Citrus-growing Areas Research Articles

Identification of Circulifer haematoceps (Hemiptera:Cicadellidae) as vector of Spiroplasma citri in the Kerman province of Iran

A study of leafhoppers collected in citrus-growing areas of Kerman province as vectors of Spiroplasma citri, the causal agent of citrus stubborn disease, was carried out by transmission tests to liquid medium and/or to healthy periwinkle plants. From seven field-collected leafhopper species only Orosius albicinctus and Circulifer haematoceps delivered S. citri into LD10 broth medium by feeding through parafilm membrane and C. haematoceps was the sole leafhopper species capable of transmitting S. citri to healthy periwinkle plants. C. haematoceps, O. albicinctus and Austroagallia sinuata collected from sesame fields in citrus- growing areas, were raised on garden stock and sesame plants in the greenhouse. For S. citri acquisition, each progeny leafhopper colony was caged on S. citri infected periwinkles. After the acquisition access period leafhoppers were transferred to healthy periwinkle plants. Detection of spiroplasmas in leafhoppers was done by PCR performed separately on insect bodies and heads. Spiroplasmas were detected in the bodies of all three leafhopper species but in the head of C. haematoceps only. Symptoms typical of S. citri infection were observed only on periwinkle plants fed on by C. haematoceps. Results indicate that among the leafhoppers species collected in Kerman province, only C. haematoceps was capable of transmitting S. citri, and the other two species while capable of acquiring S. citri, were unable to transmit it to healthy plants.

First Report of 'Candidatus Liberibacter asiaticus' Associated with Huanglongbing in Sweet Orange in Ethiopia.

Huanglongbing (HLB) is a serious disease of citrus worldwide. Three different 'Candidatus Liberibacter' species are associated with HLB: 'Ca. Liberibacter asiaticus', 'Ca. L. africanus', and 'Ca. L. americanus' (1). 'Ca. L. africanus' and its vector, Trioza erytreae, are both heat sensitive, and when present, occur in citrus when temperatures remain below 30 to 32°C. In Africa, 'Ca. L. africanus' and T. erytreae have been reported in South Africa, Zimbabwe, Malawi, Burundi, Kenya, Somalia, Ethiopia, Cameroon, and Madagascar (1). Inspection of citrus trees in orchards and budwood sources in nurseries located in the warmer citrus-growing areas of Tigray and North Wollo in northern Ethiopia revealed nearly 100 trees with symptoms of leaf yellowing with a blotchy mottle pattern, dead branches, and decreased fruit quality and yield. Two symptomatic sweet orange budwood trees and three symptomatic orchard plants were sampled in April 2009, along with three healthy-looking sweet orange plants. DNA was extracted from 200 mg of desiccated leaf midribs using the CTAB method (4) and subjected to conventional PCR using the primer pairs A2/J5 (2) and OI2/23S1 (3) that amplify the ribosomal protein gene in the rplKAJL-rpoBC operon and the 16S/23S ribosomal intergenic regions, respectively, of 'Ca. L. africanus' and 'Ca. L. asiaticus'. Positive PCR reactions were obtained for all five symptomatic samples with both primer pairs. PCR amplicons of 703 bp (A2/J5) and 892 bp (OI2/23S) recovered from two of these samples were purified, cloned, and sequenced. BLAST analysis revealed that the nucleotide sequences we obtained for the ribosomal protein (GenBank Accessions Nos. GQ890155 and GQ890156) shared 100% identity with each other and 99% identity with sequences of 'Ca. L. asiaticus' from Brazil (DQ471904), Indonesia (AB480161), China (DQ157277), and Florida (CP001677). Similarly, the 16S/23S ribosomal intergenic sequences (GU296538 and GU296539) shared 100% identity with each other and 99% identity with homologous 'Ca. L. asiaticus' sequences from Brazil (DQ471903), Indonesia (AB480102), China (DQ778016), and Florida (CP001677) and contained two tRNA genes as occurs in 'Ca. L. asiaticus' but not in 'Ca. L. africanus' (3). To our knowledge, this is the first report of 'Ca. L. asiaticus' in Africa. The presence of 'Ca. L. asiaticus' is a threat for warmer citrus-growing areas of Africa that are less favorable for 'Ca. L. africanus' and T. erytreae. In areas where 'Ca. L. asiaticus' was confirmed, symptomatic trees must be promptly eradicated and surveys to determine spread of the disease and its vectors are necessary.

