Abstract
The prevalence of Huanglongbing (HLB) disease, also known as citrus greening, has compelled the citrus industry to change management practices to increase production. However, these changes, such as enhanced nutrition and irrigation programs, have caused weed proliferation, subsequently leading to increased use of herbicides. Thus, our study evaluated a popular preemergence herbicide active ingredient, diuron, for nontarget impacts on young Hamlin and Valencia sweet orange (Citrus sinensis L. Osbek) trees in two commercial orchards in southwest Florida. The treatments included the preemergence application of diuron at three rates (1.8, 3.6, and 7.3 kg a.i./ha), a weed-checked control (using postemergence herbicides: glufosinate ammonium + saflufenacil), and a nontreated control. The treatments were applied twice (in Fall 2021 and Spring 2022) in a randomized complete block with four replicates. Results indicate that over a 5-month period, the application of diuron generally had no significant impact on citrus root growth. Further, over a 2-month observation period for Hamlin and a 4-month observation period for Valencia, it was found that diuron application had no notable effect on fruit detachment force. Valencia trees treated with diuron high showed higher HLB disease severity at location 1. In addition, Hamlin trees treated with diuron low and medium showed higher fruit drop (∼19% more) than the untreated control at location 1. However, this trend was inconsistent across the locations and cultivars. This result suggests that increased disease severity and fruit drop were not associated with diuron treatment. Thus, our study finds diuron as a tree-safe option for preemergence weed suppression in citrus production, as long as it is used in accordance with the recommended dosage and restrictions stated on the herbicide label.
Since its discovery in the early 1950s, diuron [International Union of Pure and Applied Chemistry name: 3-(3,4-dichlorophenyl)-1,1-dimethylurea] has been used extensively for the weed management of citrus crops (Castillo et al. 2006). It is the most used preemergence herbicide (i.e., applied before weed emergence to prevent weed seed germination and establishment) in citrus production (USDA 2020). Diuron is a substituted urea that belongs to the phenyl amide family and phenyl urea subclass (Weed Science Society of America mode of action group number: 7). It suppresses weed seed emergence and inhibits seedling growth (Kanissery et al. 2022a; Wessels and van der Veen 1956). Diuron provides pre- and early postemergence control of annual grasses and broadleaf weeds in citrus orchards.
Because of its peculiar physicochemical properties (e.g., high water solubility of 36.4 mg/L, high mobility as evidenced by a leaching index of 2.1, and a relatively low adsorption coefficient value of 196), diuron can be transported systemically to the citrus root zone (Dong et al. 2023; Fava et al. 2006; Landry et al. 2004). Prior studies have shown that the diuron is moderately persistent in the soil with a half-life ranging from 15 to 150 d (Dores et al. 2009; Giacomazzi and Cochet 2004; Wang et al. 2017). Furthermore, diuron herbicide exhibits increased mobility in sandy soils, primarily because of their reduced herbicide retention capacity (Di Bernardo Dantas et al. 2011). For example, diuron residues have been identified in deeper soil profiles where fruit tree roots can be exposed (Fava et al. 2006; Landry et al. 2006; Tworkoski and Miller 2001). After root absorption, diuron can move via the transpiration stream and accumulate in the plant’s roots, stem, leaves, and fruits (Jin et al. 2017). When this happens, diuron interrupts the ‘Hill reaction’ of plant photosynthesis by preventing electron transport from the primary acceptor plastoquinone (PQ) to the secondary acceptor Qb. This is accomplished when the diuron herbicide displaces PQ from the Qb-binding site on the D1 protein of photosystem II (PSII). This suppression of electron transport from PII to photosystem I (PSI) during plant photosynthesis inhibits the formation of high-energy compounds [e.g., adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH)]. ATP and NADPH are used in carbon fixation and various biochemical reactions (Jin et al. 2017; Munekage and Shikanai 2005; NCBI 2023). This implies that once taken up by the roots, diuron affects their physiology and performance in susceptible plants, including targeted weeds and nontargeted crops such as citrus (Bayer and Yamaguchi 1965; Kanissery et al. 2021, 2022b; Liu et al. 1978; Nash 1968; Pascal-Lorber et al. 2010; Smith and Sheets 1967).
