Comparison of insecticide sprays in alternate rows and in all rows on Asian citrus psyllid control and Huanglongbing progress in sweet orange orchards.

One strategy to reduce Huanglongbing (HLB) is controlling its insect vector, the Asian citrus psyllid (ACP) Diaphorina citri, by preventive insecticide sprays. The recommendation is to spray insecticide in all rows (conventional spray - CONV), but some growers empirically spray in alternate rows (ALT) to increase the spray frequency without increasing the operating cost. Therefore, this work compared the effect of ALT with CONV on the ACP population and HLB incidence. The spray deposition (amount of metallic copper per leaf area), coverage (percentage of water-sensitive paper area covered by spray), and efficacy (ACP mortality) of each treatment were also evaluated on both sides of the trees. Two field trials were performed: Trial #1 compared ALT every 7 days (ALT7) with CONV every 14 days (CONV14), and trial #2 compared different spray frequencies of ALT with CONV every 7 days (CONV7). In trial #1, no differences were observed in the ACP population or HLB progress between ALT7 and CONV14 after 5 years. In trial #2, ALT7 presented the highest percentage of ACP and cumulative HLB incidence than CONV7 and ALT every 3 to 4 days, after 2 years. Hence, when the frequency of ALT was half the frequency of CONV, similar results were observed. Spray deposition, coverage, and efficacy were similar between tree sides in CONV, but they were uneven in ALT, resulting in higher values on the tree side that directly received the spray. Insecticide spray should be performed with the frequency enough to keep new shoot protected during their growth.

Brevipalpus yothersi Baker (Tenuipalpidae) development in sweet orange plants is influenced by previous mite infestation and the presence of shelters.

Citrus leprosis is the most important viral disease affecting citrus. The disease is caused predominantly by CiLV-C and is transmitted by Brevipalpus yothersi Baker mites. This study brings some insight into the colonization of B. yothersi in citrus [(Citrus × sinensis (L.) Osbeck (Rutaceae)] previously infested by viruliferous or non-viruliferous B. yothersi. It also assesses the putative role of shelters on the behavior of B. yothersi. Expression of PR1 and PR4 genes, markers of plant defense mechanisms, were evaluated by RT-qPCR to correlate the role of the plant hormonal changes during the tri-trophic virus-mite-plant interplay. A previous infestation with either non-viruliferous and viruliferous mites positively influenced oviposition and the number of adult individuals in the resulting populations. Mite populations were higher on branches that had received a previous mite infestation than branches that did not. There was an increase in the expression of PR4, a marker gene in the jasmonic acid (JA) pathway, in the treatment with non-viruliferous mites, indicating a response from the plant to their feeding. Conversely, an induced expression of PR1, a marker gene in the salicylic acid (SA) pathway, was observed mainly in the treatment with viruliferous mites, which suggests the activation of a plant response against the pathogen. The earlier mite infestation, as well as the presence of leprosis lesions and a gypsum mixture as artificial shelters, all fostered the growth of the B. yothersi populations after the second infestation, regardless of the presence or absence of CiLV-C. Furthermore, it is suggested that B. yothersi feeding actually induces the JA pathway in plants. At the same time, the CiLV-C represses the JA pathway and induces the SA pathway, which benefits the mite vector.

Risk-based regionalization approach for area-wide management of HLB vectors in the Mediterranean Basin.

Huanglongbing (HLB) is one of the most devastating citrus diseases worldwide. It is associated with the non-culture bacteria Candidatus Liberibacter spp., which can be transmitted by grafting and/or the psyllid vectors Diaphorina citri (ACP) and Trioza erytreae (AfCP). Although HLB has not been reported in the Mediterranean Basin to date, both vectors are present, and thus represent a serious threat to the citrus industry in this region. Resistant citrus cultivars or effective therapeutic treatments are not currently available for HLB. Nevertheless, area-wide pest management via coordinated management efforts over large areas has been implemented in Brazil, China and the USA for HLB control. This study proposes an open access flexible methodology to address area-wide management of both HLB vectors in the Mediterranean Basin. Based on a risk-based approach which considers climatic information and other variables that may influence vector introduction and spread, such as conventional, organic, abandoned and residential citrus areas as well as transportation corridors, an area-wide management division in pest management areas (PMAs) is proposed. The size and location of these PMAs were estimated by means of a hierarchical clustering algorithm with spatial constraints whose performance was assessed under different configuration scenarios. This proposal may assist policymakers and the citrus industry of the citrus-growing areas of the Mediterranean Basin in risk management planning in the case of the spread of HLB vectors or a possible introduction of the disease. Additionally, it may be a valuable resource to inform opinion dynamic models, enabling the identification of pivotal factors for the success of control measures.