Candidatus Liberibacter asiaticus and Huanglongbing Effects on Citrus Seeds and Seedlings

Huanglongbing (HLB) is a devastating disease of citrus and threatens the citrus industry worldwide. The suspected causal agent of the disease is a phloem-limited bacterium of the genus Candidatus Liberibacter transmitted through insect vector or grafting with diseased budwood. Currently, most seed source trees for citrus rootstock propagation are located outdoors and unprotected from disease transmission. In addition, fruit from HLB-affected scion varieties in Florida containing seeds enter the commercial trade and move into other citrus-growing areas. The objective of this study was to determine how Ca. L. asiaticus infection affects seed quality and seedling development and whether the disease appears in seedlings grown from infected fruit. Two experiments were conducted involving thousands of seedlings produced from seeds from infected rootstock seed source trees and ‘Valencia’ sweet orange trees, respectively. Infection of trees and fruit with Ca. L. asiaticus significantly reduced seed weight, seed germination, and seedling height. Seedlings did not develop symptoms typical of HLB throughout the experiment. Polymerase chain reaction (PCR) analysis initially identified two of 686 rootstock seedlings and three of 431 sweet orange seedlings positive for the pathogen when they were very young. Resampling and PCR analysis of these five seedlings at older ages consistently indicated they were negative for the pathogen and none of these plants ever developed symptoms of HLB. It is suggested that Ca. L. asiaticus may have been translocated into some part of the embryo during seed development but that it was not present in cells or tissue, which permitted replication or disease development as the seedling grew.

Pollen Tube Behavior in Different Mandarin Hybrids

Seedlessness is an important trait in the evaluation of commercial mandarin for fresh consumption. However, in the last decade, the presence of seeds in fruit of cultivars considered as seedless has become a problem in different citrus-growing areas because the commercial value is depreciated. Seeds have appeared concomitantly with the introduction of new cultivars that appear to be cross-compatible. To overcome this problem, different strategies have been explored, but a definitive answer is still elusive. The search for alternatives contrasts with how little is known about the basis of the problem: the pollen-pistil incompatibility reaction in mandarin, and the intercompatibility relationship between different cultivars. In this work, we characterized the pollen-pistil incompatibility in the two commonly grown mandarin cultivars Fortune and Nova, and evaluated the intercompatibility relationship between six cultivars with different genetic origins; these cultivars included Fortune (Citrus clementina Hort. ex Tan. × Citrus reticulata Blanco), Nova [(Citrus paradisi Macf. × C. reticulata) × C. clementina), Comune Clementine (C. clementina), Avana apireno (C. reticulata), Primosole (Citrus unshiu Marcov. × C. reticulata), and Simeto (C. unshiu × Citrus deliciosa Ten.). Following the controlled hand pollination in the field, we evaluated pollen tube performance after self- and cross-pollination, as well as intercultivar compatibility by observing pollen tube growth. The results show the self-incompatibility of these cultivars with the pollen tube having been arrested in the style, which explains their seedless condition when planted in solid blocks. The study of intercompatibility indicates a different pollen tube behavior depending on the genotype, as well as on the cross combinations. These results provide a basis to evaluate self- and intercompatibility in citrus, and the effect of close planting of some cultivars.

Evaluation of Mn concentration provided by soil in citrus-growing regions

Manganese (Mn) is an essential nutrient element in citrus growing and Mn deficiency causes some problems related with physiological and morphological structure. Spatial evaluation of Mn obtained from soils in citrus-growing areas is the main objective of this paper. For this purpose, a citrus-growing region in Turkey has been selected and three effective estimation methods: kriging, neural–fuzzy modelling, and fuzzy interval arithmetic have been considered for the spatial evaluations. The model works primarily focus on the model accuracy and smoothing degree of estimations. In addition, error analysis and comparative assessments, which present the advantages and drawbacks of the models, are conducted in the paper. The results and performance evaluations prove the superiorities of soft computing approach in this evaluation.

