Citrus Huanglongbing (HLB) and the Brazilian Efforts to Overcome the Disease

Abstract

Recorded in Brazil since 2004, Huanglongbing (HLB), or simply Greening, is the most impacting disease in citrus production worldwide. It induces qualitative and quantitative losses in production, but also can be lethal to the infected citrus plant. The disease is caused by phloem-limited bacteria with three identified species, Candidatus Liberibacter asiaticus (CLas), Ca. L. africanus (CLaf), and Ca. L. americanus (CLam). CLas and CLam are reported in Brazil, being vectored by the Asian citrus psyllid Diaphorina citri. HLB-infected citrus exhibit blotchy mottling with green islands on leaves, and small and lopsided fruits. As the infection progresses, excessive fruit drop and dead branches are frequently observed, shortening the trees' lifespan and reducing fruit yield and quality. HLB occurrence is restricted to the States of São Paulo, Minas Gerais, Mato Grosso do Sul and Paraná, which corresponds to more than 85% of citrus production in Brazil. Although citrus production is concentrated in these states, commercial citrus groves are present in 85% of the microregions of Brazil, reinforcing the economic and social importance of citrus for the country. Thus, in addition to the economic damage already caused to the national citrus industry, the spread of HLB to other regions of Brazil represents a great risk to the economic and food security of these non-affected regions. For this reason, the management of HLB is supported by legislative force, both at the federal and state levels. In 2008, based on scientific knowledge at the time, the Brazilian Ministry of Agriculture published a Normative Instruction 53 (IN53) which established criteria related to HLB surveillance and control, imposing the obligation to carry out periodic phytosanitary inspections, eradication of symptomatic hosts and planting HLB-free nursery trees. The implementation of IN53 has already resulted in the eradication of 55.5 million HLB-infected citrus plants in the State of São Paulo alone in the period from 2007 to 2019. In May 2021, IN53 was revoked and Portaria 317 was published. This new policy establishes the National Programme for HLB Prevention and Control. It addresses the problem according to the HLB status in each State, with or without the disease, and sets the general guidelines for local control actions.<br/> In view of the economic importance and complexity of this pathosystem, the disease has required concerted mobilization of the research community and inter-institutional cooperation, which has been working intensively to understand all the components directly and indirectly involved with the pathosystem. Also, a major cooperation effort is needed to gather all the information to develop and/or improve technologies and cultural practices to fight the disease better and reduce its impacts as much as possible. Here we present some of these efforts which are being carried out to develop short, medium and long term solutions for HLB control and management in areas of occurrence, as well as for surveillance and risk mitigation in non-affected areas.

Huanglongbing Pandemic: Current Challenges and Emerging Management Strategies

“…The ENPs’ formulation containing essential micronutrients, such as phosphate and salicylate, is shown to reduce nutrient deficiency symptoms [ 355 ]. A high-density planting with dwarf citrus rootstocks like the ‘Flying Dragon’ trifoliate orange, which has much lower cumulative HLB incidence than the ‘Rangpur’ lime, can be employed to compensate for the HLB impact [ 333 , 335 ]. It was speculated that the intercropping of guava plants in citrus orchards reduces infestations of the psyllid population, termed the “guava effect” on HLB [ 356 ].…”

“…Mango ( Mangifera indica ) is another plant that serves as a psyllid deterrent, whereas orange jasmine ( Murraya paniculata ) is the most attractive host for the psyllid. The appropriate utilization of such promising intercrops results in just 3–20% HLB incidence, compared to 76% incidence in citrus without intercropping [ 333 ]. As a result, proper planting of both psyllid-repellant and attractive plants across the entire grove may reduce the HLB effect.…”

“…If a high-density planting strategy is used around the core orchard, it leads to the control of secondary HLB infections within the citrus groves. This could be due to a “dilution effect” for pathogen inoculum sources arriving via psyllid vector from outside-infected orchards, as high-density orchards have relatively more trees per area than traditional plantings [ 333 ]. The orchard can be built by using a higher planting density (up to 200 m) along the farm boundaries to anticipate productivity and reduce HLB impact ( Figure 12 ).…”

“…Currently, the Brazilian citrus planting density has been stabilized around 615–640 trees/ha; the density was increased from 337 to 676 trees/ha. A high-density planting approach with dwarf citrus rootstocks like the ‘Flying Dragon’ trifoliate orange, which has less shoot flush abundance and much lower cumulative HLB incidence than the ‘Rangpur’ lime, can be employed to improve HLB management [ 333 , 335 ].…”