury. For example, if the injured trees are more or less confined to the edge of tree rows, an herbicide may be drifting from neighboring application areas. If the symptoms are randomly scattered throughout the grove without a distinct pattern, it may suggest an issue related to the root uptake of chemicals from herbicide hot spots in soil.
RULE OUT OTHER CAUSES
Be sure to rule out disease, pest, environmental or other stress factors as the cause of injury symptoms. Distorted leaves on newer plant growth can be caused by pests like aphids and thrips or even frost injury. Yellowing and tissue death can be caused by nutrient deficiency and drought but may look like herbicide injury.
Also, remember that some citrus varieties have characteristics that may look like herbicide symptoms. For example, some grapefruit are naturally variegated and can mimic yellowing on the foliage due to herbicide exposure.
If herbicides are suspected of causing the symptoms from visual analysis and review of application history, soil and/or plant tissues may be analyzed for herbicide residues at a laboratory. In most cases, such analysis would help confirm the diagnosis. The plant diagnostic clinic at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center (SWFREC) is available to assist with herbicide-related injury diagnosis. Visit swfrec.ifas.ufl.edu/programs/veg-plant-path/plant-clinic for more information and to submit a sample.
Most importantly, applicators should follow safe practices to avoid unintentional injuries to citrus during herbicide sprays. See edis.ifas.ufl.edu/publication/AE246 for more information on this topic.
Ramdas Kanissery is an assistant professor, Nirmal Timilsina is a master’s graduate student, and Ruby Tiwari is a Ph.D. graduate student — all at the UF/IFAS SWFREC in Immokalee.
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