Citrus viroid V: Occurrence, Host Range, Diagnosis, and Identification of New Variants

The recently described Citrus viroid V (CVd-V) has been proposed as a new species of the genus Apscaviroid within the family Pospiviroidae. Analysis of 64 samples from different citrus-growing areas has shown that CVd-V is present in the United States, Spain, Nepal, and the Sultanate of Oman. CVd-V found in six sweet orange sources from the Sultanate of Oman was identical to the reference CVd-V variant, whereas three new variants with sequence identities of 98.6% (CVd-VCA), 97.3% (CVd-VST), and 94.9% (CVd-VNE) were identified in sources from California, Spain, and Nepal, respectively. These results suggest that this viroid has not emerged recently and that it is relatively widespread. Transmission assays to sweet orange, mandarin, and mandarin hybrids, clementine, satsuma, lemon, sour orange, Tahiti lime, Palestine sweet lime, calamondin, bergamot, and kumquat have shown that all these citrus species and citrus relatives are hosts for CVd-V. Several indexing approaches, including slot blot, northern blot hybridization, and reverse transcription-polymerase chain reaction, have been evaluated for detecting CVd-V, either using Etrog citron as an amplification host or directly from commercial species and cultivars.

First Report from Syria of Citrus tristeza virus in Citrus spp.

During the spring of 2006, the main Syrian citrus-growing areas of Lattakia (Jableh, Aledyye, Eseelya, Siano, and Hresoon provinces) and Tartous (Almintar, Aljammase, Karto, Majdaloonelbahr, Yahmour, Amreet, Althawra, and Safita provinces) were surveyed to assess the presence of Citrus tristeza virus (CTV). Eight nurseries (approximately 130 plants per nursery), two budwood source fields (approximately 230 trees per field), and 19 groves (approximately 60 trees per grove) containing the main citrus varieties were visually inspected and sampled for serological assays. The hierarchical sampling method was carried out in each selected grove (2). Infected samples were collected from two nurseries, two budwood source fields, and six groves. Stems and leaf petioles from nursery trees and flower explants from the groves were collected and analyzed for CTV by direct tissue blot immunoassay (DTBIA) with the commercial kit from Plantprint (Valencia, Spain). Of 2,653 samples tested, 89 (4%) CTV-infected plants were detected. Five citrus varieties were found to be infected and Meyer lemon (Citrus limon 'Meyer') had the highest incidence at 16%. Numerous sweet orange varieties (Citrus sinensis L.) were found to be highly infected in the field, but only the Washington navel sweet orange was found to be infected in the nurseries. No clear CTV symptoms were observed during the survey. Samples that were positive for CTV by DTBIA were also positive by biological indexing on Mexican lime (C. aurantifolia) and immunocapture-reverse transcription-PCR as described by Nolasco et al. (3). Coat protein gene sequences obtained from five selected clones of a Syrian CTV isolate (GenBank Accession No. EU626555) showed more than 99 and 98% nucleotide sequence identity to a Jordanian CTV isolate (GenBank Accession No. AY550252) and the VT isolate (GenBank Accession No. U56902), respectively. Almost all infected samples induced moderate vein clearing symptoms when grafted to Mexican lime. Symptoms of vein clearing, leaf cupping, stunting, and stem pitting on Mexican lime were induced by graft transmission of CTV from one Valencia sample from the Tartous area. The viral inoculum is widely and randomly distributed in commercial groves, especially in the southern Tartous area and in some nurseries. To our knowledge, this is the first report of CTV in Syria. However, CTV was reported from the neighboring citrus-growing countries of Lebanon, Turkey, and Jordan (1), and the severe seedling yellows strain is present in this area, which poses a potential threat to Syrian citriculture.

