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Herbal extracts: For green control of citrus Huanglongbing

  • Yuning Li , Maosheng Zeng , Zehua Liu , Yigang Lin und Hanhong Xu EMAIL logo
Veröffentlicht/Copyright: 24. Oktober 2025
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Open Agriculture
Aus der Zeitschrift Open Agriculture Band 10 Heft 1

Abstract

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), poses a substantial threat to global citrus agriculture. Current management strategies (planting disease-free nursery trees, implementing effective psyllid control, and removing all symptomatic trees) are inadequate for remedying diseased trees or alleviating economic losses. Therefore, new alternative methods for HLB control are urgently needed. In this study, the ethanol extracts from four traditional Chinese medicinal herbs: Sophora japonica (SJ), Lonicera japonica Thunb. (LJT), Taraxaci herba (TH), and Scutellaria baicalensis Georgi (SBG), were selected for the treatment assay of HLB. Analysis of flavonoid content revealed that SJ extract possesses the highest concentration, followed by LJT, TH, and SBG extracts. The potted studies demonstrated that all extracts reduced the CLas titers, with the SJ extract achieving the greatest significant reduction at a depletion rate of 79.05%. Field tests confirmed the remarkable efficacy of SJ extract, achieving a 99.1% reduction in CLas titers after 150 days. Physiological studies revealed that all extracts increased photosynthetic pigment concentrations, reduced starch/soluble sugar accumulation, and mitigated reactive oxygen species oxidative damage in HLB-affected plants, with SJ extract showing statistical significance relative to other treatments. Furthermore, herb extracts mitigated soil acidification, elevating pH beyond control levels, with SJ exhibiting the most prolonged effect. A comparative assessment of SJ extract and rutin, an established anti-HLB flavonoid, on HLB cure was performed, revealing no significant difference in efficacy. These findings establish SJ extract and flavonoid-rich botanicals as promising eco-friendly, environmentally sustainable alternatives for HLB management.

Keywords: citrus Huanglongbing; herbal extracts; CLas titers; physiological changes

1 Introduction

Citrus holds a significant status among global commercial crops, with the largest cultivation area worldwide [1]. The cultivation and production of citrus are beset by several dangers, with the detrimental impact of citrus Huanglongbing (HLB) being especially significant, severely impeding the citrus industry [2,3]. HLB infection impedes the growth and development of the entire citrus tree, resulting in leaf chlorosis and fruit deformity. Citrus HLB is induced by Candidatus Liberibacter asiaticus (CLas) [4,5]. The Asian citrus psyllid, Diaphorina citri, is the principal vector for disseminating this destructive disease [6]. Given that CLas predominantly inhabits the phloem tissue, which is difficult for bactericides to penetrate, managing citrus HLB presents a significant problem [7,8]. The primary approaches for managing citrus HLB are chemical control, biological control, and agronomic control. However, each method possesses inherent limits. Chemical control employs antibiotics and pesticides to regulate the disease and its vectors; however, prolonged usage may result in resistance, environmental contamination, and possible health hazards to humans [9]. Furthermore, once the trees are afflicted, the sickness is incurable [10]. Biological control entails the introduction of natural predators or competitors to diminish the population of disease vectors; nevertheless, its efficacy may vary between ecosystems, and it cannot entirely eradicate the illness [11]. Agricultural treatment approaches, including pruning and the removal of diseased trees, offer temporary respite but do not fundamentally eradicate the disease, which frequently reemerges [12].

Herbal extracts have garnered heightened interest as a natural remedy in recent years compared to conventional techniques. Herbs in traditional Chinese medicine are rich in bioactive molecules, notably those exhibiting antioxidant, antibacterial, and anti-inflammatory properties. Additionally, these natural constituents are more biodegradable in the environment, reducing the potential for pollution. Consequently, employing herbal extracts as a novel approach for managing citrus HLB not only adheres to sustainable development principles but also diminishes pesticide application, thereby safeguarding ecological equilibrium. Flavonoids are among the most significant bioactive natural constituents of herbal medicines. Flavonoids, as secondary metabolites of plants, are prevalent in numerous herbs and have potent antioxidant, anti-inflammatory, and antibacterial properties. Research indicates that flavonoids can efficiently neutralize reactive oxygen species (ROS), diminish the buildup of free radicals such as hydrogen peroxide, and mitigate immunological responses and oxidative damage resulting from pathogen infections in plants [9]. Consequently, flavonoids exhibit considerable promise in mitigating citrus HLB symptoms and bolstering the disease resistance of citrus trees.

Sophora japonica (SJ) is abundant in flavonoids, particularly rutin, which possesses potent antioxidant qualities, efficiently neutralizing free radicals and safeguarding cells from oxidative harm. The flavonoids in SJ exhibit potent antibacterial effects, suppressing the proliferation of diverse pathogens, hence rendering it extensively utilized for antimicrobial and antioxidant therapies [13,14]. Lonicera japonica Thunb. (LJT) is a prevalent medicinal herb rich in flavonoids, including luteoloside, which have antibacterial, antiviral, and antioxidant properties. Its antioxidant capabilities render it an efficacious natural agent for mitigating cellular damage induced by ROS and immunological responses, potentially contributing positively to the alleviation of symptoms associated with citrus HLB [15,16]. Taraxaci herba (TH), an herb containing several medicinal components, is abundant in flavonoids such as luteolin and its glycosides, quercetin, and others. These flavonoids possess potent antioxidant properties and can modulate the plant’s immunological response, so augmenting its disease resistance [17–19]. Scutellaria baicalensis Georgi (SBG) is a widely utilized herb in traditional medicine, abundant in baicalin and baicalein, flavonoid chemicals known for their notable antioxidant, anti-inflammatory, and antibacterial characteristics. Baicalein possesses a robust capacity to scavenge free radicals, hence mitigating oxidative stress induced by pathogens and enhancing plant health [20,21,22].