Despite favorable weed suppression, recent anecdotal evidence suggests an increase in preharvest fruit drop and reduced citrus yield associated with popular preemergence herbicides such as diuron (Anonymous personal communication, 18 Jun 2021). Diuron herbicide residues have been found to persist in the root zone for more than a year and potentially harm plants that are not its intended targets (Marriage et al. 1975). One study found that pomegranate grown in the sandy soil of Florida showed unacceptable phytotoxicity (e.g., leaf burning, leaf drop, and tree death) at the labeled rate of diuron (Castle et al. 2011). Another study suggests that diuron can cause severe damage to grapevines and peach trees grown in sandy soil (Itoh and Manabe 1997). In addition, high rates of diuron reportedly reduced the vigor and yield of young apple trees (Hogue and Neilsen 1988) and peach trees (Majek and Welker 1990). In contrast, however, other studies have suggested that diuron residues do not affect fruit trees, such as peaches and apples (Foy et al. 1996; Heeney et al. 1981). The potential impacts of diuron on citrus root growth, Huanglongbing (HLB) disease severity, and fruit yield are not fully understood. HLB, also known as citrus greening disease, is a devastating disease that has significantly reduced citrus production in Florida. Candidatus Liberibacter asiaticus (CLas), a phloem-limited bacterium transmitted by the Asian citrus psyllid Diaphorina citri, is the putative causative agent of HLB disease in Florida.
Weed management is a crucial and common practice for the profitability of citrus orchards (Kanissery et al. 2022a). Herbicides are known to reduce weeds, improve access to trees, and increase crop yield; however, rigorous weed control has often resulted in a nonjudicious use of herbicides at increased rates and frequencies (Tiwari et al. 2022). Despite the extensive use of diuron in citrus orchards, there is limited information available concerning the nontarget effects of diuron on citrus trees, particularly those affected by HLB disease. Most reports on the adverse effects of these herbicides in citrus merely focus on visual herbicide injury symptoms. Although herbicide manufacturers may conduct generic studies to gather crop-safety data on specific herbicides, citrus growers have access to limited information on the effects of these chemicals on trees. Thus, this study aimed to investigate the impact of preemergence herbicide diuron on root growth, HLB disease severity, and yield in citrus trees grown in the sandy soils of Florida.
Material and Methods
Field experiments
Two field trials were conducted in commercial citrus orchards in southwest Florida during the production years of 2021 and 2022. Location 1 is an orchard in Felda Ridge, FL (26°33′25.4″N, 81°28′18.2″W), and location 2 is an orchard in Orange Hammock, FL (26°31′24.6″N, 81°27′52.3″W). There was no prior history of diuron application at either study location. Three-year-old citrus trees were selected for this experiment. Hamlin and Valencia sweet orange (Citrus sinensis L. Osbek) cultivars were chosen for this study because they are Florida’s major juice-producing citrus cultivars. Hamlin and Valencia represent 28.67% and 59.62% of the total citrus acreage of Florida, respectively (USDA 2022). Table 1 provides details of the citrus rootstocks and cultivars used in the study locations. Before the first application of treatments, soil samples were sent to Waters agricultural laboratories (Camille, GA) to measure the pH, organic matter, and silt+clay% (Table 1). The study used three levels of diuron herbicide application rate (low, medium, and high), a weed-checked control, and an untreated control. The treatments were arranged in a randomized complete block design with four replications. Citrus production in southwest Florida has a subtropical to tropical climate where mean daily temperatures during the experiment varied between 6 °C and 27 °C. The long-term average rainfall during the study period was recorded as 2.44 mm per day (Florida Automated Weather Network, FAWN 2022).
Summary of experimental parameters, including citrus cultivar used, soil characteristics, and treatment application dates for the trial locations.
