A diagnostic medium for the semi-selective isolation and enumeration of <Emphasis Type="Italic">Xanthomonas axonopodis</Emphasis>pv. <Emphasis Type="Italic">vignicola</Emphasis>

A semi-selective medium for isolation of Xanthomonas axonopodis pv. vignicola from cowpea (Vigna unguiculata) plant and soil samples was developed. Twelve carbon and five nitrogen sources were tested with four strains of X. axonopodispv.vignicola, and 25 antibiotics were screened against saprophytes. -cellobiose (10g) was selected as the optimal carbon source. Among the antibiotics, cefazoline inhibited growth of most of the saprophytes with little effect on strains of the pathogen. ,-methionine enhanced growth of X. axonopodispv.vignicola. Boric acid along with ammonium chloride suppressed growth of Pseudomonas fluorescens. The semi-selective medium designated as cefazoline-cellobiose-methionine (CCM) medium contained K₂HPO₄ 1.34g, KH₂PO₄ 0.4g, MgSO₄ 0.3g, H₃BO₃ 0.2g, NH₄Cl 1.0g, -cellobiose 10g, cycloheximide 0.2g, ,-methionine 1.0g, cefazoline 10mg and agar 14g per l of water (pH 7.2). Colonies of X. axonopodispv.vignicola on CCM medium were whitish, round, raised and 0.2–1.8mm in diameter 96h after incubation. CCM medium generally inhibited growth of Pantoea agglomerans, Bacillus subtilis and saprophytes isolated from cowpea leaves. Colonies of Pseudomonas fluorescens and a saprophytic bacterium, which were not completely suppressed by CCM, could be differentiated from X. axonopodispv.vignicola by their smaller size and different color. The CCM medium proved useful for isolation of X. axonopodispv.vignicola from cowpea plant and soil samples. This is the first report of a semi-selective medium developed for detection of X. axonopodispv.vignicola.     

Persistence of <Emphasis Type="Italic">Xanthomonas axonopodis</Emphasis> pv. <Emphasis Type="Italic">vignicola</Emphasis> in weeds and crop debris and identification of <Emphasis Type="Italic">Sphenostylis stenocarpa</Emphasis> as a potential new host

The survival of Xanthomonas axonopodis pv. vignicola, incitant of cowpea bacterial blight and pustule, in residues of infested cowpea leaves was studied in the field in the forest savanna transition zone of South Benin and under variable controlled conditions. The pathogen survived for up to 60 days when placed on the soil surface, and up to 45 days buried at depths of 10 and 20 cm. In the glasshouse, bacteria survived in residue mixed with soil for at least 2 months in dry soil and less than 2 months in moist soil. The pathogen survived at least 30 days in the field after spray-inoculation on the weed species Euphorbia heterophylla, Digitaria horizontalis and Synedrella nodiflora; 20 days on Panicum subalbidum; 10 days on Euphorbia hirta; and 5 days on Talinum triangulare. After leaf-infiltration under glasshouse conditions, the pathogen was detected after 90 days in D. horizontalis; 75 days in T. triangulare, P. subalbidum and S. nodiflora; 60 days in E. hirta, and 30 days in E. heterophylla. Among 12 legume species tested as alternative hosts of X. axonopodis pv. vignicola, only Sphenostylis stenocarpa (African yam bean) showed typical symptoms of cowpea bacterial blight in a glasshouse experiment following artificial inoculation. This is the first time this legume species has been identified as a potential host of X. axonopodis pv.vignicola. Crop residue and weeds are likely sources of primary inoculum when planting two consecutive cowpea crops per year and they probably play a role in dissemination of the pathogen during the cropping season. The alternate host may form a bridge for primary inoculum between cropping seasons.     

Structural conservation of the <Emphasis Type="Italic">hrp</Emphasis> gene cluster in <Emphasis Type="Italic">Xanthomons oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzae</Emphasis>

The clustered hrp genes encoding the type III secretion system in the Japanese strains MAFF301237 and MAFF311018 of Xanthomonas oryzae pv. oryzae were sequenced and compared. The strains differ in their pathogenicity, location, and year of isolation. A 30-kbp sequence comprising 29 open reading frames (ORFs) was identical in its structural arrangement in both strains but differed from X. campestris pv. campestris, X. axonopodis pv. citri, and X. axonopodis pv. glycines in certain genes located between the hpaB-hrpF interspace region. The DNA sequence and the putative amino acid sequence in each ORF was also identical in both X. oryzae pv. oryzae strains as were the PIP boxes and the relative sequences. These facts clearly showed that the structure of the hrp gene cluster in X. oryzae pv. oryzae is unique.     