Pest risk assessment made by France on Citrus chlorotic dwarf virus considered by France as harmful in the French overseas departments of French Guiana, Guadeloupe, Martinique and Réunion - Scientific Opinion of the Panel on Plant Health

Following a request from the European Commission, the EFSA Panel on Plant Health was asked to deliver a scientific opinion on 30 pest risk assessments made by France on organisms which are considered by France as harmful in four French overseas departments, i.e. French Guiana, Guadeloupe, Martinique and Reunion. In particular, the Panel was asked whether these organisms can be considered as harmful organisms for the endangered area of the above departments, in the meaning of the definition mentioned in Article 2.1.(e) of Directive 2000/29/EC and thus potentially eligible for addition to the list of harmful organisms in Directive 2000/29/EC. This document presents the opinion of the Panel on Plant Health on the simplified[1] pest risk assessment conducted by France on Citrus chlorotic dwarf virus, with French Guiana, Guadeloupe, Martinique and Reunion considered as endangered area. The Panel will use the name Citrus chlorotic dwarf (CCD) agent in the following text, as the causal agent of citrus chlorotic dwarf disease has not yet been identified. CCD is a virus-like disease that can affect almost all citrus species and cultivars. The causal agent is transmitted by the bayberry whitefly Parabemisia myricae Kuwana (Homoptera: Aleyrodidae) in a persistent or semi-persistent manner. In addition CCD is transmitted by grafting and can be experimentally transmitted by mechanical inoculation. The Panel examined in detail the risk assessment provided, and considered the accuracy and quality of the information provided and of the methods applied for pest risk assessment purposes. The review was based on the principles and terminology of the International Standard on Phytosanitary Measures ISPM No. 11[2] [Pest risk analysis for quarantine pests including analysis of environmental risks and living modified organisms (2004)] and the terminology of ISPM No. 5 [Glossary of Phytosanitary Terms (2007)] by the International Plant Protection Convention (FAO, 2007). The simplified assessment provides limited evidence to justify the ratings given in the assessment. It concludes that the phytosanitary risk associated with CCD disease is low to moderate. After analysis of the French document, the references cited and additional references and in consultation with experts in Turkey, the Panel considers that the overall phytosanitary risk posed by the CCD agent is underestimated in the assessment provided. The Panel concludes that: * Although the causal agent of the CCD disease is not yet characterised, it produces distinct and consistent symptoms. * The CCD agent is graft transmissible and can be reliably detected although not identified by biological indexing. * The disease is currently restricted to the eastern Mediterranean region of Turkey * The CCD agent is effectively transmitted in a semi-persistent or persistent manner by the bayberry whitefly, Parabemisia myricae, which is present in Guadeloupe. Uncertainty exists over the presence of this vector in the other French overseas departments under consideration. * The main pathway of entry is through the import, from Turkey, of citrus plants or budwood for multiplication. * If the CCD agent was to be introduced, it would remain in the French overseas departments for at least as long as the infected material is cultivated and propagated and, at least in Guadeloupe, it is likely to spread to other citrus plants through vector activity. * Severe losses are reported in almost all citrus species with the exception of sweet orange which shows tolerance to the disease but may, however, act as source of inoculum. * Once the CCD agent is introduced in a citrus growing area, it would be difficult if not impossible to control and/or to eradicate. * Preventing entry of citrus plant material coming from the affected area would reduce the risks to the PRA area[3] to a negligible level. * While the CCD agent can be detected by bioassays as a disease causing agent, its unequivocal identification is pending. * A significant level of uncertainty also exists over the presence of P.myricae in the French overseas departments other than Guadeloupe. Overall, the Panel considers the CCD agent appropriate for analysis of risk management options for the French overseas departments of French Guiana, Guadeloupe, Martinique and Reunion and thus potentially eligible for addition to the list of harmful organisms in Directive 2000/29/EC. The Panel notes, in addition, that the CCD agent may pose a risk to other citrus-growing areas in the European Union, particularly areas where the vector is present (e.g. Spain), and which lie closer to Turkey, the area where the disease is present. The Panel therefore recommends further assessment of the risks posed for the wider PRA area of the EU

First Report of Dodder Transmission of 'Candidatus Liberibacter asiaticus' to Tomato (Lycopersicon esculentum).