Treatment application and orchard management
Three weeks before the treatment applications, the emerged weeds in the study locations were first cleaned up with a grower-standard postemergent herbicide mix of glufosinate ammonium (Trade name: Rely 280®; Bayer Crop Science, Durham, NC, USA) and saflufenacil (Trade name: Treevix®; BASF corporation, Durham, NC, USA) to ensure minimum weed coverage in the tree rows. The commercial diuron product (Trade name: Diuron® 4L; Alligare LLC, Opelika, AL, USA) was applied at three rate levels: low (1X: 1.8 kg a.i./ha), medium (2X: 3.6 kg a.i./ha), and high (3X: 7.3 kg a.i./ha). These application rates span the recommended range of rates for diuron in Florida citrus (Kanissery et al. 2022a). Treatments were applied to the tree rows using a handheld backpack sprayer with an ATR 80 ALBUZ - Hollow cone spray nozzle (Chemical Containers, Inc., Sebring, FL, USA) at a carrier volume of 75 L per acre. Untreated control plots were sprayed with water (pH 6.75), and a grower-standard postemergent herbicide mixture of glufosinate ammonium and saflufenacil was used to manage the emerged weeds in weed-checked control plots. At least five citrus trees were separated by a two-tree buffer per experimental plot. Treatments were applied twice: once in the early fall (September/October) of 2021 and once in the early spring (January) of 2022. As per the recommendations from the University of Florida, the nutrition, irrigation, and disease management practices were followed throughout the study (Diepenbrock et al. 2022).
Data collection
Root growth.
Disease severity.
At three time points [1, before the first herbicide treatment application in early Fall 2021 (5 Sep 2021); 2, before the second herbicide treatment application in Spring 2022 (20 Dec 2021); and 3, at the end of the experiment in late Spring 2022 (18 Mar 2022)], at least four fully expanded mature leaves were randomly collected from the middle tree of each experimental plot for CLas detection. For each treatment, leaf samples were taken from four replicates. Leaf petioles were excised and minced, and 100 mg of tissue was weighed and lyophilized overnight in a FreeZone 6 freeze-dry system (Labconco, Kansas City, MO, USA). The dried leaf tissue was pulverized using a mini bead beater with steel beads (Biospec products, Inc., Bartlesville, OK, USA). Deoxyribonucleic acid (DNA) from the pulverized tissue samples (100 mg) was extracted using the Wizard Magnetic 96 DNA Plant System (Promega Corporation, Madison, WI, USA) according to the manufacturer’s instructions. A microplate reader (Synergy HTX Multimode Reader; Biotek Instruments, Inc, Winooski, VT, USA) was used to quantify DNA, which was then normalized to 10 ng/L. Samples were tested for the presence of CLas by using an ABI®PRISM 7500 Real-Time PCR system (Applied Biosystems, Inc., Carlsbad, CA, USA) according to the methods of Li et al. (2006). Samples were considered CLas positive if their Ct value was ≤ 36.
Fruit detachment force.
Five fruits with the peduncle were randomly harvested from the middle three trees of each experimental plot. For each treatment, fruits were harvested from four replicates. Hamlin fruits were harvested three times: 53 d before harvest (DBH) followed by 27 DBH and 1 DBH. Similarly, Valencia fruits were harvested three times: 137 DBH, followed by 33 DBH, and 1 DBH. The fruit detachment force (FDF) was determined as described previously (Gairhe et al. 2022) using a digital pull force gauge (Force One; Wagner Instruments, Greenwich, CT, USA). A clamp was connected to the digital gauge, which held the fruit. Outward-facing peduncles were then pulled horizontally using pliers. The force required to separate each fruit from its peduncle was recorded as Kilogram force (KgF).
Preharvest fruit drop.
Data analysis
Statistical analysis was conducted using R programming (R version 2022.07.1 + 576). The response variables, including RGR, HLB severity (measured by Ct value), FDF, and preharvest fruit drop, were analyzed in relation to different treatments. The treatments comprised an untreated control, a weed-checked control, and three diuron treatments at different rates (low: 1.8 kg a.i./ha, medium: 3.6 kg a.i./ha, and high: 7.3 kg a.i./ha), served as the independent variables. Data were subjected to a one-way analysis of variance (ANOVA). To meet the ANOVA assumptions, arcsine square root transformation was performed for RGR and preharvest fruit drop. Similarly, a log transformation was applied for FDF and HLB severity (Ct value) analysis data. Treatment means were compared using Tukey’s honestly significant difference test at a 5% significance level.