Impacts of huanglongbing on fruit yield and quality and on flushing dynamics of Sicilian lemon trees.

Introduction: The severe Asian form of huanglongbing (HLB), a vascular disease associated with the phloem-limited bacterium 'Candidatus Liberibacter asiaticus', is transmitted by the Asian citrus psyllid (ACP) Diaphorina citri. Disease impacts are known for sweet oranges and acid limes but not lemons. Methods: In a five-year study (2017-2021) we compared yield and fruit quality between naturally-infected and healthy 5-yr-old trees of Sicilian lemon 'Femminello', and shoot phenology on both lemon and 'Valencia' orange, both grafted onto 'Swingle' citrumelo, grown in southeastern São Paulo State, Brazil. HLB severity (percentage of tree canopy area with HLB symptoms) was assessed every 3-4 months, fruit yield and quality in May (2017 to 2019) or June/July (2020-2021), and vegetative and reproductive shoots fortnightly on 50-cm-long branches. The development of ACP on one-year-old seedlings of five lemon varieties, 'Tahiti' acid lime, 'Valencia' orange, and orange jasmine was evaluated. Results: Symptoms increased from 11% in 2017 to 64% in 2021, and a monomolecular model estimated 10 years for symptoms to occupy >90% of the tree canopy. On average, production of trees with symptom on 20%, 50% or 80% of the canopy respectively dropped by 18%, 38%, and 53% compared to healthy trees. Fruits of symptomatic branches of lemons were 4.22% lighter and the number of dropped fruits did not correlate with symptom severity. Flushing on symptomatic branches started earlier by 15 to 55 days as compared to the healthy branches of lemon and orange. On diseased trees, vegetative and reproductive shoots respectively increased by 24.5% and 17.5% on lemon and by 67.2% and 70.6% on sweet orange, but fruit set was reduced by 12.9% and 19.7% on lemon and orange trees, respectively. ACP reproduced similarly on all tested plants. Discussion: The fast symptom progress, significant yield reduction, and earlier flushing on diseased trees, providing conditions highly favorable for the pathogen to spread, reinforce the need of prompt diseased tree removal and frequent ACP preventive control to manage HLB in lemons as in any other citrus crop.

Impact of diseases and pests on premature fruit drop in sweet orange orchards in São Paulo state citrus belt, Brazil.

BACKGROUND: Despite technical improvements in the citrus chain and leadership in orange production achieved in the past decades, premature fruit drop remains a major component of crop loss in São Paulo state citrus belt, the largest sweet orange production area in the world. The present study aimed to determine, during five consecutive seasons, the impact of the diseases and pests on premature fruit drop in the orange belt. RESULTS: Fruit drop due to the main diseases and pests averaged approximately 11.0%, which corresponded to approximately 63% of the annual fruit drop. The average fruit drop rate due to fruit borer and fruit flies combined was 4.0%, Huanglongbing (HLB) 3.3%, black spot 2.6%, leprosis 1.0% and citrus canker 0.3%. The average amount of fruit drop (million 40.8 kg boxes) and value of crop losses (million US$ dollars), in five seasons, were 12.7 and 66.2 for fruit borer/fruit flies, 11.0 and 57.9 for HLB, 8.1 and 42.2 for black spot, 3.1 and 15.6 for leprosis, and 0.9 and 4.9 for citrus canker, respectively. CONCLUSION: Fruit borer and fruit flies (combined), HLB, black spot, leprosis and citrus canker are, in this order, the main diseases and pests in the orange belt of São Paulo state. All of these causes significantly increased the overall fruit drop rate in the evaluated seasons. The results will contribute to the development of the Brazilian citrus industry, while showing to other citrus-growing regions the potential that diseases and pests have to jeopardize production. © 2022 Society of Chemical Industry.

Lethal and sublethal effects of insecticides on the survival and reproduction of Brevipalpus yothersi (Acari: Tenuipalpidae).

