Space-time variability of citrus leprosis as strategic planning for crop management

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.

Stress response and tolerance mechanisms of spirobudiclofen exposure based on multiomics in Panonychus citri (Acari: Tetranychidae)

The toxicity of insecticides used in the field decreases gradually to sublethal concentrations over time. Therefore, it is necessary to study sublethal effects of pesticides for controlling population explosion. Panonychus citri is a global pest which control is based on insecticides. This study explores the stress responses of spirobudiclofen on the P. citri. Spirobudiclofen significantly inhibited survival and reproduction of P. citri, and the effects aggravated as concentration increased. The transcriptomes and metabolomes of spirobudiclofen-treated and control were compared to characterize spirobudiclofen molecular mechanism. Transcriptomics indicated stress induced by spirobudiclofen stimulated immune defense, antioxidative system, cuticle formation, and lipid metabolism, as deduced from RNA-seq analysis. Meanwhile, our study found that tolerance metabolism in P. citri was regulated by promoting the metabolism of glycerophospholipids, glycine, serine, and threonine. The results of this study can provide a basis for exploring the adaptation strategies of P. citri to spirobudiclofen stress.

Citrus Bright Spot Virus: A New Dichorhavirus, Transmitted by Brevipalpus azores, Causing Citrus Leprosis Disease in Brazil

Citrus leprosis (CL) is the main viral disease affecting the Brazilian citriculture. Sweet orange (Citrus sinensis L. Osbeck) trees affected by CL were identified in small orchards in Southern Brazil. Rod-like particles of 40 × 100 nm and electron lucent viroplasm were observed in the nucleus of infected cells in symptomatic tissues. RNA extracts from three plants, which proved negative by RT-PCR for known CL-causing viruses, were analyzed by high throughput sequencing and Sanger sequencing after RT-PCR. The genomes of bi-segmented ss(-)RNA viruses, with ORFs in a typical organization of members of the genus Dichorhavirus, were recovered. These genomes shared 98-99% nt sequence identity among them but <73% with those of known dichorhavirids, a value below the threshold for new species demarcation within that genus. Phylogenetically, the three haplotypes of the new virus called citrus bright spot virus (CiBSV) are clustered with citrus leprosis virus N, which is a dichorhavirus transmitted by Brevipalpus phoenicis sensu stricto. In CiBSV-infected citrus plants, B. papayensis and B. azores were found, but the virus could only be transmitted to Arabidopsis plants by B. azores. The study provides the first evidence of the role of B. azores as a viral vector and supports the assignment of CiBSV to the tentative new species Dichorhavirus australis.

Resistance to spirodiclofen in Brevipalpus yothersi (Acari: Tenuipalpidae) from Brazilian citrus groves: detection, monitoring, and population performance

BACKGROUND

Brevipalpus yothersi mite is the main vector of Citrus leprosis virus (CiLV), the causal agent of citrus leprosis disease. The acaricide spirodiclofen has been widely used to control this mite. However, failures in control using spirodiclofen have been frequently reported by citrus growers. In this study, we estimated the diagnostic concentration to monitor the resistance to spirodiclofen of B. yothersi populations collected in nine citrus groves in Brazil. We then selected the B. yothersi population that showed lowest mortality with the estimated diagnostic concentration of spirodiclofen to characterize the frequency of resistant individuals, as well as demographic and life table parameters.

RESULTS

Variability was higher between populations in terms of susceptibility to spirodiclofen. The frequency of resistant eggs between populations ranged from 0.7% to 85.8%. The resistance ratio of B. yothersi to spirodiclofen was low to moderate. Survival rates of the immature stage, total adult longevity, oviposition days, and female fecundity were lower in the resistant strain. Furthermore, net reproduction rate, intrinsic rate of increase, finite rate of increase, and mean length of a generation were also lower in the R strain.

CONCLUSION

Although variations in resistance to spirodiclofen were observed between populations, the resistance ratio was low to moderate. Such data can be useful in the development of resistance management strategies for B. yothersi in Brazilian citrus groves. © 2021 Society of Chemical Industry.

Sublethal effects of spirodiclofen on biological and demographic parameters of the citrus leprosis mite Brevipalpus yothersi (Acari: Tenuipalpidae)

BACKGROUND

Brevipalpus yothersi Baker is one of the main vectors of citrus leprosis. Knowing the biology of this mite species when under chemical control is extremely important to understand its population dynamics, and then to solve problems of management of this pest. Therefore, we assessed the effects of one sublethal dose of spirodiclofen (0.48 ppm) on biological and demographic parameters of B. yothersi under laboratory and greenhouse conditions, comparing them with non-exposed mite population.

