Spatial patterns of leprosis and its mite vector in commercial citrus groves in Brazil

In order to describe the dissemination of Citrus leprosis virus (CiLV) and Brevipalpus phoenicis , the Tenuipalpidae mite which transmits it, as well as to generate data for future development of better sampling and disease control procedures, analyses were conducted at three levels of spatial hierarchy. Over 100 distribution maps of mite-infested plants and plants with CiLV symptoms were constructed after evaluation of 174 sweet orange commercial grove blocks from four citrus regions of São Paulo State, Brazil. Spatial correlation between maps of mite-infested plants and those with CiLV symptoms was very low and mostly not significant. Spatial dependency between adjacent plants was incipient as ordinary runs analysis showed that percentages of aggregated sequences within or across rows were very low for both mite-infested and diseased plants. Index of dispersion ( D ) values for all quadrat sizes suggested aggregation of plants with symptoms within quadrats, but much less aggregation for mite-infested plants. Values of log( A ) and b were higher than 0 and 1, respectively, indicating a general and significant aggregation of infested/diseased plants inside quadrats. However, the degree of aggregation of plants with symptoms was higher than that of infested plants. Aggregation in each grove was positively correlated to the incidence of infested/diseased plants. Spatial autocorrelation also showed a higher strength of aggregation for plants with symptoms than for infested ones. This is the first time that spatial patterns of leprosis and B. phoenicis have been described.     

The influence of citrus rootstocks on the relationship between the mite Brevipalpus phoenicis and citrus leprosis disease

BACKGROUND: Leprosis is one of the most serious citrus plant diseases. Leprosis‐affected plants, especially sweet orange [Citrus sinensis (L.) Osbeck], which is the most widely cultivated citrus fruit worldwide, show reduced photosynthetic capacity and severe defoliation. The aim was to evaluate the relationship between the Brevipalpus phoenicis (Geijskes) vector mite and citrus leprosis disease in Pera sweet orange plants grafted on different rootstocks. Data were analysed using numerical classification and conventional statistical analysis (ANOVA). RESULTS: Both viruliferous and non‐viruliferous B. phoenicis populations increased in number on plants maintained at low soil water content. Among the evaluated rootstocks, Sunki mandarin proved least favourable for mite population increase. Furthermore, the viruliferous mite population increased more rapidly than the non‐viruliferous mite population. CONCLUSION: The Cleopatra rootstock showed low variability in leaf nitrogen content, low mite number and low leprosis severity, thereby producing the most favourable results. Under appropriate moisture conditions, the Sunki rootstock produced the best results, showing the least severe leprosis. Rangpur lime rootstock grown in soil with varying water content showed the greatest variation in foliar nitrogen content, mite number and severity of leprosis and is considered to be the most difficult rootstock to manage. Copyright © 2012 Society of Chemical Industry     

Inheritance and heritability of resistance to citrus leprosis

ABSTRACT The genetic inheritance of resistance to leprosis, the most important viral disease of citrus in Brazil, was characterized through the phenotypic assessment of 143 hybrids resulting from crosses between tangor 'Murcott' (Citrus sinensis x C. reticulata) and sweet orange 'Pêra' (C. sinensis), considered to be resistant and susceptible to the disease, respectively. All plants were grafted onto Rangpur lime (C. limonia) and inoculated with Citrus leprosis virus, cytoplasmic type through the infestation with viruliferous mites, Brevipalpus phoenicis. The experiments were arranged in a completely randomized block design with 10 replicates. Incidence and severity of the disease in leaves and stems as well as plant growth parameters (plant height and stem diameter) were recorded for 3 years after the infestation with the viruliferous mites. The average values of all variables were analyzed using principal component analysis, discriminant factorial analysis, estimation of the clonal repeatability coefficients, and frequency of the distributions of the average values for each measured variable. The principal component analysis resulted in the identification of at least two groups with resistance and susceptibility to leprosis, respectively. About 99% of all hybrids were correctly classified according to the discriminant factorial analysis. The broad-sense heritability coefficients for characteristics associated with incidence and severity of leprosis ranged from 0.88 to 0.96. The data suggest that the inheritance of resistance to leprosis may be controlled by only a few genes.     