We wonder whether herbal extracts can be applied for green control of citrus HLB. Given that HLB symptoms arise from systemic acquired resistance triggered by effector protein-induced immunity and ROS stimulated by CLas, antioxidative agents may mitigate hydrogen peroxide levels in phloem tissue, thereby diminishing this immune response and facilitating the restoration of citrus health [23,24]. Consequently, flavonoids, potent antioxidative polyphenolic chemicals derived from secondary plant metabolites, were designated as the target actives for extraction. Four flavonoid-rich herbs, SJ, LJT, TH, and SBG, were extracted using ethanol and water to produce flavonoids, and the therapeutic efficacy of the crude extracts against citrus HLB was evaluated. We aim to establish a scientific foundation for the efficacy of herbal extracts in managing citrus HLB by examining the CLas content and physiological parameters of citrus subjected to herbal extract therapies. Herbal extracts are expected to emerge as a novel technique for the environmentally friendly management of citrus HLB and offer potential candidates for therapeutic medicines against HLB.

2 Materials and methods

2.1 Materials

2.1.1 Medicine

SJ, LJT, TH, and SBG were all purchased from Guangzhou Nanbeixing Chinese Herbal Pieces Co., Ltd.

2.1.2 Regents

The reagents used and reagent manufacturers in experiments are shown in Table 1.

Table 1

Reagents and reagent manufacturers

Regent Manufacturer
0.1 mmol/L DPPH ethanol solution Fuzhou Feijing Biotechnology Co., Ltd
Anhydrous ethanol (analytical grade) Tianjin Fuyu Fine Chemical Co., Ltd
1.0 M PBS (pH 7.0), 1.0 M KI, 0.6% TBA, 10% (W/V) TCA Aladdin Biochemical Technology Co., Ltd
Sodium nitrite (analytical grade), aluminum nitrate (analytical grade) Tianjin Yongda Chemical Reagents Co., Ltd
Sodium hydroxide (analytical grade) Fuchen (Tianjin) Chemical Reagents Co., Ltd
Sulfuric acid (analytical grade) Guangzhou Chemical Reagents Factory
Rutin hydrate (95%) Sahn Chemical Technology (Shanghai) Co., Ltd
Starch reagent kit Solvay Biotechnology Co., Ltd
Plant DNA extraction kit D2485-05 Omega Bio-Tek, USA
Real-time fluorescent PCR primers, probes Shenggong Biotechnology Co., Ltd
PerfectStart™ II Probe qPCR SuperMix Quanshi Jin Biotechnology Co., Ltd
30% hydrogen peroxide, chloroform (analytical grade), isoamyl alcohol (analytical grade), β-mercaptoethanol (analytical grade) Guangzhou Jianyang Biotechnology Co., Ltd

2.2 Methods

2.2.1 Extraction of herbs

200 g of each herb (SJ, LJT, TH, and SBG) were subjected to a drying oven at 55°C for 48 h. Subsequently, they were crushed and subjected to a 40-mesh sieve. The sieved components were combined with anhydrous ethanol, 70% ethanol, and water at a ratio of 1:10 (w:w), respectively. The suspensions were subsequently sonicated for 1 h and filtered. The filtrates were evaporated under vacuum to achieve a viscous consistency. Subsequently, they were dispensed and transported to a freeze dryer for desiccation to yield pastes. The pastes were incorporated into 50% ethanol at a concentration of 2.5 g/L for subsequent utilization [25].

2.2.2 Determination of total flavonoid content

The method for total flavonoid content analysis was referred to that described in the study by Ramani et al. [26]. The total flavonoid content was assessed using the NaNO2-AI(NO3)3 colorimetric technique. 1.0 mL of 5% NaNO2 was incorporated into 2.0 mL of a 2.5 g/L herbal extract solution (2.5 g/L solution of 50% ethanol diluted with water). The solution was well agitated and allowed to rest for 6 min. Subsequently, 1.0 mL of 10% AI(NO3)3 was incorporated. The acquired solution was thoroughly agitated and allowed to rest for 6 min. Subsequently, 10 mL of 4% NaOH and anhydrous ethanol were combined to achieve a total volume of 25 mL. The resulting solution was thoroughly agitated and allowed to rest for 15 min. The absorbance at 510 nm was measured using an N5000/N5000Plus UV-Vis spectrophotometer. The total flavonoid content was determined using the absorbance standard curve of rutin, a specific flavonoid. Every sample was replicated thrice.

2.2.3 Determination of DPPH free radical scavenging capacity

The DPPH free radical scavenging ability was assessed according to the literature [27]. The subsequent solutions were prepared: Solution 1: 0.5 mL of 2.5 g/L herbal extract, 1.0 mL of 0.1 mmol/L DPPH ethanol solution, and 1.5 mL of 40% ethanol; Solution 2: 0.5 mL of anhydrous ethanol, 1.0 mL of 0.1 mmol/L DPPH ethanol solution, and 1.5 mL of 40% ethanol; and Solution 3: 1.5 mL of anhydrous ethanol and 1.5 mL of 40% ethanol. All solutions were well agitated and allowed to rest for 30 min to confirm the completion of the reaction. The absorbance of solution 1, solution 2, and solution 3 at 517 nm was measured and documented as A1, A2, and A3, respectively. The DPPH free radical scavenging rate was determined using the subsequent equation. Each sample was replicated thrice.

(1) DPPH free radical scavenging rate = 1 − A 1 − A 3 A 2 × 100 % .

2.2.4 HLB inoculation

The plant components exhibiting HLB in the indoor experiment were acquired through grafting. The HLB bud grafting procedure as cited in the literature [28]. Infected buds were grafted into healthy Hongjiang orange trees by ventral grafting, with one infected bud per plant. The grafting location is situated on the primary stem, 5–10 cm above the soil surface. All infected buds originated from the same branch afflicted with HLB and were transplanted at 1-month intervals for a total of four instances.

2.2.5 Plant trial

The indoor experiment was conducted at South China Agricultural University in Guangzhou City, Guangdong Province, China (113°21′34″E, 23°10′9″N), utilizing annual Hongjiang orange trees as the test subjects. Herbal extracts were administered weekly for a duration of 6 months. For each treatment group, one plant received 200 mL of 2.5 g/L herbal extract by foliar application and 200 mL of 2.5 g/L herbal extract through root irrigation. Three biological replicates were performed in each treatment group. Distilled water served as the control.