Citrus huanglongbing (HLB) is one of the most devastating diseases of citrus worldwide. The disease is associated with three different species of fastidious α-proteobacteria, namely 'Candidatus Liberibacter asiaticus', Ca. L. americanus, and Ca. L. africanus (1). 'Ca. L. asiaticus' was first detected in South Florida in 2005 and has spread throughout the citrus-growing areas of Florida. 'Ca. L. asiaticus' is transmitted naturally by the Asian citrus psyllid, Diaphorina citri, and can also be transmitted by graft propagation and via various species of dodder (Cuscuta). HLB affects most if not all citrus and citrus relatives within the family Rutaceae (2), including the ornamental shrub Murraya paniculata (4). In addition, 'Ca. L. asiaticus' and 'Ca. L. americanus' can infect tobacco (Nicotiana xanthi) and periwinkle (Catharanthus roseus) (1,4). Here we report that 'Ca. L. asiaticus' can infect tomato (Lycopersicon esculentum) cvs. Manapal and FL47. Manapal and FL 47 plants grown from seed were placed adjacent to 'Ca. L. asiaticus'-infected sweet orange (Citrus sinensis) plants with dodder (Cuscuta pentagona) already well established on them. Young dodder shoots still connected to the citrus were draped over the tomato plants and subsequently also became attached to the tomato stems. After 1 month, the tomato plants were detached from the citrus and most of the dodder removed. One month later, these tomato plants started to show vein clearing and subsequently the mature leaves became thicker and leathery. Some leaves showed blotchy mottle symptoms and some fruits became lopsided in a manner similar to HLB symptom expression on citrus. PCR amplification of the 'Ca. L. asiaticus' 16S rDNA with primers OI1/OI2c and the β-operon with primers A2/J5 (1) revealed the presence of 'Ca. L. asiaticus' DNA. Sequence analysis confirmed that the sequences of the cloned amplicons were identical to those from the HLB-infected citrus source plant. Both conventional and quantitative real-time PCR (3) revealed a much lower abundance of 'Ca. L. asiaticus' DNA in tomato as compared with 'Ca. L. asiaticus'-infected citrus or periwinkle, indicating that 'Ca. L. asiaticus' bacteria multiplied at a lower titer in these tomato cultivars.

A selective PCR-based method for the identification of Phytophthora hibernalis Carne

Preharvest and postharvest brown rot of citrus fruit is responsible for important economic losses thoroughout the world. The disease is commonly caused by several species of Phytophthora. In Spain, citrus brown rot is mainly caused by P. citrophthora, but in the last years outbreaks of the disease in lemon and sweet orange caused by P. hibernalis Carne have been recorded in northwestern citrus-growing areas. In this work, a PCR method has been developed for the diagnostic of P. hibernalis either from isolated mycelia or directly from fruit lesions. One specific primer pair, PHIB1 and PHIB2, was designed from the nucleotide sequences of ITS1 and ITS2 regions. The two primers amplified a 407-bp fragment from the genomic DNA of P. hibernalis that was sequenced (Acc. No. AY827556). Results demonstrated that PCR amplification of ITS regions by primers PHIB1 and PHIB2 followed by DNA sequencing can provide a rapid, selective and reliable identification of P. hibernalis.

VIRUS AND VIRUS-LIKE DISEASES OF CITRUS IN EPIRUS

In 2005 a survey was conducted in the main citrusgrowing areas of Epirus. Commercial groves and nurseries were inspected for symptoms of virus and viruslike diseases and a total of 123 samples were collected. Molecular hybridisation was used to test for Citrus tristeza virus (CTV), Citrus psorosis virus (CPsV), Citrus infectious variegation virus (CVV), Citrus exocortis viroid (CEVd) and Hop stunt viroid (HSVd). Although the results are from a low number of samples, they give a significant insight into the sanitary conditions of the Greek citrus industry, disclosing the presence of CTV mild strains in the North-western area.

Biology and Management of the Asian Citrus Psyllid, Diaphorina citri Kuwayama, in Florida Citrus

ENY-739, a 7-page illustrated fact sheet by Michael E. Rogers and Philip A. Stansly, provides information about the biology of this vector of citrus greening disease to aid growers in implementing integrated pest management (IPM) practices for suppressing the population in citrus-growing areas of Florida. It includes sections on identification and biology, psyllid feeding damage, pathogen transmission, management, and selected references. Published by the UF Department of Entomology and Nematology, June 2006.