柑桔体细胞杂种对柑桔脚腐病的抗病性研究

 三七[Panax notoginseng(Burk) F.H.Chen,1945]是我国的名贵药材之一,与三七有关的各种病害的研究报道较多,但根结线虫危害三七,引起三七根结线虫病尚未见报道。     

柑桔裂皮病发生情况鉴定

 柑桔裂皮病(exocortis)是以枳、枳橙和檫檬作砧木的柑桔树的严重病害,它的病原是一种类病毒^[4]。有的毒系引起砧木部树皮纵向开裂或翘裂,植株矮化,树势衰退;有的毒系仅引起植株矮化。此病除了嫁接传播外,亦通过修枝剪等与韧皮部接触传播。病害的鉴定,以利用指示植物为主。敏感的指示植物是Etrog香橼的USDCS 60-13选系和亚利桑那861选系^[1、2]。此外,亚利桑那861-S-1选系更为敏感^[3]。它不仅适用于鉴定一般毒系,亦适用于鉴定弱毒系。防治措施以通过茎尖嫁接脱毒或鉴定选择无病母树繁殖无毒苗木为主,亦要注意防止通过工具等的田间传播。     

Citrus viroid II variants associated with ‘Gummy Bark’ disease

A viroid etiology for citrus gummy bark (CGB) disease of sweet orange is supported by the similarity of symptom expression to cachexia disease of mandarins and tangelos caused by the hop stunt viroid (HSVd) related citrus viroid II (CVd-II), as well as the detection of CVd-II variants in CGB infected Washington navel and Dörtyol sweet orange, a Turkish cultivar. A survey was made of 67 clones of CVd-II related variants recovered from severe CGB symptomatic and non-symptomatic trees of the same cultivars growing in close proximity. Only CVd-IIa, a non-cachexia inducing variant, was found in non-symptomatic Washington navel trees and no CVd-II variants were recovered from the Dörtyol control. CGB infected sources contained a number of CVd-II related variants with the predominant species detected closely related to CVd-IIc, a known cachexia inducing viroid. Biological activity of representactive variants from CGB sources was determined by transmission to citron (Citrus medica) as well as by bioassay on the indexing host for cachexia, Parson's Special mandarin (Citrus reticulata).     

Spatial patterns of the Citrus leprosis virus and its associated mite vector in systems without intervention

Leprosis is caused by the Citrus leprosis virus cytoplasmic type and is vectored by the mite Brevipalpus yothersi. Miticide applications, which cost $54 million annually, are based on inspection for the presence of mites. The aim of the present study was to characterize the spatial patterns of B. yothersi-infested trees and trees with leprosis symptoms for further improvement in sampling and disease control. The presence of mites and the occurrence of leprosis were assessed over two years in 1160 Valencia trees and 720 Natal trees in a commercial sweet orange grove in Sao Paulo State, Brazil. To assess the natural growth and dispersal of mites and leprosis, mite populations were not controlled during the experimental period. Maps of mite-infested trees and trees with leprosis symptoms were analysed at three different levels of spatial hierarchy using complementary methods, i.e. among adjacent trees within and across rows, within quadrats, and the strength and orientation of aggregation among quadrats. The study showed that the spatial patterns of virus-infected and mite-infested trees were different, with a strong aggregation pattern of trees with leprosis symptoms that increased over time. Conversely, the spatial pattern of B. yothersi showed randomness or weak aggregation at all three spatial hierarchical levels. Disease incidence increased steadily in plots of both cultivars, unlike in mite-infested trees where incidence fluctuated over time. These results have important implications for the development of better management strategies for leprosis. Sampling methods and action thresholds for mite control should consider primary disease inoculum in addition to the incidence of mites.     

Molecular characterization of Citrus yellow mosaic badnavirus (CMBV) isolates revealed the presence of two distinct strains infecting citrus in India

Citrus yellow mosaic badnavirus (CMBV) is a non-enveloped, bacilliform DNA virus and the etiologic agent of yellow mosaic disease of citrus in India. The disease was initially reported from the southern parts of India and has now spread to other parts of the country. It is a serious disease of sweet orange (Citrus sinensis) in southern India, where it causes significant yield losses. During a recent survey of citrus groves in the Nagpur region, central India, characteristic mosaic symptoms were observed in mandarin orange (Citrus reticulata) and sweet orange. Virus transmission studies, electron microscopy, PCR amplification and sequencing of cloned PCR products from samples showing mosaic symptoms confirmed the presence of a badnavirus. The CMBV–Nagpur isolate could be transmitted to the Rangpur lime (C. limonia) and acid lime (Citrus aurantifolia) by graft inoculation. Sequence analysis of a segment of ORF-III region and intergenic region (IR) of the viral genome revealed that CMBV–Nagpur isolate formed a distinct clade along with some previously reported isolates that are known to infect acid lime and Rangpur lime. CMBV isolates that infect citrus species other than the acid lime and Rangpur lime formed a second clade. Based on the transmission studies and phylogenetic analyses, it was concluded that at least two strains of CMBV exist in India currently.     