Results
Root growth rate
At location 1, the average RGR of Hamlin citrus ranged from 0.10 cm/d for diuron high to 0.54 cm/d for untreated control (Table 2). At location 2, the average RGR of Hamlin citrus ranged from 0.07 cm/d for diuron low to 0.29 cm/d for untreated control. For Valencia citrus, the average RGR ranged from 0.004 cm/d for diuron high to 1.10 cm/d for untreated control at location 1. At location 2, the average RGR of Valencia citrus ranged from 0.03 cm/d for diuron high to 0.59 cm/d for weed-checked control. Although the RGR of Hamlin and Valencia differed numerically between the treatments, there was no statistically significant difference between the different rates of diuron and controls at both locations.
Average root growth rate (cm/d) for Hamlin and Valencia citrus measured in untreated control, weed-checked control, and three diuron treatments (low: 1.8, medium: 3.6, and high: 7.3 kg·ha−1 a.i.).i
Disease severity
CLas was found in all trees (i.e., Ct <36), confirming that all trees were HLB infected before this study. A general reduction in Ct values for all treatments was found after the first diuron application in the Hamlin citrus cultivar compared with pretreatment Ct values; however, the CLas in herbicide-treated Hamlin trees was not significantly different from the controls at the second diuron herbicide application regardless of location or rates (Fig. 1). Despite a further reduction in Ct values of Hamlin citrus after the second diuron application, there was no significant difference in CLas between the different rates of diuron and controls at both locations for Hamlin citrus.
Hamlin citrus disease severity for 6 months at (A) Location 1 and (B) Location 2. Point 0 represents the pretreatment Ct values before the first herbicide treatment application (5 Sep 2021), point 1 before the second herbicide treatment application (20 Dec 2021), and point 2 at the end of the experiment (18 Mar 2022). Letters above the bar, if present, represent significant differences between the treatments at that time point, based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Hamlin citrus disease severity for 6 months at (A) Location 1 and (B) Location 2. Point 0 represents the pretreatment Ct values before the first herbicide treatment application (5 Sep 2021), point 1 before the second herbicide treatment application (20 Dec 2021), and point 2 at the end of the experiment (18 Mar 2022). Letters above the bar, if present, represent significant differences between the treatments at that time point, based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Hamlin citrus disease severity for 6 months at (A) Location 1 and (B) Location 2. Point 0 represents the pretreatment Ct values before the first herbicide treatment application (5 Sep 2021), point 1 before the second herbicide treatment application (20 Dec 2021), and point 2 at the end of the experiment (18 Mar 2022). Letters above the bar, if present, represent significant differences between the treatments at that time point, based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
A similar CLas titer was found in Valencia citrus trees after the first diuron application compared with the pretreatment Ct values; however, the CLas in herbicide-treated Valencia trees were not significantly different from the controls, regardless of location or rates (Fig. 2). The decrease in the Ct value of Valencia trees after the second diuron application for all treatments, including controls, indicates an increase in CLas titer at the time of harvest. Valencia trees treated with diuron at a high rate showed a significantly lower Ct value at location 1 than the trees treated with weed-checked control. At location 2, all treatments showed statistically similar Ct values after the second diuron application.
Valencia citrus disease severity for 6 months at (A) Location 1 and (B) Location 2. Point 0 represents the pretreatment Ct values before the first herbicide treatment application (5 Sep 2021), point 1 before the second herbicide treatment application (20 Dec 2021), and point 2 at the end of the experiment (18 Mar 2022). Letters above the bar, if present, represent significant differences between the treatments at that time point, based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Valencia citrus disease severity for 6 months at (A) Location 1 and (B) Location 2. Point 0 represents the pretreatment Ct values before the first herbicide treatment application (5 Sep 2021), point 1 before the second herbicide treatment application (20 Dec 2021), and point 2 at the end of the experiment (18 Mar 2022). Letters above the bar, if present, represent significant differences between the treatments at that time point, based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Valencia citrus disease severity for 6 months at (A) Location 1 and (B) Location 2. Point 0 represents the pretreatment Ct values before the first herbicide treatment application (5 Sep 2021), point 1 before the second herbicide treatment application (20 Dec 2021), and point 2 at the end of the experiment (18 Mar 2022). Letters above the bar, if present, represent significant differences between the treatments at that time point, based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Overall, the results of quantitative polymerase chain reaction analysis before and after treatments indicated no significant CLas titer change, thus, suggesting that diuron treatments had no effect on the naturally occurring CLas titer fluctuation or HLB disease severity during the experimental period.
