Abstract
Simple SummaryCandidatus Liberibacter spp. associated with citrus huanglongbing (HLB) comprise species Ca. L. africanus (Laf), Ca. L. asiaticus (Las), and Ca. L. americanus (Lam). While Laf is vectored by the psyllid Trioza erytreae, Las and Lam are vectored by Diaphorina citri. Las spread by D. citri is a major factor for the incidence of HLB-affected citrus in Asia and South, Central, and North America. Recently, both D. citri and Las were also detected in continental Africa, while T. erytrea was found in Portugal and Spain. D. citri that feed as nymphs in citrus harboring Las are better vectors when become adults than adults that acquire Las. Planting nursery trees free of Ca. Liberibacter spp., removing diseased symptomatic citrus trees, and controlling the psyllid population are management practices able to keep infection at acceptable rates. Psyllid population levels, as well as the percentage of individuals that carry Liberibacter, are important factors to succeed against HLB. Area-wide HLB management has been used in Brazil since 2009 and, in this context, psyllid population increase is monitored for control. The same integrated system was used to collect psyllids from traps over a three-year period, as a large-scale resource to address the effect of local management on the percentage of D. citri harboring Las. Good HLB management practices resulted in lower numbers of psyllids with Las. Location and regional disease incidence also affected the percentage of psyllids with Las, ranging from 33 to 74.6%.Huanglongbing (HLB) is a citrus disease of worldwide importance, associated with the presence of Candidatus Liberibacter asiaticus (Las) and vectored by the psyllid Diaphorina citri in Asia and the Americas. To properly manage HLB, removal of inoculum sources and control of the psyllid are undertaken. We evaluated the percentage of the psyllid population with Las, sampled from yellow sticky traps over a three-year period and its relationship with insect population, regions, season of the year, and HLB management in citrus areas in the southwestern, central, and northern regions of São Paulo (SP) and southwestern region of Minas Gerais states, Brazil. In each reading, up to 50 psyllids per region were collected and detection of Las in individual psyllids were made by quantitative polymerase chain reaction. The percentage of psyllids with Las—an average of 65.3%—was constant throughout the year in the southwestern region of SP state, while showing an increase from spring to autumn when sampled from central to northern regions. The proportion of psyllids carrying Las from each region and year period were compared by a proportion test and spectral density analysis. The proportion of psyllids carrying Las evaluated in the same region in different seasons presented statistical differences in central (Araraquara) and southwestern (Santa Cruz do Rio Pardo) regions in 2015, with higher values in the first semester (summer and autumn) than in the second semester (winter and spring). Orchards with poor HLB management had higher incidence of psyllids with Las. Spectral density analysis indicated that good management areas had 50% less relevant peaks of psyllids with Las than in areas with poor HLB management practices. The relationship between the percentage of psyllids carrying Las and the number of captured psyllids in the region in a given time denotes the most critical intake time for HLB spread in citrus orchards. The reduction in the population of psyllids carrying Las is a direct benefit from the use of good management practices.
Highlights
Citrus is one of the most cultivated fruit trees and Huanglongbing (HLB) is a worldwide disease severely threatening its production and profitability [1,2]
Associated bacteria, such as phytoplasmas [7,8], are not known to be associated with psyllids [9] and L. africanus (Laf) is not found in Brazil [10], studies on the interaction between Liberibacter asiaticus (Las) and D. citri are of capital importance
9873 samples were employed for analysis, where Las was detected in 6449 psyllids or 65.3% of the individuals (Table 1)
Summary
Citrus is one of the most cultivated fruit trees and Huanglongbing (HLB) is a worldwide disease severely threatening its production and profitability [1,2]. HLB is associated with the Liberibacters, Candidatus Liberibacter asiaticus (Las), Ca. L. africanus (Laf), and Ca. L. americanus (Lam). Las has wider occurrence than Laf and Lam and as in the case of Lam, is transmitted by the psyllid Diaphorina citri (Hemiptera: Liviidae) [3,4]. In Brazil, Lam was reported to be the prevalent Liberibacter along with minor presence of Las in 2004 [5,6], there was a shift in the occurrence of Las and currently, it has, by far, been the most prevalent Liberibacter found in citrus orchards for several years [2]. Since other HLB associated bacteria, such as phytoplasmas [7,8], are not known to be associated with psyllids [9] and Laf is not found in Brazil [10], studies on the interaction between Las and D. citri are of capital importance
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