The field study was conducted at an orchard situated in Huidong County, Huizhou City, Guangdong Province, China (114°55′8″E, 23°3′35″N). The 7-year-old Orah mandarins (Citrus reticulata cv. Orah) were used for the experiments. This species was identified to be the susceptible variety of CLas, the pathogen of HLB [29]. The orchard was managed with good water and fertilizer management. The prior survey indicated that the infection rate of HLB in citrus trees across the orchard was approximately 25%. 15 trees afflicted with HLB were designated as experimental participants, organized into five groups (SJ, LJT, TH, SBG, and control group [CK]), each comprising three trees. The herbal extracts were administered biweekly for a duration of 5 months. For each treatment group, 10 L of 2.5 g/L herbal extract were applied via foliar spray, and 10 L of 2.5 g/L herbal extract were administered through root irrigation per tree each time. Three biological replicates were performed in each treatment group. Distilled water served as a control.

To evaluate the therapeutic effects of rutin and SJ extracts, the annual Hongjiang orange trees were treated every 24 h for 5 days. For the herbal extract treatment group, 200 mL of 2.5 g/L herbal extract was applied via foliar spray, and 200 mL of 2.5 g/L herbal extract was administered through root irrigation per tree. For the rutin treatment group, 200 mL of 2.5 g/L rutin solution was applied via foliar spray, and 200 mL of 2.5 g/L herbal extract was administered to the roots via irrigation per tree. Three biological replicates were performed in each treatment group.

2.2.6 Measurement of CLas concentration

The DNA of the citrus leaf blades was extracted utilizing the HP Plant DNA Kit from OMEGA BIO-TEK, with each sample being replicated three times. The total plant DNA collected was evaluated for purity and concentration using a nucleic acid quantifier (OD260/280, NanoDrop, Thermo Fisher Scientific). Real-time fluorescence quantitative PCR was conducted to quantify the concentration of CLas (expressed as the number of CLas copies per nanogram of DNA, copies/ng DNA), in accordance with the literature [30]. CLas concentration values for samples from several locations were presented as mean values ± standard error. The CLas reduction rate was calculated as: Reduction Rate ( % ) = 1 − C post C pre × 100 % . Among them, Cpre = CLas concentration before treatment; Cpost = CLas concentration after treatment.

2.2.7 Measurement of starch content

The method for starch content analysis was referred to that described in previous literature [31]. Starch content was assessed utilizing the Starch Content Assay Kit (Solarbio, Cat#BC0700) from Solarbio Life Sciences (Beijing, China) in accordance with the manufacturer’s instructions. Each treatment was subjected to three biological replicates.

2.2.8 Measurement of hydrogen peroxide content

The hydrogen peroxide content analysis was assessed according to the literature [32]. 0.1 g of leaf midrib was rapidly excised and placed into liquid nitrogen for grinding. The ground material was thereafter placed into a centrifuge tube. 1.5 mL of 0.1% trichloroacetic acid (TCA) was introduced into the tube to homogenize the sample. The suspension was subsequently centrifuged for 20 min at 12,000 rpm at 4°C. 0.5 mL of supernatant was transferred into a fresh centrifuge tube. Subsequently, 0.5 mL of 1.0 M PBS (pH 7.0) and 1.0 mL of potassium iodide (KI) were included. The solution was thoroughly agitated and stored in a dark environment at ambient temperature. After 2 h, the absorbance at 390 nm of the solution was assessed using an N5000/N5000Plus UV-Vis spectrophotometer, using 0.1% TCA as the reference standard. Each sample underwent three repetitions. The concentration of hydrogen peroxide was determined using the standard curve of its absorbance.

2.2.9 Measurement of malondialdehyde (MDA) content

0.5 g leaves were fragmented and placed in a mortar. Subsequently, 1.0 mL of 10% TCA and a little quantity of quartz sand were introduced into the mortar. The sample was pulverized into a uniform slurry, and 4.0 mL of TCA was included for additional grinding. The resultant homogeneous slurry was transferred to centrifuge tubes and subjected to centrifugation at 4,000 rpm for 10 min. 2.0 mL of supernatant were pipetted into 2.0 mL of 0.6% thiobarbituric acid (TBA) solution, with a control consisting of 2.0 mL of water combined with 2.0 mL of 0.6% TBA. The amalgamation was subjected to a boiling water bath for 15 min. After cooling, the suspension was subjected to centrifugation once again. The absorbance at 450 nm (A 450), 532 nm (A 532), and 600 nm (A 600) of the supernatant was quantified using an N5000/N5000Plus UV-Vis spectrophotometer. Each sample underwent three repetitions. The MDA concentration was determined using the subsequent equation.

(2) MDA concentration μ mol L = 6.45 × ( A 532 − A 600 ) − 0.56 × A 450 .

The MDA content was calculated according to the following equation:

(3) MDA content μ mol g = MDA concentration × 4 × 10 − 3 0.5 × 2 5 .

2.2.10 Measurement of photosynthetic pigment content

The content of photosynthetic pigments was quantified according to the literature [33]. Fresh leaves were rinsed and desiccated, and the midrib was excised. Samples of 0.2 g were excised and thereafter introduced into 25 mL of 95% ethanol. The amalgamation was thoroughly agitated and allowed to rest in obscurity. After 24 h, the combination underwent sonication for 20 min and was then allowed to rest in darkness for a further 24 h. Each sample underwent three repetitions. The absorbance at 470 nm (A 470), 649 nm (A 649), and 665 nm (A 665) of the supernatant was quantified using an N5000/N5000Plus UV-Vis spectrophotometer. The concentrations of chlorophyll a (C a), chlorophyll b (C b), total chlorophyll (C a+b), and carotenoid (C c) were determined using the subsequent equations.

(4) C a mg L = 13.95 × A 665 − 6.88 × A 649 ,

(5) C b mg L = 24.96 × A 649 − 7.32 × A 665 ,

(6) C a + b mg L = 18.08 × A 649 + 6.63 × A 665 ,

(7) C c mg L = 1000 × A 470 − 2.05 × C a − 114.8 × C b 245 ,

(8) Chloroplast pigment content mg g = C c × 0.025 0.2 .