Fruit detachment force
The FDF of Hamlin citrus increased as the fruit developed and decreased as the fruit matured, which is a typical fruit development and maturation pattern (Alferez et al. 2021; Tang et al. 2019). Valencia fruits demonstrated a relatively higher detachment force than Hamlin fruits at both locations. Although citrus FDF differed numerically between the treatments, there was no statistically significant difference between the treatments in Hamlin and Valencia citrus FDF regardless of time point or location (Tables 3 and 4).
Average fruit detachment force (FDF) for Hamlin citrus measured in untreated control, weed-checked control, and three diuron treatments (low: 1.8, medium: 3.6, and high: 7.3 kg·ha−1 a.i.).i
Average fruit detachment force (FDF) for Valencia citrus measured in untreated control, weed-checked control, and three diuron treatments (low: 1.8, medium: 3.6, and high: 7.3 kg·ha−1 a.i.).i
Fruit drop
The total preharvest fruit drop per tree ranged from 50% to 70% for Hamlin trees and 22% to 35% for Valencia trees. Significant differences were observed between the treatments for Hamlin preharvest fruit drop at location 1, where the citrus trees treated with low and medium rates showed statistically higher fruit drops (∼19% more) than the untreated control (Fig. 3A). However, a statistically significant difference between the diuron rates and controls was not observed at location 2 (Fig. 3B). Similarly, in Valencia trees, there was no statistically significant difference between the treatments for preharvest fruit drop data at both locations (Fig. 4).
Hamlin citrus preharvest fruit drop at (A) Location 1 and (B) Location 2. The total fruit drop percentage is calculated as [cumulative dropped fruit number/total fruit number (i.e., sum of dropped fruit and attached fruit on the tree at harvest)] * 100. Letters above the bar, if present, represent significant differences between the treatments based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Hamlin citrus preharvest fruit drop at (A) Location 1 and (B) Location 2. The total fruit drop percentage is calculated as [cumulative dropped fruit number/total fruit number (i.e., sum of dropped fruit and attached fruit on the tree at harvest)] * 100. Letters above the bar, if present, represent significant differences between the treatments based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Hamlin citrus preharvest fruit drop at (A) Location 1 and (B) Location 2. The total fruit drop percentage is calculated as [cumulative dropped fruit number/total fruit number (i.e., sum of dropped fruit and attached fruit on the tree at harvest)] * 100. Letters above the bar, if present, represent significant differences between the treatments based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Valencia citrus preharvest fruit drop at (A) Location 1 and (B) Location 2. The total fruit drop percentage is calculated as [cumulative dropped fruit number/total fruit number (i.e., sum of dropped fruit and attached fruit on the tree at harvest)] * 100. Letters above the bar, if present, represent significant differences between the treatments based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Valencia citrus preharvest fruit drop at (A) Location 1 and (B) Location 2. The total fruit drop percentage is calculated as [cumulative dropped fruit number/total fruit number (i.e., sum of dropped fruit and attached fruit on the tree at harvest)] * 100. Letters above the bar, if present, represent significant differences between the treatments based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Valencia citrus preharvest fruit drop at (A) Location 1 and (B) Location 2. The total fruit drop percentage is calculated as [cumulative dropped fruit number/total fruit number (i.e., sum of dropped fruit and attached fruit on the tree at harvest)] * 100. Letters above the bar, if present, represent significant differences between the treatments based on Tukey’s honestly significant difference test (α = 0.05). Error bars represent standard error (n = 4).