Elimination of Citrus psorosis virus by somatic embryogenesis from stigma and style cultures

Somatic embryogenesis was used to eliminate Citrus psorosis virus (CPsV) from three citrus species (common mandarin, sweet orange and Dweet tangor), all of which regenerated somatic embryos with different embryogenic potential from stigma and style explants. CPsV was detected by double antibody sandwich‐indirect‐enzyme‐linked immunosorbent assay (DASI-ELISA) in explants and embryogenic callus, but was not detected in any of the plants obtained from somatic embryos, even 24 months after regeneration. Loss of juvenile characters (disappearance of thorns) was observed in the first year of growth and was retained in plants propagated by grafting from thornless stems. Somatic embryogenesis appears to be a very promising technique for the production of healthy citrus stocks.     

First record of the citrus red mite <Emphasis Type="Italic">Panonychus citri</Emphasis> in Cyprus

A new phytophagous mite species, Panonychus citri (McGregor) (Acari: Tetranychidae) (the citrus red mite), was found in Cyprus during spring 2008, infesting citrus plantations. Mites occurred in large numbers, mainly in the canopy of orange, lemon and mandarin trees. The adults and larvae preferred the upper surfaces of mature leaves of the above citrus species, although they occurred on both leaf surfaces. Orange, lemon and mandarin trees were greatly affected by this mite, as its high population densities under hot and dry conditions in the spring caused heavy leaf drop (firing) and twig dieback.     

Separation and interference of strains from a citrus tristeza virus isolate evidenced by biological activity and double-stranded RNA (dsRNA) analysis

Separation of strains of citrus tristeza virus (CTV), differentiated by their double-stranded RNA (dsRNA) profiles, was obtained by graft-inoculating citron plants from a Mexican lime that had been recently aphid- or graft-inoculated with a mild CTV isolate (T-385). Up to 24 sub-isolates with differing dsRNA profiles were obtained from the aphid-inoculated lime. Some of these sub-isolates induced stronger symptoms in several citrus species than the original T-385 isolate. One sub-isolate, T-385-33, was mild in Mexican lime, but induced stem pitting on sweet orange. Inoculation of this isolate on Mexican lime, sour orange and Eureka lemon induced mild or no symptoms when inoculum was taken from citron, but very severe symptoms when the inoculum was from sweet orange. Mexican lime and sweet orange plants co-inoculated with T-385-33 from sweet orange in combination with the other 23 sub-isolates showed mild symptoms. The results obtained suggest that there is natural cross-protection among sub-isolates in the original T-385 isolate.     

Field effectiveness of fosetyl-AI against citrus foot rot and brown rot1

The effectiveness of fosetyl-A1 against citrus foot rot caused by Phytophthora citrophthora has been evaluated in a 25-year-old orchard of sweet orange cv. Tarocco, showing severe symptoms of the disease and in a 10-year-old orchard of the clementine-type mandarin cv. Monreal, apparently healthy. All the trees were grafted on sour orange. In both orchards, three sprays at 200 g a.i. per 100 1 were applied in May, July and September for 3 years. The results were evaluated on the basis of yield and fruit quality. Trees of cv. Tarocco had yield increased by 25–44% whereas cv. Monreal yielded 3–16% higher than the unsprayed trees. Fruits of orange cv. Tarocco were collected from trees sprayed with fosetyl-A1 and plunged in a water suspension of P. citrophthora. Fruits treated 11 days before inoculation showed an infection rate of 8.7%) whereas those unsprayed were 77% infected. Fruits inoculated 21 days after the treatment with fosetyl-A1 were 32% infected whereas those unsprayed were 91%, infected. In other trials, trees of volkamer lemon were sprayed with a conidial and mycelial suspension of P. citrophthora at different times after application of fosetyl-A1. The number of infected fruits and leaves in the unsprayed trees was very high (up to total leaf drop) and decreased sharply with the number of sprays (1 to 3).     

Effect of temperature and leaf wetness duration on infection of sweet oranges by Asiatic citrus canker

Asiatic citrus canker, caused by Xanthomonas smithii ssp. citri , formerly X. axonopodis pv. citri , is one of the most serious phytosanitary problems in Brazilian citrus crops. Experiments were conducted under controlled conditions to assess the influence of temperature and leaf wetness duration on infection and subsequent symptom development of citrus canker in sweet orange cvs Hamlin, Natal, Pera and Valencia. The quantified variables were incubation period, disease incidence, disease severity, mean lesion density and mean lesion size at temperatures of 12, 15, 20, 25, 30, 35, 40 and 42°C, and leaf wetness durations of 0, 4, 8, 12, 16, 20 and 24 h. Symptoms did not develop at 42°C. A generalized beta function showed a good fit to the temperature data, severity being highest in the range 30–35°C. The relationship between citrus canker severity and leaf wetness duration was explained by a monomolecular model, with the greatest severity occurring at 24 h of leaf wetness, with 4 h of wetness being the minimum duration sufficient to cause 100% incidence at optimal temperatures of 25–35°C. Mean lesion density behaved similarly to disease severity in relation to temperature variation and leaf wetness duration. A combined monomolecular-beta generalized model fitted disease severity, mean lesion density or lesion size as a function of both temperature and duration of leaf wetness. The estimated minimum and maximum temperatures for the occurrence of disease were 12°C and 40°C, respectively.     

