Could tree density play a role in managing Citrus Huanglongbing epidemics? (original) (raw)
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Tropical Plant Pathology, 2017
Huanglongbing (HLB), the most damaging disease of the citrus industry, is present in the Brazilian states of São Paulo, Minas Gerais and Paraná. The risks of introduction and spread of this A1 quarantine pest in citrus groves of the Rio Grande do Sul (RS) state were assessed based on a structured questionnaire composed of 27 questions applied to 163 growers in 35 municipalities. The locations were defined according to the economic relevance of citriculture, and the sample size (number of interviewed farmers) was proportional to the cultivated area in each municipality. A multi-criteria method was used to prioritize and assign weights to specific criteria associated with the different risks. Each farm was assigned to one of the four risk categories: low (0 to 0.25), medium (0.25 to 0.50), high (0.50 to 0.75) and very high (0.75 to 1.0). The estimated risks of HLB introduction and spread in RS were mostly medium to high across the regions and farms within regions. Two factors, the presence of HLB in neighbor countries and citrus fruit transport from other regions mostly accounted for the risk of introduction of HLB into RS. Following an eventual introduction, the lack of knowledge about this pathosystem and the little or no use of insecticides were identified as the main contributors of disease spread within the state.
Frequency of symptomatic trees removal in small citrus blocks on citrus huanglongbing epidemics
Crop Protection, 2013
Among the recommended measures for citrus Huanglongbing (HLB) management, the systematic elimination of symptomatic trees is the most argued and difficult to be accepted and accomplished by citrus growers. Elimination of recently affected HLB trees represents a short term yield loss and cost increase due to the need of frequent inspections and removal operations. This work aimed to evaluate the effect of different frequencies of inoculum reduction applied at individual citrus blocks scale (or local inoculum reduction) on HLB temporal progress. Eight experiments were carried out in new planted and older citrus blocks with 504e1290 trees/plot. In all experiments, inspections to detect symptomatic trees were done in a fortnightly or monthly frequency. The treatments of frequencies of local inoculum reduction varied from fortnightly to 6 months. Annual disease progress rate was estimated by logistic model for each plot. No difference on HLB progress rates among treatments was observed, except in experiments 1 and 3 where less frequent tree removal resulted in higher disease progress rate. This ineffectiveness of local inoculum reduction on the disease progress rate was explained by the higher weight of primary spread on HLB epidemics than the secondary spread within plots associated with small size and narrow shape of treated plots (except for experiments 1 and 3), high dispersal capacity of HLB-insect vector among plots and groves, and strong control of psyllid within the plots (except for experiment 1, with poor insecticide spray program). Also, the high amount of inspections to detect symptomatic trees before the eradication treatment, which reduced the escapes (asymptomatic and non visual detectable diseased trees) contributed for these results. It is important to note that these results were obtained with only small citrus plots (0.8e2.9 ha) and they cannot be extended to larger groves and farms amenable to HLB management by the symptomatic tree removal and vector control.