Distribution and management ofPhaeoramularialeaf and fruit spot disease of citrus in Ethiopia

Introduction. Citrus is the most important fruit crop widely grown by small-scale and commercial farms in Ethiopia. However, productivity of citrus in many parts of the country is threatened by Phaeoramularia leaf and fruit spot disease caused by Phaeoramularia angolensis. Surveys and field experiments were conducted to map the geographical distribution, and to study the efficacy of different fungicides for the control of this disease. Materials and methods. Surveys were conducted in the major citrus-growing areas (south, south-west and north-west) of Ethiopia. Random sampling techniques were used for sample collection. The efficacy of different fungicides against P. angolensis was studied under a hot-spot area, based on disease incidence and severity, and marketable fruit yield. Results and discussion. Phaeoramularia leaf and fruit spot disease of citrus has been widespread throughout the south, south-west, and north-west of Ethiopia, cau- sing complete crop failure. This disease was more severe in smallholder farmers' fields as compared with the commercial orchards. The severity of P. angolensis was varied between different citrus species and areas with different agro-ecologies. The south and south-west part of the country, cha- racterized by high and frequent rainfall with high humidity, were severely affected by the disease. Field application of Benlate, Score and Cuproxat significantly reduced the incidence and severity of the disease and, thereby, increased marketable fruit yield. Conclusion. Phaeoramularia leaf and fruit spot disease of citrus has become widespread in many parts of Ethiopia within a short period. Further investigation on P. angolensis biology and detailed epidemiological studies are mandatory to develop a model, and, thereby, to facilitate forecasting systems for effective and sus- tainable management of leaf and fruit spot disease of citrus in Ethiopia. Ethiopia / Citrus / plant diseases / blotches / Phaeoramularia angolensis / geographical distribution / disease control

Establishment and dispersal of the parasitoids Ageniaspis citricola Logvinovskaya (Hymenoptera: Encyrtidae) and Semielacher petiolatus Girault (Hymenoptera: Eulophidae), introduced into Tunisia for the biological control of Phyllocnistis citrella (Lepidoptera: Gracillariidae)

A programme for the biological control of the citrus leafminer Phyllocnistis citrella Stainton (Lepidoptera, Gracillariidae) was carried out in Tunisia between 1996 and 1998. Two exotic parasitoids, Ageniaspis citricola (Logvinovskaya) (Hymenoptera, Encyrtidae) and Semielacher petiolatus Girault (Hymenoptera, Eulophidae), were introduced from Australia, mass-reared in a greenhouse on Citrus aurantium L. seedlings infested with P. citrella, and released in different orchards located in the major citrus-growing area of Tunisia. In 1996, A. citricola was recovered in 39.13% of release sites, reaching a maximum percentage of parasitism of 28%. In 1997, the parasitoid was recovered in a third of all release points, with an average percentage of parasitism of 5%. However, the wasp was unable to survive the winter 1998, and has become extinct. The non-establishment of A. citricola may have been due to the more arid climatic conditions present during the rainless summer and most of the autumn in Tunisia. By contrast, the establishment, dispersal, and reduction in citrus leafminer population observed with S. petiolatus between 1997 and 1998 shows this species to be well-adapted to the Tunisian climate. The average percentage parasitism of P. citrella by S. petiolatus increased from 6.6% in 1997 to 15% in 1998. Moreover, S. petiolatus was established in 76% of release points in 1997 and in 82% in 1998. It spread as much as 30 km from some release points, with no apparent decline in parasitism levels with distance. Semielacher petiolatus is considered to be an effective biological control of the leafminer in Tunisia. Its biological control action would supplement the action of the native parasitoids Pnigalio sp. and Cirrospilus pictus Nees (Hymenoptera, Eulophidae).