Evolution and status of Oecophylla smaragdina (Fabricius) as a pest control agent in citrus in the Mekong Delta,Vietnam

Citrus farmers in the Mekong Delta have a long tradition of managing the weaver ant Oecophylla smaragdina (Fabricius). From 1994 to 1998, insecticide use increased significantly (P< 0.01) from 66% to 84% in orchards where O. smaragdina occurred. In 1998, ca 75% of the sweet orange (Citrus sinensis) and 25% of the Tieu mandarin (C. reticulata) orchards had large O. smaragdina populations, due to lower pesticide pressure in the first crop. In orchards with O. smaragdina, farmers sprayed less frequently and used fewer highly hazardous insecticides. Major insecticides used in sweet orange were monocrotophos and alpha-cypermethrin, and those used in Tieu mandarin were methidathion, imidacloprid and fenpropathrin. Expenditure on pesticides was reduced by half when O. smaragdina was abundant, without affecting either the yield or the farmers' income. Therefore, O. smaragdina husbandry is a good example of a traditional practice which should be further promoted as an important component of sustainable citrus production. The experience of those farmers who use no or few pesticides should be drawn upon in developing farmer training programmes or mass media tools to promote IPM in citrus. Farmers practicing ant husbandry were significantly older than those not doing so.     

柑桔碎叶病毒的发生与初步鉴定

 采用特洛亚枳橙(Troyer citrange),卡里佐枳橙(Carrizo citrange)、鲁斯克枳橙(Rusk citrange)和厚皮来檬(Citrus excelsa)作指示植物,鉴定出黄岩栽培的柑桔本地早,少核本地早、槾桔、早桔、朱红、乳桔、椪柑.北京柠檬、兴津温州及采自温州的柳橙等10个品种,无论枳砧或构头橙砧的都感染有碎叶病毒(Tatter Leaf Virus)枳橙的症状是叶片出现黄白色斑点,皱缩,畸形,茎下出现黄白色条斑,扭曲;厚皮来檬的症状是叶片黄白色斑点,皱缩、畸形.田间栽培的构头橙砧柑桔品种比枳砧的表现耐柑桔碎叶病毒。枳橙的TLV病叶和田间本地早(积)叶片、花瓣能汁液磨擦豇豆(Vigna sp.)叶,并能回接豇豆,接种3—5天后,豇豆叶即出现红褐色(木占)斑,电镜初步观察到为450—900nm长杆状病毒粒子。     

Uncontrolled Citrus psorosis virus infection in Citrus sinensis transgenic plants expressing a viral 24K-derived hairpin that does not trigger RNA silencing

Citrus psorosis virus (CPsV) is the causal agent of psorosis disease of citrus. Pineapple sweet orange plants were transformed with a hairpin construct derived from the viral 24k gene (lines ihp24K). Contrary to expectations, these lines did not trigger efficient RNA silencing, and when infected with CPsV they showed a phenotype of exacerbated symptoms with a persistent and homogeneous infection without the recovery observed in non-transgenic plants. Ihp24K lines did not behave similarly when challenged with Citrus tristeza virus. All these results indicate that hypersusceptibility is likely related to the specific action of 24K-derived hairpin over CPsV multiplication.     

<Emphasis Type="Italic">Indian citrus ringspot virus:</Emphasis> localization of virus in seed tissues and evidence for lack of seed transmission

Indian citrus ringspot disease is an important viral disease in kinnow mandarin orchards where disease incidence up to 100% has been recorded. The disease is caused by Indian citrus ringspot virus (ICRSV), a positive sense flexuous RNA virus. The transmission of ICRSV is generally through budwood. Association of ICRSV with pollens of naturally infected flowers from cv. ‘Kinnow’ mandarins has been shown previously and this study demonstrates the presence of ICRSV in seed tissues. DAC-ELISA revealed the presence of virus in seed coats but not in embryo and endosperm of seeds collected from the fruits of ICRSV-infected Kinnow plants. Of the infected seed coats, 18% were found to harbor the virus. The seedlings in the grow-out test did not show any symptom for 2 years and the virus could not be detected in seedlings by DAC-ELISA and RT-PCR. The present study indicated that ICRSV could be localized in the testa of seeds but its transmission to progeny was not observed.