2.2.11 Measurement of soil pH

The soil pH was determined according to the literature [34]. Three spots were randomly chosen diagonally for each plant, extending from the trunk of the tree to two-thirds of the crown’s edge. Thirty grams of mixed soil samples were weighed from each station, and 120 mL of water was added. The suspension was thoroughly mixed, agitated for 5 min, and allowed to rest for 40 min. The pH was determined using a pH meter, with three measurements recorded for each soil sample, and the average calculated for statistical analysis.

2.3 Statistical analysis

Root diameter measurements were repeated five times, and other data were repeated three times, and errors were analyzed with standard deviations. The p-values were calculated by one-way analysis of variance (ANOVA) followed by a Student t-test for validation analysis using GraphPad Prism (version 8.0). All bar graphs were analyzed by ANOVA (one-way) and plotted using GraphPad Prism (version 8.0) software. All line graphs were performed using OriginPro (version Learning Edition) software to perform data statistics and plot graphs. All tables were performed using Excel (version 2016) for data statistics and plotted as tables.

3 Result

To mitigate the environmental impact of organic solvents during the extraction process, and considering the strong polarity of flavonoids, we employed ethanol and water for herb extraction. Table 2 illustrates that the extraction rate for total compounds using 70% ethanol and water was markedly superior to that of anhydrous ethanol, indicating that the inclusion of water enhances the extraction of additional substances. The total flavonoid content was subsequently quantified. Figure 1a illustrates that the total flavonoids extracted from the four herbs with 70% ethanol significantly exceeded those extracted with anhydrous ethanol and water. Considering the extraction efficiency for total compounds, 70% ethanol is more effective for extracting the four herbs compared to anhydrous ethanol and water. The extraction of SJ yielded the highest total flavonoid concentration at 205.53 mg using 70% ethanol (200 g of herbs), followed by LJT at 118.30 mg, TH at 32.77 mg, and SBG at 17.27 mg, respectively.

Table 2

Extraction rates of ethanolic extracts of four herbs

Herb Extraction rate (%)
Anhydrous ethanol 70% ethanol Water
SJ 17.8 39.1 25.6
LJT 10.9 31.7 26.8
TH 6.3 21.3 17.2
SBG 4.2 27.5 32.6

Note: SJ: S. japonica, LJT: L. japonica, TH: T. herba, SBG: S. baicalensis.

Figure 1 Total flavonoid concentration and free radical scavenging efficacy (DPPH) of four herbal preparations. (a) Total flavonoid content collected from four herbal extracts using various solvent conditions. 1 denotes herbs extracted using 70% ethanol, 2 denotes herbs extracted using anhydrous ethanol, and 3 denotes herbs extracted using water. (b) Free radical scavenging activity of four herbal extracts using the DPPH technique. SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 1

Total flavonoid concentration and free radical scavenging efficacy (DPPH) of four herbal preparations. (a) Total flavonoid content collected from four herbal extracts using various solvent conditions. 1 denotes herbs extracted using 70% ethanol, 2 denotes herbs extracted using anhydrous ethanol, and 3 denotes herbs extracted using water. (b) Free radical scavenging activity of four herbal extracts using the DPPH technique. SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

The antioxidative capabilities of the four extracts were evaluated using the DPPH free radical scavenging rate. The antioxidative capacities of the four extracts exhibited no significant differences, as depicted in Figure 1b. However, a marked reduction in antioxidative capacities was noted in the flavonoid contents relative to the other three extracts, underscoring considerable variation in flavonoid content among them. These data suggest that flavonoids are not the sole antioxidative compounds present in the extracts.

To ascertain the efficacy of herbal extracts in inhibiting the growth of CLas, Hongjiang orange trees were initially employed to examine the therapeutic effects of these extracts in controlled indoor studies. As illustrated in Figure 2a, CLas was identified in the plants of all groups at 120 days, as infected buds were grafted monthly. Nonetheless, the CLas contents varied among the five groups. The CLas levels in citrus treated with herbal extracts were lower than that in the control group, indicating the therapeutic efficacy of the four herbal extracts. Of the four herbal extracts, SJ extracts demonstrated the greatest therapeutic efficacy, with CLas concentrations reaching 1,551 copies/ng DNA. SBG demonstrates the least therapeutic efficacy. The CLas levels diminished following the cessation of CLas inoculation. For the SJ group, the abatement rate at 180 days was 79.03%, over double the abatement rates observed in CK, indicating exceptional therapeutic efficacy (Figure 2b).

Figure 2 Efficacy of four herbs against CLas in a controlled indoor experiment. (a) Dynamics of CLas content. (b) Reduction rate of CLas contents (from 120 to 180 days). (c) Correlation between the abatement rates of CLas and the total flavonoid concentrations of four herbal preparations. (d) Correlation between the abatement rates of CLas and the antioxidant capabilities of four herbal extracts. The abatement rates of CLas are all comparative to CK. The data were analyzed utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk indicates that the observed difference is statistically significant (**P < 0.01), whereas “ns” denotes no significant difference. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 2

Efficacy of four herbs against CLas in a controlled indoor experiment. (a) Dynamics of CLas content. (b) Reduction rate of CLas contents (from 120 to 180 days). (c) Correlation between the abatement rates of CLas and the total flavonoid concentrations of four herbal preparations. (d) Correlation between the abatement rates of CLas and the antioxidant capabilities of four herbal extracts. The abatement rates of CLas are all comparative to CK. The data were analyzed utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk indicates that the observed difference is statistically significant (**P < 0.01), whereas “ns” denotes no significant difference. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

We seek to understand the factors that affect the therapeutic efficacy of herbal extracts on citrus HLB. Consequently, the correlation between the abatement rates of CLas and the total flavonoid contents or antioxidative capacities was examined. Figure 2c illustrates a positive association between the abatement rates of CLas and the total flavonoid contents of the four herbal extracts. No discernible link existed between the abatement rates of CLas and the antioxidative powers of the four herbal extracts (Figure 2d). Despite the similar antioxidative powers of the four extracts, SJ, being the richest in flavonoids, exhibited the most effective therapeutic activity. These findings indicated that not all antioxidative compounds are effective in treating citrus HLB, and flavonoids possess activities beyond their antioxidant properties. Furthermore, the flavonoid content may hold greater significance than the specific types of flavonoids in the regulation of citrus HLB.