The overuse of insecticides to control vector insects such as Diaphorina citri Kuwayama in citrus groves has altered the population dynamics of pest mites. Among phytophagous mites, population outbreaks of citrus leprosis mite, Brevipalpus yothersi Baker, have been increasingly intense and frequent in Brazilian citrus groves. Despite the great importance of the B. yothersi mite for citrus production, the lethal and sublethal effects of insecticides on this mite have not yet been studied. Therefore, in this study, the effects of insecticides commonly used for D. citri control on B. yothersi mortality, reproduction, and instantaneous growth rate were assessed. For this, two experiments were carried out, one under controlled conditions and another in a greenhouse. The insecticides tested were beta-cyfluthrin, bifenthrin, buprofezin, chlorpyrifos, dimethoate, pyriproxyfen, and thiamethoxam at 0 (control), 0.0625, 0.125, 0.25, 0.5, 1, and twofold the recommended insecticide concentration for D. citri control. The pyriproxyfen insecticide provided high mortality of B. yothersi even at low concentrations. Furthermore, this insecticide negatively interfered with the reproduction of this mite. Beta-cyfluthrin, bifenthrin, buprofezin, chlorpyrifos, dimethoate, and thiamethoxam, in the tested concentrations, showed low impact on citrus leprosis mite. Regarding the reproduction of the mite, no significant increase in fecundity was observed on B. yothersi females exposed to insecticide residues, regardless of the concentration tested. Therefore, the application of these insecticides in the management of pest insects is unlikely to promote an increase in the citrus leprosis mite population.

Huanglongbing and Citrus Variegated Chlorosis Integrated Management Based on Favorable Periods for Vector Population Increase and Symptom Expression.

Huanglongbing (HLB, associated with Candidatus Liberibacter asiaticus and transmitted by the Asian citrus psyllid Diaphorina citri) and citrus variegated chlorosis (CVC, caused by Xylella fastidiosa subsp. pauca and transmitted by sharpshooter species) have been managed by vector control and removal of symptomatic trees. Although vectors and new symptomatic trees can be detected year round, peaks of vector populations are higher in spring and summer, and the most symptomatic trees are found in autumn and winter. This work aimed to compare the management of both diseases during these favorable periods. The experiment was conducted during 5 years in a commercial orchard and had a three-by-two factorial design. The factor "vector control" had three levels: monthly vector control year round (VCYR), monthly vector control in spring and summer (VCSS), and vector control when a threshold level of 10% occupancy was detected (VCOT). The factor "inoculum removal" had two levels: monthly eradication year round (TEYR) and monthly eradication in autumn and winter (TEAW). Host flush, both HLB and CVC vector populations, and the number of symptomatic citrus plants were visually assessed. The level of vectors over the seasons, as measured using the average area under the curve (AUC), was similar for all treatments with the exception of psyllid abundance, which was approximately 4.5 times higher for VCSS than for other treatments. For both diseases, no difference in the average AUC of disease progress and disease final incidence was observed. VCOT or adjusted VCSS associated with TEAW could be integrated for sustainable citrus production.

Region-Wide Comprehensive Implementation of Roguing Infected Trees, Tree Replacement, and Insecticide Applications Successfully Controls Citrus Huanglongbing.

Huanglongbing (HLB) is a devastating citrus disease worldwide. A three-pronged approach to controlling HLB has been suggested, namely, removal of HLB-symptomatic trees, psyllid control, and replacement with HLB-free trees. However, such a strategy did not lead to successful HLB control in many citrus-producing regions, such as Florida. We hypothesize that this is because of the small-scale or incomprehensive implementation of the program; conversely, a comprehensive implementation of such a strategy at the regional level can successfully control HLB. To test our hypothesis, we investigated the effects of region-wide comprehensive implementation of this scheme to control HLB in Gannan region, China, with a total planted citrus acreage of over 110,000 ha from 2013 to 2019. With the region-wide implementation of comprehensive HLB management, the overall HLB incidence in Gannan decreased from 19.71% in 2014 to 3.86% in 2019. A partial implementation of such a program (without a comprehensive inoculum removal) at the regional level in Brazil resulted in HLB incidence increasing from 1.89% in 2010 to 19.02% in 2019. Using dynamic regression model analyses with data from both Brazil and China, we constructed a model to predict HLB incidence when all three components were applied at 100%. It was predicated that in a region-wide comprehensive implementation of such a program, HLB incidence would be controlled to a level of less than 1%. We conducted economic feasibility analyses and showed that average net profits were positive for groves that implemented the comprehensive strategy, but groves that did not implement it had negative net profits over a 10-year period. Overall, the key for the three-pronged program to successfully control HLB is the large scale (region-wide) and comprehensiveness in implementation. This study provides valuable information to control HLB and other economically important endemic diseases worldwide.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.