RESULTS

Under laboratory conditions, where citrus fruits were used as a substrate, the duration of developmental stages (pre-egg-laying and egg-laying) and mite longevity showed no differences between treatments. However, the number of laid eggs increased in acaricide-treated fruits. In the greenhouse experiment, where citrus plants were used as a substrate, the instantaneous growth rate (ri) of mites was positive in all untreated plots. In contrast, in treated plants, ri was negative in 12 experimental units, and mite populations were suppressed in five of them. Moreover, mite population dynamics had a positive ri in three treated plants.

CONCLUSIONS

Spirodiclofen sublethal dose had no negative effect on the offspring of B. yothersi females. However, it is not safe for citrus leprosis mite since it may increase egg number per female in some conditions. Biological and demographic differences could influence mite population dynamics in the field, requiring appropriate management strategies to improve citrus leprosis control. © 2019 Society of Chemical Industry.

Spatiotemporal association between the mite Brevipalpus yothersi and Citrus leprosis virus C in orange orchards

Citrus leprosis virus C (CiLV-C) is an economically important pathogen and the main causative agent of leprosis disease in citrus orchards. The main vector of this disease, the mite Brevipalpus yothersi, is widely distributed in Mexican orchards on a wide range of citrus species. Despite the importance of both the virus and the mite, field studies recording their occurrence and co-occurrence are practically non-existent. We systematically sampled orange orchards for both CiLV-C and B. yothersi throughout the year. The distribution of the CiLV-C and B. yothersi was evaluated on each sampling occasion and their spatiotemporal associations were determined. Specifically, 100-112 orange trees, distributed in 18 rows (five or six trees per row), were sampled monthly between March 2017 and February 2018 (11 sampling dates). Twenty leaves per tree were sampled on each occasion. The number of mites per tree and the percentage of leaves per tree with disease symptoms were recorded. On each sampling occasion, spatiotemporal associations between mites and disease were determined using the Spatial Analysis by Distance Indices (SADIE) method. CiLV-C and B. yothersi were identified using molecular methods. Throughout the study, the distribution of CiLV-C was aggregated and the distribution of B. yothersi was random. No association was found between the virus and the mite on any of the sampling dates. In total, 173 mites were collected, but only 43 mites were found to be carrying CiLV-C. The reason for this lack of association between the virus and the mite, as well as the impact of our findings on the epidemiology of the disease in orange orchards, are discussed.

Bioactivity of an oxymatrine-based commercial formulation against Brevipalpus yothersi Baker and its effects on predatory mites in citrus groves

The acaricidal bioactivity of an oxymatrine-based commercial formulation against Brevipalpus yothersi Baker (Acari: Tenuipalpidae), a vector mite of the Citrus leprosis virus (CiLV), and its impact on predatory mites were assessed. For this purpose, laboratory and field assays using bioacaricide concentrations ranging from 0.5 to 2.0 mg L^-1 of oxymatrine were performed during the years from 2015 to 2016. Laboratory results showed that the oxymatrine-based commercial formulation does not cause deleterious effects on B. yothersi eggs; however, it causes high larval mortality. For adult females, the bioacaricide caused high acute toxicity and residual effect for at least 5 days after application. In the field, the bioacaricide exhibited high acaricidal activity against B. yothersi, with efficacy levels similar to that of synthetic acaricide spirodiclofen (48 mg L^-1) until 49 days after the application. The application of the bioacaricide did not negatively affect the population levels of phytoseiid predatory mites. Therefore, our results suggest that the oxymatrine-based commercial formulation is an important tool for management of the citrus leprosis mite in citrus groves.

Draft Genome Assembly of the False Spider Mite Brevipalpus yothersi

The false spider mite Brevipalpus yothersi infests a broad host plant range and has become one of the most economically important species within the genus Brevipalpus. This phytophagous mite inflicts damage by both feeding on plants and transmitting plant viruses. Here, we report the first draft genome sequence of the false spider mite, which is also the first plant virus mite vector to be sequenced. The ∼72 Mb genome (sequenced at 42× coverage) encodes ∼16,000 predicted protein-coding genes.