The therapeutic effects of herbal extracts on citrus HLB in the field were further investigated. The primary symptoms of plants afflicted by citrus HLB are leaf chlorosis and root hypertrophy. Figures 3a–f and 4a and b illustrate the field symptoms of plants subjected to four varieties of traditional Chinese medicine ethanol extracts, HLB-negative plants, and CK-treated plants prior to treatment, 50 days post-treatment, and the symptoms of leaves and roots 150 days post-treatment. Fifty days post-treatment, all treated plants, with the exception of the HLB-negative specimens, exhibited varied degrees of chlorosis in their leaves. The plants treated with locust seed ethanol extract exhibited a markedly decreased level of chlorosis. One hundred and fifty days post-treatment, the foliar and root symptoms across all treatments exhibited variable degrees of alteration. In comparison to the CK treatment and HLB-negative plants, those treated with SJ ethanol extract and LJT ethanol extract exhibited a large decrease in leaf chlorosis and a significant reduction in root swelling. The plants subjected to SBG and TH ethanol extracts exhibited pronounced leaf chlorosis, but root swelling was marginally mitigated. Additionally, the CLas concentrations in leaves and roots following treatments were examined (Figure 4c and d). CLas levels diminished across all groups but exhibited a more pronounced reduction following treatment with the four herbal preparations. SJ extracts demonstrated the most effective reduction of CLas concentration, achieving abatement rates of 97.0 and 99.1% after 150 days, which was considerably superior to CK (Figure 4e and f). The impact of LJT extract was comparable to that of SJ extract. Given the elevated extraction rate of SJ, it is posited that SJ possesses the most significant potential as a raw material for the extraction aimed at treating citrus HLB across all four species.

Figure 3 Comprehensive plant symptoms across the field resulting from the use of four traditional Chinese medicine extracts in the prevention and treatment of citrus HLB. (a) Comparison of field-wide symptoms in HLB-negative trees before and after a 50-day period. (b) Comparison of plant symptoms across the field prior to and after 50 days of SJ treatment. (c) Comparison of plant symptoms across the field prior to and following 50 days of LJT treatment. (d) Comparison of plant symptoms across the field prior to and following 50 days of TH treatment. (e) Comparison of plant symptoms across the field prior to and following 50 days of SBG treatment. (f) Comparison of field-wide plant symptoms of CK trees prior to and following a 50-day period. The field experimental orchard is located in Xiashenlong (114°55′8 ″E, 23°3′35″N), Duozhu Town, Huidong County, Huizhou City, Guangdong Province, China. The tested plants are 7-year-old fertile orange. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group. All the photos in Figure 3 are original photos taken by the author.
Figure 3

Comprehensive plant symptoms across the field resulting from the use of four traditional Chinese medicine extracts in the prevention and treatment of citrus HLB. (a) Comparison of field-wide symptoms in HLB-negative trees before and after a 50-day period. (b) Comparison of plant symptoms across the field prior to and after 50 days of SJ treatment. (c) Comparison of plant symptoms across the field prior to and following 50 days of LJT treatment. (d) Comparison of plant symptoms across the field prior to and following 50 days of TH treatment. (e) Comparison of plant symptoms across the field prior to and following 50 days of SBG treatment. (f) Comparison of field-wide plant symptoms of CK trees prior to and following a 50-day period. The field experimental orchard is located in Xiashenlong (114°55′8 ″E, 23°3′35″N), Duozhu Town, Huidong County, Huizhou City, Guangdong Province, China. The tested plants are 7-year-old fertile orange. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group. All the photos in Figure 3 are original photos taken by the author.

Figure 4 Impact of four herbal extracts on the management of citrus HLB in a field study. (a) Symptoms of citrus leaves. The images in the upper row depict leaves prior to the administration of herbal extracts, while the images in the lower row illustrate leaves following a 150-day treatment period. Healthy denotes HLB-negative plant, whereas CK signifies HLB-positive plant. The herbal extracts were administered biweekly for a duration of 5 months. (b) Diameter of fibrous roots of citrus under various treatments in the field. 1 signifies prior to application, whereas 2 denotes after 150 days of application. Healthy denotes HLB-negative plant, whereas CK signifies HLB-positive plant. The herbal extracts were administered biweekly for a duration of 5 months. The diameter of the fibrous roots of the plants across various treatments was assessed at five distinct sites per plant. Dynamics of CLas content in citrus leaves (c) and roots (d). Rate of reduction of CLas content in citrus leaves (e) and roots (f). (g) Correlation between the abatement rates of CLas and the total flavonoid concentrations of four herbal preparations. (h) Correlation between the abatement rates of CLas and the antioxidant capabilities of four herbal extracts. The abatement rates of CLas are all related to CK, which denotes HLB-positive plants. Data were evaluated utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk indicates that the observed difference is statistically significant (**P < 0.01, ***P < 0.001, ****P < 0.0001), whereas “ns” denotes no significant difference. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 4

Impact of four herbal extracts on the management of citrus HLB in a field study. (a) Symptoms of citrus leaves. The images in the upper row depict leaves prior to the administration of herbal extracts, while the images in the lower row illustrate leaves following a 150-day treatment period. Healthy denotes HLB-negative plant, whereas CK signifies HLB-positive plant. The herbal extracts were administered biweekly for a duration of 5 months. (b) Diameter of fibrous roots of citrus under various treatments in the field. 1 signifies prior to application, whereas 2 denotes after 150 days of application. Healthy denotes HLB-negative plant, whereas CK signifies HLB-positive plant. The herbal extracts were administered biweekly for a duration of 5 months. The diameter of the fibrous roots of the plants across various treatments was assessed at five distinct sites per plant. Dynamics of CLas content in citrus leaves (c) and roots (d). Rate of reduction of CLas content in citrus leaves (e) and roots (f). (g) Correlation between the abatement rates of CLas and the total flavonoid concentrations of four herbal preparations. (h) Correlation between the abatement rates of CLas and the antioxidant capabilities of four herbal extracts. The abatement rates of CLas are all related to CK, which denotes HLB-positive plants. Data were evaluated utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk indicates that the observed difference is statistically significant (**P < 0.01, ***P < 0.001, ****P < 0.0001), whereas “ns” denotes no significant difference. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

The significant significance of flavonoids in extracts for HLB management was confirmed by investigating the correlation between the abatement rates of CLas and the total flavonoid contents or antioxidative capacities in field trials. Figure 4g and h illustrate a positive association between the abatement rates of CLas and the overall flavonoid contents. Consequently, we proposed establishing flavonoid concentration as a criterion for the rapid assessment of plant extracts for HLB management.

