elocation-id: e3600
Huanglongbing (HLB), also known as greening disease or citrus greening, is a bacterial disease that affects citrus worldwide. This disease causes yellowing of the leaves, deformities in the fruits and eventually, the death of the tree. Currently, there is no cure for Huanglongbing; therefore, control focuses on preventing and mitigating the disease. The present research aimed to discuss the different control methods that are currently applied to control Huanglongbing, among which antibiotics, nutrients, and thermotherapy stand out. Antibiotics have been shown to be effective in controlling Huanglongbing in controlled studies. Nevertheless, their widespread use is limited due to concerns about antibiotic resistance. On the other hand, the application of certain nutrients, such as zinc and phosphorus, has shown promising results in improving the tolerance of citrus to Huanglongbing. These nutrients strengthen the plant’s immune system and make it more resistant to infection. Finally, thermotherapy has proven to be an effective method to eliminate the Huanglongbing bacteria. Controlling Huanglongbing in citrus requires an integrated approach that combines different methods, such as the use of specific antibiotics, nutrient application, and thermotherapy. Continued research is needed to develop more effective, sustainable, and economically viable control methods to protect this important agricultural industry.
Candidatus Liberibacter, Diaphorina citri, Huanglongbing.
Candidatus Liberibacter is the bacterium associated with Huanglongbing (HLB), which is the most destructive disease of citrus trees, causing the loss of millions of trees in almost all citrus-growing areas of the world; in Florida, United States of America, it has reduced the citrus area by 40% and production by 49%, causing a loss of one billion dollars (da-Graça et al., 2016). The first records of HLB disease date back to the early eighteenth century in India; likewise, in 2004, the bacterium was reported in Brazil and in 2005, in South Florida. HLB has reduced citrus production in many countries; for example, in the United States of America, it has reduced production by 81% and killed more than 60 million citrus trees in Africa and Asia (Huang et al., 2018; Shimwela et al., 2018).
Worldwide, Mexico is the sixth largest producer of citrus fruits, with a production of 6 634 000 t (da-Graça et al., 2016), behind China (32 705 000 t) (Song et al., 2018), Brazil (16 555 000 t) (Shimwela et al., 2018), India (9 755 000 t) (Huang et al., 2018), the United States of America (7 829 000 t) (da-Graça et al., 2016) and Spain (6 882 000 t) (Liu et al., 2019); however, recently, citrus production in Mexico has been affected by the presence of pests and diseases, among which Huanglongbing (HLB) or ‘yellow dragon’ stands out (Robles-González et al., 2017; Robles-González et al., 2018).
HLB is transmitted by two different routes, insects (Diaphorina citri and Trioza erytreae) (Figure 1) and through the use of infected grafts, which is why it has been classified as a systemic disease; the lack of HLB-resistant cultivars and the rapid spread of this disease has caused its control to become difficult in many countries (Song et al., 2018). Populations of Diaphorina citri show seasonal dynamics, with peaks of abundance during the warmest and driest months of the year (25 °C and 30 °C) (da-Graça et al., 2016).
The fruits with this disease are smaller (Figure 2), production decreases, there is seed abortion, uneven color of the peel, decrease in total soluble solids, calyx spot and fruit abortion; consequently, over the years, it will cause the tree to die in a period of 8 years; this time could vary depending on the type of crop, the age of the tree, its nutrition, etc. (Li et al., 2019).
The symptoms of HLB disease vary depending on the age of the tree and cultivar; in some cases, they can be confused with nutritional disorders; however, the main symptoms are asymmetric chlorosis in the leaf (irregular spots), yellow veins with obstruction of vascular bundles, chlorotic patterns, deficient in green or with a slight spot (green islands) (Pereira et al., 2011; Liu et al., 2019) (Figure 3).
The aim of this paper is to discuss the different control methods that are currently applied to control HLB.
The antibiotics used to control the disease are diverse and the applications are mostly by injection into the trunk. Studies by Zhang et al. (2011) concluded that ampicillin at 200 mg L-1 showed moderate efficacy for the control of Candidatus spp. in oranges, reducing the incidence of the disease by 50% and severity by 30%. Nonetheless, no significant effect on fruit quality was observed.
On the other hand, Vincent et al. (2022) conducted a research where they examined the effects of a variety of adjuvants on the absorption of oxytetracycline and streptomycin by foliar application. They also compared the efficiency of foliar application of oxytetracycline and streptomycin with trunk injection. They included extremely low levels of oxytetracycline and streptomycin in leaves that were coated during foliar application, indicating that neither streptomycin nor oxytetracycline were successfully administered systemically by foliar application, even after mixing with adjuvants (Zhang et al., 2014; Shin et al., 2016; Habiba et al., 2018; Ghosh et al., 2018).
Among other treatments used against diseases of bacterial and phytoplasmic origin, oxytetracycline has been used successfully in emergency situations; in addition, Slinsky (2016) proposes good practices (such as proper irrigation, fertilization, and weed control to maintain the health of trees and make them more resistant to diseases) in the management of varieties tolerant or resistant to bacteria as an efficient method of control. A practice like this represents an effective method for HLB control (Yang et al., 2016; Archer et al., 2023).
Mineral nutrients are necessary for the growth and development of plants and microorganisms and are important factors in plant-pathogen interactions (da-Graça et al., 2016). Nutrient deficiencies or excesses in plants can affect their susceptibility to this disease through the metabolic changes of plants, thus creating a more favorable environment for the development of the disease (Gilani et al., 2018).
Zinc is an essential micronutrient that is involved in various physiological processes in plants, including defense against disease. It has been shown that the application of zinc can reduce the incidence and severity of HLB in citrus trees (Habiba et al., 2018). Boron is another important micronutrient that plays a crucial role in the health of citrus plants.
The application of boron can improve nutrient absorption, strengthen cell walls, and increase resistance to stress, which can help plants better tolerate HLB (Gilani et al., 2018). Calcium is an essential macronutrient for the structural development and physiological function of citrus plants. The application of calcium can improve the integrity of cell walls and increase resistance to stress, which can help plants better tolerate HLB (Ahmad et al., 2011; Gilani et al., 2018; Zhang et al., 2018; Killiny et al., 2019).
Thermotherapy acts on the bacterium Candidatus spp. in two main ways: cell damage: elevated temperatures can damage the cell membrane of the bacterium, leading to the loss of its ability to reproduce and survive; and the denaturation of enzymes: enzymes essential for the metabolism of the bacterium are denatured when exposed to elevated temperatures, which interrupts their functioning and leads to the death of the bacterium (Fan et al., 2016; Hussain et al., 2018). Abdulridha et al. (2018) indicated that continuous thermal exposure at 55 °C was sufficient to eliminate or significantly reduce the titer of Candidatus spp. in citrus seedlings affected by HLB; in contrast, Yang et al. (2016) mentioned that in healthy two-year-old plants of Citrus paradisi Macfad infected with Candidatus spp. exposed to temperatures of 40 °C, 42 °C or 45 °C in growth chambers at a constant relative humidity of 85% and alternating (Figure 4).
Currently, methods of controlling the bacteria are effective, but they do not guarantee that the plant is free from contracting the disease again, so the application of antibiotics, integrated nutrient management, and thermotherapy do not guarantee trees completely free of the pathogen.
The attack and spread of the bacterium is worrying, particularly in Mexico, added to the lack of or little research to control the disease in the plant, so the disease continues to spread, decreases production considerably and with it, the economic losses are significant. It is necessary to continue with the research work to determine the best control method focused on the control of the bacterium and the vector, promoting the use of tolerant and resistant cultivars to this disease.
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