Citation: HortScience 58, 12; 10.21273/HORTSCI17359-23
Discussion
The total root length of Hamlin and Valencia citrus trees for all treatments, including controls, was low, most probably due to HLB. Previous studies have consistently shown a decline in the root systems in HLB-affected citrus trees compared with healthy trees (Atta et al. 2020; Graham et al. 2013; Hamido et al. 2019; Kumar et al. 2018). For instance, in the case of 3-year-old Hamlin and 4-year-old Valencia citrus trees with HLB, it has been observed that they lose more than 37% of their fibrous root system. This loss is attributed to HLB-induced root damage and the inhibition of new root growth, as documented in studies by Graham et al. (2013) and Johnson et al. (2014). The results of our study on RGR indicate that diuron herbicide showed no adverse effect on citrus root growth under any of the conditions we evaluated in this study. Our findings agree with Futch and Singh (1997), in which diuron applied at four different rates did not cause significant differences in the root weight of two citrus rootstocks: Swingle and Carrizo, though this was before the discovery of HLB in Florida. Soil-applied diuron can leach to the root zone and be rapidly absorbed by the citrus roots (Hirose 1979). Once absorbed, diuron (a phenylurea herbicide) is transported from the roots to the plant leaves, where it inhibits the Hill reaction during plant photosynthesis (Bayer and Yamaguchi 1965; Monaco et al. 2002). This rapid absorption and vascular transport may be the underlying reason why diuron did not show a negative effect on root growth in our study. Furthermore, it is important to acknowledge that variability in rootstocks of the cultivars in our experiments could potentially influence these observations.
At location 1, Valencia citrus trees treated with a high rate of diuron exhibited a lower Ct value, indicating a higher bacterial titer, in contrast to trees in the weed-checked control plots. However, when compared with the trees in the untreated control, Valencia trees treated with diuron at all rate levels displayed Ct values that were statistically similar. It is worth noting that this trend was observed exclusively at this particular location. Other factors, such as environment, climate, and Asian citrus psyllid population (Diaphorina citri, insect vector of HLB disease) could have influenced the difference in HLB disease severity. Nevertheless, regardless of location or cultivar, all other diuron applications showed no effect on disease severity. Because limited literature is available, further research is necessary to correlate herbicide effects on HLB bacterial infection.
The lack of significant differences in both citrus cultivars implies that diuron, when applied within the label rates, does not affect citrus FDF. In general, low FDF was observed as fruits reached maturity. Low FDF indicates that fruits could be separated from the branches with comparatively less force (Alferez et al. 2021; Tang et al. 2019; Vashisth et al. 2019).
The differences observed between the treatments for preharvest fruit drop did not follow any trends. For example, at location 1, Hamlin orange trees treated with low and medium diuron rates showed higher fruit drop than the control; however, this trend for fruit drop in Hamlin orange trees was not observed in the other trial location. A previous study on muscadine grapes has also reported inconsistent fruit shattering before harvest due to diuron application (Lane and Daniell 1973). In addition, there was no significant difference in preharvest fruit drop in Valencia orange trees due to diuron application in either location. Similar results were reported by an experiment conducted on highbush blueberries, where diuron applied for weed management did not negatively affect the productivity of blueberry plants (Welker and Brogdon 1968). Overall, diuron applied for preemergence weed management in citrus was not found to affect preharvest fruit drop when used within the labeled range of product rates.
Differences in soil composition between the two locations may help account for the subtle variations in disease severity and fruit drop observed in Hamlins between the locations. Analysis of the soil (Table 1) indicates a slightly greater silt and clay content in location 1 when compared with location 2, implying a possible increase in diuron retention (Sheng et al. 2001) and a subsequent decrease in its leaching below the root zone. This potential for extended diuron residue presence warrants further exploration, particularly in the context of the relatively higher Ct values and fruit drop rate observed for the Hamlin cultivar at location 1. Consequently, it is crucial to undertake additional research to understand the environmental fate and mobility of diuron in the sandy soils typical of citrus production in Florida. Furthermore, investigating diuron’s uptake and translocation within citrus plants is essential for a comprehensive assessment of its impact on citrus root development, overall health, and productivity.
Conclusions
Diuron is an extensively used nonselective herbicide for preemergence suppression of weeds in citrus orchards. In this study, diuron was applied in two consecutive seasons for preemergence weed management in Hamlin and Valencia citrus orchards. Diuron was not found to affect RGR and fruit detachment force in any of these citrus cultivars tested. Although we detected some trends in CLas titer and fruit drop analysis, these were inconsistent between the study locations. Thus, it seems most likely that these trends were not associated with diuron treatments but with other unknown factors that are usually associated with the HLB disease cycle. Based on our observations, we conclude that diuron is a crop-safe weed management tool in citrus production. However, we suggest carefully following the recommended product rate and other precautions mentioned on the label when using preemergence herbicides, such as diuron, in citrus orchards to control weeds.
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