We additionally assessed the therapeutic effects of herbal extracts by examining the alterations in the physiological parameters of the plants. Figure 5 illustrates that the citruses in the control group exhibited elevated starch concentrations in both the roots and the leaves. Following treatment with these four herbal extracts, a notable reduction in starch content was seen in both the roots and leaves of affected citrus plants. The starch content was classified from lowest to highest as SJ, LJT, TH, and SBG, reflecting the identical sequence noted for flavonoid content. This alignment indicates that flavonoids may possess functions beyond their antioxidative characteristics. The SJ group exhibited the most substantial reductions in root and leaf starch levels, with declines of 45.2 and 49.2%, respectively, compared to CK. The data indicate that SJ extract is efficient in alleviating the starch obstruction induced by HLB.

Figure 5 Degree of starch deposition in citrus in field trial. (a) Starch contents of leaves. (b) Starch contents of roots. Data were analyzed using one-way analysis of variance (ANOVA) and was validated using the Student t-test. An asterisk refers to that the observed difference is statistically significant (****P < 0.0001), ns refers to no significant difference. Healthy: The trees without CLas infection, CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 5

Degree of starch deposition in citrus in field trial. (a) Starch contents of leaves. (b) Starch contents of roots. Data were analyzed using one-way analysis of variance (ANOVA) and was validated using the Student t-test. An asterisk refers to that the observed difference is statistically significant (****P < 0.0001), ns refers to no significant difference. Healthy: The trees without CLas infection, CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

Another significant physiological indication is photosynthetic pigments. We quantified the photosynthetic pigment concentrations in the leaves of various groups following 100 days of treatment. Figure 6 illustrates that all four herbal extracts enhanced the levels of chlorophyll a, chlorophyll b, and carotenoid in the leaves. The most significant increases in photosynthetic pigment concentration were noted in the group treated with SJ extract relative to the control group. The augmentation of photosynthetic pigments signifies a degree of chloroplast structural regeneration, conducive to the recovery of photosynthesis and overall plant vitality.

Figure 6 Variations in leaf photosynthetic pigment concentrations during the field trial. (a) Alterations in leaf chlorophyll a concentration. (b) Alterations in leaf chlorophyll b concentration. (c) Variations in total chlorophyll concentrations in leaves. (d) Alterations in carotenoid concentrations inside foliage. Data were evaluated utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk denotes that the observed difference is statistically significant (***P < 0.001, ****P < 0.0001). CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 6

Variations in leaf photosynthetic pigment concentrations during the field trial. (a) Alterations in leaf chlorophyll a concentration. (b) Alterations in leaf chlorophyll b concentration. (c) Variations in total chlorophyll concentrations in leaves. (d) Alterations in carotenoid concentrations inside foliage. Data were evaluated utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk denotes that the observed difference is statistically significant (***P < 0.001, ****P < 0.0001). CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

Alongside alterations in physiological indicators, the oxidative damage to citrus leaves was mitigated by herbal extracts. Figure 7a illustrates that the leaves of the control group plants possessed a greater concentration of hydrogen peroxide. The concentration of hydrogen peroxide diminished following treatment with herbal extracts. In the four herbal extract groups, the hydrogen peroxide content in the leaves was ranked in descending order as follows: SJ, LJT, TH, and SBG. This ranking corresponds with the order of flavonoid content, indicating that the reduction in hydrogen peroxide levels may be attributed to the antioxidant properties of flavonoids. MDA exhibited a similar pattern in its concentrations (Figure 7b). The MDA content in the leaves of the SJ group exhibited the most significant drop among the four herbal extract groups, demonstrating a 43.8% decrease relative to the CK group. These findings indicate that SJ extract effectively mitigates oxidative damage in plant leaves induced by HLB.

Figure 7 Magnitude of oxidative damage in leaves during field testing. (a) Hydrogen peroxide contents of leaves. (b) MDA contents of leaves. Data were analyzed using one-way analysis of variance (ANOVA) and was validated using the Student t-test. An asterisk indicates that the observed difference is statistically significant (*P < 0.1, **P < 0.01, ***P < 0.001, ****P < 0.0001). ns indicates no significant difference. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 7

Magnitude of oxidative damage in leaves during field testing. (a) Hydrogen peroxide contents of leaves. (b) MDA contents of leaves. Data were analyzed using one-way analysis of variance (ANOVA) and was validated using the Student t-test. An asterisk indicates that the observed difference is statistically significant (*P < 0.1, **P < 0.01, ***P < 0.001, ****P < 0.0001). ns indicates no significant difference. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

The pH of the soil surrounding trees afflicted with citrus HLB was typically lower than that of the soil around healthy trees, resulting in soil acidification issues. Consequently, the alterations in soil pH were also assessed in this investigation. Figure 8 illustrates that the four herbal extracts considerably increased the soil pH to approximately 6.4–6.6 after 7 days of treatment, while the control group maintained a soil pH of around 6.0. Consequently, the four herbal extracts can enhance soil acidity, with the SJ therapy exhibiting the most pronounced improvement. The SJ extract showed superior performance in terms of extraction quantity, pathogen suppression, plant physiological metrics, and soil acidification mitigation, indicating its significant potential as an effective agent for controlling citrus HLB.