10.5 An approach to model the impact of Huanglongbing on citrus yield
2008
Citrus huanglongbing (HLB) is the most serious disease of citrus worldwide because once a tree is infected there is no cure and its yield is greatly compromised while the disease spreads all over the grove. Where HLB becomes endemic and there is no effective control, the disease progress in the orchard as well as the evolution of symptom severity throughout the tree canopy can be relatively fast, greatly reducing the economic life of affected orchards (1,11). In many countries where HLB is present, it becomes a limiting factor for citrus production. Even knowing that, it has been difficult to convince citrus farmers and regulatory agencies to take part in global HLB management programs, because HLB management is based on elimination of diseased trees and reduction of vector population demands continuous efforts and is costly. Thus, the aim of this work is to better characterize the importance of the disease and the impact of absence of any control on expected yield of affected orchards through a simple model that takes into account the disease incidence progress, disease severity progress and disease severity-fruit yield relationship according to the age of trees at first symptom onset. The approach was to estimate the incidence of symptomatic trees (y) and the disease severity on each symptomatic tree (s) every year. For that, we assumed that in younger blocks, the disease incidence and severity progress is faster than in older ones. The factors that supported this were: i) more frequent and intensive new shoots in younger trees attract a higher population of HLB psyllid vector that allows higher infection rates; ii) small canopy size leads to relative higher initial severity of symptoms and faster distribution of the pathogen into the tree. The HLB incidence progress curves have been well described by Gompertz model [y = exp(-(ln(y o )).exp(-r G .t))] (9), where y is the proportion of symptomatic trees at time t (years), y o is the proportion of symptomatic trees at the first appearance of symptomatic trees, and r G is the annual rate of disease incidence progress. Based on literature (3,5-9) and field observations of the disease progress without control, a different value of r G was used for each tree age class at the appearance of first symptomatic trees. The r G values considered for 0-2, 3-5, 6-10 and >10 years old affected groves were respectively 1.300, 0.650, 0.325 and 0.244. According to this, the disease incidence reaches 50% (y = 0.5) in less than two years after the appearance of first symptomatic trees in 0-2 years old block, and in about ten years after the appearance of first symptomatic trees in blocks older than ten years ). Despite there are only comments (1,2,10-12) but no multi years assessments describing the progress of HLB severity in an affected tree, we decided to use the Logistic model [s = 1/(1+((1/s o )-1).exp((-r L .t)))], where s is the proportion of HLB symptomatic area of tree canopy at time t (years), s o is the proportion of HLB symptomatic canopy area at the onset of first symptoms in the tree, and r L is the annual rate of disease severity progress in affected tree. Values of r L and s o were empiric guesses from field observation in Sao Paulo. The considered s o values were respectively 0.2, 0.1, 0.05 and 0.025 for trees which the first symptoms appeared when they were 0-2, 3-5, 6-10 and >10 years old. The r L values considered were 3.68 for 0-2 years old trees; 1.84 for 3-5 years old trees; 0.92 for 6-10 years old trees; and 0.69 for trees older than 10 years. According to this, for trees which the first symptoms onset occurred when they were 0-2 years old it takes about two years for the HLB severity to reach 100% (s = 1.0), while
Field Evaluation of Integrated Management for Mitigating Citrus Huanglongbing in Florida
Frontiers in Plant Science, 2019
Citrus huanglongbing (HLB) is extremely difficult to control because the psyllid-transmitted bacterial pathogen resides inside the citrus phloem and the disease is systemic. In Florida, the nine billion dollar citrus industry has been significantly impacted by severe HLB epidemics. To combat citrus HLB, in this study we implemented an integrated strategy that includes chemotherapy, thermotherapy, and additional nutrition treatment in three different field trials over three consecutive years. In these trials, only trees already showing HLB symptoms with Ct values ranging from 25.1 to 27.7 were selected for treatments. To assess the complex interactions, we used several methods for evaluating the effectiveness of integrated management, including the slopes (b) of the Ct increase (dy/dt), the pathogenic index (PI) and the decline index (DI) from Ct value and tree scores, and the therapeutic efficacies from PI and DI. This comprehensive analysis showed that most of the tested chemicals were effective to some degree in killing or suppressing the Las bacterium, with higher therapeutic efficacies seen for Grove B, where citrus trees were severely affected by HLB, and it had a higher number of psyllids, relative to Grove E and P in the first 2 years. Trunk-injected penicillin G potassium was the most effective chemical treatment in all groves, followed by Oxytetracycline Calcium Complex, and Silver Nitrate delivered as foliar sprays. Although the steam heat treatment and additional nutrition did not eliminate or suppress Las over the long term, these treatments did positively affect tree growth and recovery in the short term. Overall, our results provide new insights into HLB control method and strategy for integrated management for HLB epidemic plantations.