OVERWINTERING OF THE CITRUS LEAFMINER, PHYLLOCNISTIS CITRELLA (LEPIDOPTERA: GRACILLARIIDAE), WITHOUT DIAPAUSE IN FLORIDA

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is an important pest of citrus that has spread around the world. The citrus leafminer invaded Florida in 1993 and rapidly colonized all citrus-growing areas. Despite the fact that the citrus leafminer has been studied extensively, gaps in our knowledge of its biology remain. One issue is how the citrus leafminer overwinters. Citrus leafminers have been observed to overwinter in many countries, but their capacity to diapause has not been confirmed. To investigate whether P. citrella overwinters in diapause, we evaluated the ability of eggs to develop to adulthood on potted citrus trees when reared in both outdoor and growth chamber conditions. No pupae were found to be in diapause in the outdoor assessment in which three populations of citrus leafminer were reared in a shade house in Gainesville, Florida beginning on Oct 15, Nov 11, or Nov 22. Larval and pupal mortality significantly increased in the population set out on Nov 22 compared to the other populations. Adult longevity significantly increased in the last two populations compared to the Oct population. However, similar degree days were accumulated during the adult lifespan among the three populations. In the growth chamber assessments, citrus leafminers showed no arrestment in development during the pupal stage by short-day treatments. Development (days) from egg to adult was not different between short- and long-day conditions in both sexes. No signs of reproductive diapause were found from dissections of adult females reared under short- or long-day conditions. We concluded that P. citrella may overwinter on the few small flushes available during winter in mid and central Florida, and the relevance of these results for citrus IPM programs is discussed.

Application of a GIS–AF/RF model to assess the risk of herbicide leaching in a citrus-growing area of the Valencia Community, Spain

We integrated an index-based attenuation factor/retardation factor (AF/RF) model into a GIS to evaluate the risk of leaching of the most frequently applied herbicides (glyphosate, diuron, diquat, bromacil, simazine, linuron, terbuthylazine, and terbumeton) used in citrus orchards of the Valencia Community, Spain. The GIS-model system was applied to a region of 33,800 ha located near Valencia City. The soil and climate data required by the model were stored in an Arc/Info GIS in which the model algorithms were integrated using the AML programming language. A graphical user interface was developed to facilitate the use of the GIS-model system. The resulting simulation maps indicate that terbumeton, bromacil, and simazine herbicides have the highest risk of leaching because of their high mobility and low K(oc) (32-158 mg l(-1)). The remaining herbicides are strongly adsorbed by clay particles and organic matter, thus minimising the risk of leaching through the soil profile and into groundwater. The obtained ranking of the leaching potential of analysed herbicides is as follows, from highest to lowest risk: terbumeton>bromacil>simazine>terbuthylazine>diuron>linuron>glyphosate>diquat.

Huanglongbing: a destructive, newly-emerging, century-old disease of citrus.

A detailed account is given of the history, aetiology, biology, epidemiology, detection, geographical distribution, and control of huanglongbing (HLB), a destructive disease of citrus that represents a major threat to the world citrus industry, and is slowly invading new citrusgrowing areas. HLB, whose name in Chinese means “yellow dragon disease”, was first reported from southern China in 1919 and is now known to occur in next to 40 different Asian, African, Oceanian, South and North American countries. The agent is a phloem-restricted, non cultured, Gram-negative bacterium causing crippling diseases denoted “greening “ in South Africa, “mottle leaf” in the Philippines, “dieback” in India, “vein phloem degeneration” in Indonesia. The HLB bacterium belongs to the genus Candidatus Liberibacter, three species of which are currently known, Candidatus Liberibacter asiaticus, occurring in Asian countries and, to a lesser extent, in Brazil and the USA (Florida), Candidatus Liberibacter africanus with its subspecies “capensis”, recorded from African countries, and Candidatus Liberibacter americanus present in Brazil. The suggestion is that each liberibacter species has evolved in the continent after which it is named. HLB symptoms are virtually the same wherever the disease occurs. Infected trees show a blotchy mottle condition of the leaves that results in the development of yellow shoots, the early and very characteristic symptom of the disease. Trees are stunted, declining and bear a few, small-sized, and deformed (lop-sided) fruits, that are poorly coloured (greening) and with coloration starting at the peduncular end (colour inversion). HLB can be transmitted by grafting from citrus to citrus and by dodder to periwinkle. The psyllids Trioza erytreae and Diaphorina citri are natural vectors. Two different types of HLB are known: the heat-sensitive African form transmitted by T. erytreae, which develops at temperatures of 22-25°C, and the heat-tolerant Asian form, transmitted by D. citri, which stands temperatures well above 30°C. Although the HLB pathogen can be identified by electron microscopy, other laboratory methods are used for routine detection. ELISA with monoclonal antibodies is not recommended. Better systems are dot blot hybridization with a DNA probe, and various PCR formats (one-step, nested, multiplex) using species-specific primers based on 16S rRNA or rplKAJL-rpoBC operon sequences. Because no curative methods of HLB are available, control is preventive and largely based on inoculum elimination by removal of infected trees and chemical treatments against vectors. Strict quarantine measures must be implemented to impair further international spread of HLB agents and their vectors.