Figure 8 Variations in soil pH throughout a 30 day period in the field study. Irrigation denotes the timing of herbal extract application, whereas soil pH was assessed at 0, 7, 15, and 30 days following the initial application of herbal extracts. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.
Figure 8

Variations in soil pH throughout a 30 day period in the field study. Irrigation denotes the timing of herbal extract application, whereas soil pH was assessed at 0, 7, 15, and 30 days following the initial application of herbal extracts. CK: Control group, SJ: S. japonica treatment group, LJT: L. japonica treatment group, TH: T. herba treatment group, SBG: S. baicalensis treatment group.

To achieve a more precise evaluation of SJ extract’s efficacy, the therapeutic effects of rutin and SJ extract were evaluated. Rutin was selected as a reference compound due to its documented efficacy in managing citrus HLB. Figure 9a and b illustrate that the abatement rate of CLas following treatment with rutin was 99.6%, whereas the abatement rate after treatment with SJ extract was 86.6%. Despite the marginally inferior efficacy of SJ extract compared to rutin, the laborious and expensive extraction and purification procedure of rutin contrasts with the simplicity of obtaining SJ extract with a single extraction using 70% ethanol, which yields a high extraction rate. Consequently, SJ extract is more appropriate than rutin for promotion as a green botanical pesticide for the treatment of citrus HLB.

Figure 9 Therapeutic efficacy of Rutin and SJ extracts against CLas in a controlled indoor trial. (a) Dynamics of CLas contents. “Spray and Irrigate” refers to the time and method of application of pesticides. (b) Abatement rates of CLas content. Data were evaluated utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk denotes that the observed difference is statistically significant (**P < 0.01). Rutin: rutin treatment group; SJ: S. japonica treatment group,.
Figure 9

Therapeutic efficacy of Rutin and SJ extracts against CLas in a controlled indoor trial. (a) Dynamics of CLas contents. “Spray and Irrigate” refers to the time and method of application of pesticides. (b) Abatement rates of CLas content. Data were evaluated utilizing one-way analysis of variance (ANOVA) and confirmed through the Student t-test. An asterisk denotes that the observed difference is statistically significant (**P < 0.01). Rutin: rutin treatment group; SJ: S. japonica treatment group,.

4 Discussion

HLB is acknowledged as an immune-mediated disorder. Infection by CLas elicits both systemic and chronic immunological responses in citrus phloem tissue, encompassing callose deposition, ROS generation such as hydrogen peroxide, and the activation of immunity-related genes. Moreover, the disease enhances the expression of genes for NADPH oxidase, implicated in ROS generation, while suppressing genes for antioxidant enzymes, thus exacerbating oxidative stress induced by CLas infection [32]. Increased amounts of ROS are recognized to cause significant oxidative damage, impairing critical cellular components and ultimately resulting in cell death [35]. Herbal extracts have substantial ecological benefits compared to chemical pesticides due to their natural degradation, hence diminishing environmental contamination and promoting sustainable agriculture. Herbal extracts are acknowledged for their antibacterial and antioxidant characteristics, attributed to their bioactive ingredients such as polyphenols, rendering them a significant resource for the creation of botanical pesticides [36]. However, the studies using plant extracts for HLB control were relatively fewer. It was reported that the leaf extracts of Quercus hemisphaerica display curative effects against CLas that restore leaf physiological parameters in HLB-affected citrus trees [37]. In addition, five crude plant extracts of oregano (Origanum vulgare), Christmas bush (Alchornea cordifolia), thyme (Thymus vulgaris), cinnamon (Cinnamomum aromaticum), and turmeric (Curcuma longa) also showed the potential for controlling HLB [38]. In this study, the extracts from four herbal species exhibited the control effects on HLB, which were similar to the results reported as above. These findings suggest that the extracts of the plants above have the potential for HLB control. Of course, the specific compounds in different herbal species need to be further investigated.

Flavonoids are widely distributed in plants, and prior research has emphasized the function of flavonoids in augmenting resistance to diverse biotic and abiotic stimuli, including drought, cold, ultraviolet radiation, and toxic metals like aluminum, as well as in providing defense against fungal and microbial infections [39]. As powerful antioxidants, they can stimulate the plant’s antioxidant defense systems during stress, increasing the efficacy of antioxidant enzymes, decreasing oxidative damage, and alleviating cell death [40]. Besides, flavonoids have a dual function: they directly neutralize ROS and decrease the activity of NADPH oxidase [41]. A previous study found that the flavonoid content in citrus species is related to their degree of tolerance of HLB, and high levels of flavonoids might enhance citrus tolerance to HLB [42]. In this study, the four chosen herbal species encompass several bioactive components, including flavonoids, which demonstrate antioxidant capabilities by directly inhibiting ROS generation. This significantly mitigates harm inflicted by infections and bacteria [43]. In addition, SJ extract showed the highest cure efficacy of HLB among these extracts in indoor and field experiments. It was reported that more than 153 compounds have been isolated from SJ, and several flavonoids and isoflavonoids comprise the active constituents of SJ and exhibit antibacterial effects [44]. Along with the results of flavonoid contents in these materials, we suggest that flavonoids, potent antioxidative chemicals generated as secondary metabolites in these plants, could be the candidate bioactive components for HLB control. Similarly, considering the control effects of SJ extract, it can be popularized as a potential control drug for HLB in the future. Of course, we have realized our shortcomings, and the control effects of SJ extract on HLB should be verified across different environments and cultivars to facilitate the generalizability.

It is posited that HLB impairs the phloem sieve pore architecture in citrus, with callose deposition hindering starch transport, resulting in atypical starch buildup that compromises host cellular or subcellular structures [45,46]. Moreover, HLB infection modifies the redox equilibrium and precipitates oxidative stress [47]. The current investigation revealed that the ethanolic extracts of four herbs considerably decreased CLas concentrations in infected citrus trees and alleviated the physiological and biochemical effects of HLB in field experiments. These effects encompass diminished MDA synthesis and membrane lipid peroxidation, reduced ROS-mediated oxidative injury, and mitigation of starch and soluble sugar buildup in roots and leaves. Moreover, the herbal extracts augmented chlorophyll levels, diminished leaf chlorosis, and promoted root development, thus facilitating the restoration of citrus productivity and yield. Furthermore, the herbal extracts mitigated soil acidification problems typically linked to HLB-infected citrus farms. Prior studies demonstrated a correlation between soil acidity and the severity of HLB [48]. The current study demonstrated that the use of herbal extracts elevated soil pH, hence alleviating acidification and enhancing plant resilience to CLas. Flavonoids have been identified as the signaling molecules to attract specific microbial communities [49,50]. According to the results, it is suggested that flavonoids in the herbal extracts may also influence the variety of rhizosphere microbes and then alter the soil pH. Of course, the specific mechanism requires additional investigation.