Evaluation of Management Programs for Protecting Young Citrus Plantings from Huanglongbing
HortScience, 2013
Asiatic huanglongbing (HLB) is a devastating disease of citrus associated in North America with the bacterium ‘Candidatus Liberibacter asiaticus’ (LAS) vectored by the Asian citrus psyllid (ACP), Diaphorina citri Kuwayama. ACP management is considered a vital component of a program aimed at reducing the incidence and spread of HLB. Considerable research has been published comparing the efficacy of different insecticides for ACP control as well as on seasonal strategies for applying ACP insecticides. However, published information was largely lacking for even the most intense insecticide programs on their effectiveness for keeping HLB out of a new citrus planting in Florida citrus. We therefore conducted two replicated (individual plots 0.2 to 0.4 ha in size) experiments on protecting young citrus from HLB using different ACP management programs. An intensive insecticide program was evaluated in each experiment: eight annual calendar applications of traditional insecticides (hereafte...
The International Food and Agribusiness Management Review, 2019
This study evaluates the adoption of biological control of the Diaphorina citri and the citrus growers' willingness to pay for biopesticides. The citrus greening continues to spread over Brazilian orchards, reaching 17.89% of all citrus trees in 2015. Utilizing data from a survey with decision-making agents from the sector, this paper used a logit model to obtain the determinants and a contingent valuation method to elicit growers' willingness to pay for a biopesticide. Results showed high acceptability of growers to the biocontrol. Important factors regarding its adoption included experience in growing citrus, a high level of education and dependence on citrus production. Producers from São Paulo's southern region paid the highest value for the biopesticide. We observed that worsening of the disease has led players from citrus chain to search for new forms of insect control.
Plant Pathology, 2016
Asiatic citrus canker (ACC), caused by Xanthomonas citri subsp. citri, has been controlled by exclusion and eradication measures in São Paulo state (SP), Brazil, since it was first detected in 1957. Several protocols of eradication have been used since then, but the protocol applied from 1999 to 2009 was the most successful and well documented of these. During that time, if the incidence of trees with symptoms in a citrus block was ≤0.5%, the infected trees and the symptomless trees within a 30 m radius were removed. However, if the incidence was >0.5%, the entire block was eliminated. Subsequently, inspections were carried out to monitor the affected blocks during quarantine. Although this protocol did not eliminate the disease from SP, it suppressed the incidence of affected blocks to very low annual levels (<0.20%) for a decade. Of over 5000 foci detected, 65.1% were eliminated by removing the block, either when disease was first detected (47.8%) or at a subsequent detection (17.3%), and 28.4% were eradicated by removing trees within a 30 m radius. The latter method was not an effective stand-alone measure for ACC eradication and the 0.5% threshold was the key factor in suppression of disease statewide. Currently, a re-emergence of ACC is occurring in SP. The information presented here will aid the establishment of protocols for the eradication of ACC in other citrus-growing areas with low incidence of the disease or at risk of pathogen introduction.
PLoS Computational Biology, 2014
A spatially-explicit, stochastic model is developed for Bahia bark scaling, a threat to citrus production in north-eastern Brazil, and is used to assess epidemiological principles underlying the cost-effectiveness of disease control strategies. The model is fitted via Markov chain Monte Carlo with data augmentation to snapshots of disease spread derived from a previouslyreported multi-year experiment. Goodness-of-fit tests strongly supported the fit of the model, even though the detailed etiology of the disease is unknown and was not explicitly included in the model. Key epidemiological parameters including the infection rate, incubation period and scale of dispersal are estimated from the spread data. This allows us to scale-up the experimental results to predict the effect of the level of initial inoculum on disease progression in a typically-sized citrus grove. The efficacies of two cultural control measures are assessed: altering the spacing of host plants, and roguing symptomatic trees. Reducing planting density can slow disease spread significantly if the distance between hosts is sufficiently large. However, low density groves have fewer plants per hectare. The optimum density of productive plants is therefore recovered at an intermediate host spacing. Roguing, even when detection of symptomatic plants is imperfect, can lead to very effective control. However, scouting for disease symptoms incurs a cost. We use the model to balance the cost of scouting against the number of plants lost to disease, and show how to determine a roguing schedule that optimises profit. The trade-offs underlying the two optima we identify-the optimal host spacing and the optimal roguing scheduleare applicable to many pathosystems. Our work demonstrates how a carefully parameterised mathematical model can be used to find these optima. It also illustrates how mathematical models can be used in even this most challenging of situations in which the underlying epidemiology is ill-understood.