Evaluation of the Alter-Rater model for spray timing for control of Alternaria brown spot on Murcott tangor in Brazil

Alternaria brown spot, caused by Alternaria alternata, recently appeared in Brazil and is now causing serious damage to Murcott tangors in São Paulo State. The Alter-Rater model and other systems for timing fungicide applications as well as different fungicide programs were evaluated in two seasons in different citrus-growing areas of São Paulo State. Alternaria brown spot severely reduced yields of Murcotts in the unsprayed controls in both years. Fungicide applications reduced fruit drop and increased yields in many cases. Regression analyses showed that yield declined as disease severity increased. A copper fungicide applied according to the Alter-Rater model with a trigger value of 50 required one to two more applications than a calendar program in both locations in 2002–03. Use of the Alter-Rater 50 increased the number of fruit harvested in both locations, but did not improve fruit quality when compared to the calendar program. Use of the Alter-Rater with trigger values of 100 or 150, or the Copper model forecast fewer sprays and resulted in poor disease control and low yields. In a test in 2003–04, fungicide applications timed with the Alter-Rater 50 were no more effective than calendar sprays. Copper fungicides throughout the season or programs with copper applied early followed by dithiocarbamates or QoI fungicides later provided better disease control than a full-season mancozeb program. Murcott product in some citrus growing areas in Brazil will be possible only if the value of the crop is sufficient to justify 10 or more fungicide applications per year.

Aonidiella citrina

<i>Aonidiella citrina</i>

Is the potential of Coccidoxenoides perminutus, a mealybug parasitoid, limited by climatic or nutritional factors?

The encyrtid Coccidoxenoides perminutus is a widely distributed parasitoid of citrus mealybug (Planococcus citri). Worldwide, it has been implicated in successful biocontrol in only a few widely separated localities. C perminutus contributes little to control P. citri in field situations in south-east Queensland, Australia, but invades insectary cultures and reduces mealybug populations considerably under these controlled conditions. This discrepancy between poor field performance and good performance under controlled conditions was investigated to establish whether climatic factors inhibit the field performance of this species in the biological control of P. citri. Subsequent laboratory examination of the influence of varied humidities and temperatures on the activity levels and survival of C perminutus revealed a low tolerance for high saturation deficits (i.e., low % RH at high T degreesC) with reduced reproductive output. The influence of different food sources on adult survival and reproduction was also quantified, to establish if the adverse effects of climate could be overcome by supplementing adult diet. Neither honeydew from their mealybug hosts nor nectar from Alphitonia flowers significantly enhanced parasitoid survival. A subsequent test of five nectar species revealed a significant difference in their influence on C. perminutus survival and reproduction, with only Alpinia zerumbet proving to be as suitable as honey. The floral species that proved suitable in the laboratory need to be checked for their attractiveness to C perminutus in the field and for their ability to enhance the survival and reproductive output of parasitoids. This information suggests that the prevailing dry conditions in south-east Queensland citrus-growing areas apparently impede successful biological control of P. citri by C perminutus, but possibilities are available for habitat manipulation (by providing suitable nectar sources for adult parasitoids) to conserve and enhance C perminutus activity in the field. (C) 2004 Elsevier Inc. All rights reserved.