The prophylactic benefits of the four herbal extracts against citrus HLB were examined. This study conducted four consecutive graft inoculations of CLas onto HLB-negative citrus plants, considering the time necessary for successful grafting. The grafting was performed on the identical branch of each plant. Following the fourth inoculation, CLas was identified in all treated plants, signifying that, under the experimental conditions, the four herbal ethanolic extracts exhibited no significant preventative benefits against HLB. From 0 to 120 days, the CLas concentrations fluctuated among treatments, indicating that the herbal extracts may not directly affect CLas during pathogen invasion but instead stimulate plant defense systems, resulting in notable disparities in CLas levels among treatments. Following the cessation of inoculation, the sustained administration of herbal extracts led to a decrease in CLas levels across all treatments relative to the control (CK). Among the extracts, SJ extract exhibited the most significant reduction in CLas levels, suggesting it may provide a protective impact during the early phases of CLas invasion. In citrus-growing areas affected by HLB, horticulture methods designed to diminish oxidative stress seem to alleviate immunological damage induced by CLas. These approaches encompass the efficient use of plant growth hormones, including gibberellic acid and brassinosteroids, with antioxidant therapies. This may elucidate why the four herbal extracts in this investigation failed to inhibit CLas invasion during indoor experiments. The pronounced disparities in CLas levels among treatments post-inoculation underscore the differing capacities of the extracts to inhibit oxidative stress, aligning with field trial findings that demonstrate the efficacy of herbal extracts in mitigating ROS-induced oxidative damage.

Prior research has validated the effectiveness of rutin in the treatment of citrus HLB [30]. This study compared the ethanolic extract of SJ with rutin, both containing similar total flavonoid concentrations, to evaluate the therapeutic effects of many vs single flavonoids on HLB. The findings indicated that rutin exhibited greater efficacy than SJ extract, implying variations in the therapeutic benefits of individual vs many flavonoids at an equivalent total flavonoid concentration. This suggests that various flavonoids may vary in their effectiveness against HLB, and the therapeutic functions of flavonoids aside from rutin in SJ extract necessitate additional research. Nevertheless, SJ extract was considered a more appropriate substance than rutin for promotion as an eco-friendly botanical pesticide for the management of citrus HLB, according to a cost-benefit analysis. The market price of rutin exceeds 1,000 RMB/kg, whereas the cost of SJ extract is estimated to be around 300 RMB/kg, derived from approximately 100 RMB/kg cost of SJ, the 39.12% extraction rate of SJ, and the associated alcohol expenses. Thus, SJ extract constitutes a more economical strategy for HLB management while preserving a similar degree of effectiveness.

5 Conclusion

This study illustrates the considerable potential of ethanol extracts from traditional Chinese medicinal herbs, SJ, LJT, TH, and SBG, as environmentally friendly and sustainable solutions for the management of citrus HLB. Among the tested extracts, SJ had the strongest efficacy in both potted and field trials, demonstrating the most significant reduction in CLas pathogen titers and exhibiting statistically superior outcomes in physiological recovery. All extracts enhanced plant health by increasing photosynthetic pigment concentrations, decreasing starch and sugar accumulation, alleviating ROS-induced oxidative damage, and neutralizing soil acidity. The effectiveness of the extracts, especially SJ, was analogous to that of the recognized anti-HLB flavonoid rutin. A significant positive association was noted between the flavonoid content of the extracts and their therapeutic efficacy against HLB. These findings underscore SJ extract and other flavonoid-rich botanical preparations as the attractive candidates for the environmentally sustainable management of HLB, with SJ extract demonstrating potential for field application to alleviate the effects of this detrimental disease on citrus production.

Acknowledgments

This work was financially supported by a grant from the open competition program of top ten critical priorities of Agricultural Science and Technology Innovation for the 14th Five-Year Plan of Guangdong Province (2022SDZG06, 2023SDZG06, and 2024KJ29), grants from the South China Agricultural University National Key Laboratory of Green Pesticides, as well as additional support from the Guangdong Provincial Key Research and Development Program. The dedication and support of the laboratory staff are deeply appreciated.

  1. Funding information: This work was supported by the open competition program of top ten critical priorities of Agricultural Science and Technology Innovation for the 14th Five-Year Plan of Guangdong Province (2022SDZG06, 2023SDZG06, and 2024KJ29).

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and consented to its submission to the journal. All the work reported is original, and all authors have reviewed and approved the final version submitted for publication. Yuning Li: writing – original draft, investigation, and data curation. Maosheng Zeng: investigation, conceptualization, and data curation. Zehua Liu: validation and resources. Yigang Lin: visualization. Hanhong Xu: writing – review and editing.

  3. Conflict of interest: Authors state no conflicts of interest.

  4. Data availability statement: Data will be provided upon request by Yuning Li.

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Received: 2025-05-13
Revised: 2025-08-26
Accepted: 2025-09-10
Published Online: 2025-10-24

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/opag-2025-0470
Schlagwörter für diesen Artikel
citrus Huanglongbing; herbal extracts; CLas titers; physiological changes
Creative Commons
BY 4.0

Artikel in diesem Heft

  1. Research Articles
  2. Optimization of sustainable corn–cattle integration in Gorontalo Province using goal programming
  3. Competitiveness of Indonesia’s nutmeg in global market
  4. Toward sustainable bioproducts from lignocellulosic biomass: Influence of chemical pretreatments on liquefied walnut shells
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  6. Assessment of nutrition status of pineapple plants during ratoon season using diagnosis and recommendation integrated system
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