Detection and identification of the crucifer pathogen, <Emphasis Type="Italic">Xanthomonas campestris</Emphasis> pv. <Emphasis Type="Italic">campestris</Emphasis>, by PCR amplification of the conserved Hrp/type III secretion system gene <Emphasis Type="Italic">hrcC</Emphasis>

The development of a rapid detection method for Xanthomonas campestris pv. campestris (Xcc) in crucifer seeds and plants is essential for high-throughput certification purposes. Here we describe a diagnostic protocol for the identification/detection of Xcc by PCR amplification of fragments from the pathogenicity-associated gene hrcC. Under stringent conditions of amplification, a PCR product of 519 bp from hrcC was obtained from a collection of 46 isolates of Xcc, with the exception of two isolates from radish. No amplicons were obtained from 39 pure cultures of the phytopathogenic bacteria Xanthomonas campestris pv. cerealicola, X. campestris pv. juglandis, X. campestris pv. pelargonii, X. campestris pv. vitians, X. arboricola pv. pruni, X. axonopodis pv. phaseoli, X. axonopodis pv. vesicatoria, X. vesicatoria, Pseudomonas syringae pv. phaseolicola, P. syringae pv. syringae, P. syringae pv. tomato, P. fluorescens, P. marginalis, Pectobacterium atrosepticum, P. carotovorum subsp. carotovorum. In addition, PCR reactions were negative for fifty unidentified environmental isolates purified from the surface of crucifers. The PCR fragment was obtained from four strains previously classified as X. campestris pv. aberrans, X. campestris pv. armorociae, X. campestris pv. barbarae and X. campestris pv. incanae using pathogenicity assays. Our PCR protocol specifically detected Xcc in inoculated leaves, seeds and naturally infected leaves of crucifers.     

Investigation of Genomic Variability of Xanthomonas Arboricola Pv. juglandis by AFLP Analysis

Xanthomonas arboricola pv. juglandis is the causal agent of walnut blight, one of the most important and widespread diseases of Persian (English) walnut (Juglans regiaL.), causing severe damage to leaves, twigs and nuts. To investigate the genomic variability of X. arboricolapv. juglandis, 66 isolates obtained from different countries (England, France, Italy, The Netherlands, Romania, Spain, USA, and New Zealand) were analysed using the Amplified Fragment Length Polymorphism (AFLP) technique. EcoRI and MseI were used as restriction endonucleases. Primers with a core sequence including endonuclease recognition sites and a 3^prime-terminal cytosine selective base for MseI primer, or no selective base for EcoRI primer, were used. Data were analysed by means of a multiple correspondence analysis. A total of 76 amplified polymorphic DNA fragments were used to compute relationships among isolates. The AFLP profiles of X. arboricola pv. juglandis isolates appeared to be reliably distinguishable from X. arboricola pv. pruni and X. arboricola pv. corylina, and from other Xanthomonas species, i.e. X. campestris pv. campestris, X. fragariae, X. hortorum, X. axonopodis pv. vesicatoria. Though this pathogen is associated with one single host genus, a high level of genomic diversity was found. This diversity might be partly explained by the geographic origin. Nevertheless, isolates with different patterns were collected within one country, and similar molecular patterns were found in isolates collected at different sites. However, genetic diversity might be influenced by exchanging vegetative material from different countries. Mixing of X. arboricola pv. juglandis isolates might have partly concealed the influence of the geographic location from which the bacteria were isolated.     

Differentiation of Xanthomonas campestris pv. Phaseoli from Xanthomonas campestris pv. Phaseoli var. Fuscans by PFGE and RFLP

Xanthomonas campestris pv. phaseoli and X. campestris pv. phaseoli var. fuscans, the causal agents of the common and fuscous bacterial blight of beans, appear to be phenotypically identical except that the latter can produce a melanin-like pigment in culture. Ten isolates of X. campestris pv. phaseoli and 12 isolates of X. campestris pv. phaseoli var. fuscans were examined using pulsed-field gel electrophoresis (PFGE) and restriction fragment length polymorphism (RFLP). The average genome sizes for X. campestris pv. phaseoli and X. campestris pv. phaseoli var. fuscans were 3850.6±48.9 and 3584.3±68.1kb respectively. The genetic relatedness of the isolates was determined from macrorestriction patterns generated using XbaI. Cluster analysis indicated that the non-fuscous and fuscous strains are distinct. RFLP results, based on the highly conserved hrp genes and a pectate lyase gene from Xanthomonas, also indicated that the two bacteria are genetically different. The results obtained in this study suggest that this pathovar can be segregated into two subgroups under a recently proposed reclassification of the Xanthomonas genus.     

Identification and Characterisation of Xanthomonas campestris pv. campestris Strains from Tanzania by Pathogenicity Tests, Biolog, rep-PCR and Fatty Acid Methyl Ester Analysis

Black rot, caused by Xanthomonas campestris pv. campestris (Xcc), is a major disease constraint to cabbage production by smallholder farmers in Africa. Variability exists within the pathogen, and yet differentiation of Xcc strains from other closely-related xanthomonads attacking crucifers is often difficult. The Biolog system, fatty acid methyl ester analysis using microbial identification system (MIS), rep-PCR and pathogenicity tests were used to identify and characterise Xcc strains from Tanzania. Great diversity was observed among Xcc strains in their Biolog and rep-PCR profiles. Specific rep-PCR genomic fingerprints were linked to some geographical areas in the country. Most of the Xcc strains were clustered in two groups based on their fatty acid profiles and symptom expression in cabbage although some deviant strains were found. Each of the methods allowed a degree of identification from species, pathovar to the strain level. Biolog and MIS identified all Xcc strains at least to the genus level. Additionally, Biolog identified 47% of Xcc strains to the pathovar and 43% to strain level, whereas MIS identified 43% of the strains to pathovar level. In the absence of a database, the utility of rep-PCR for routine diagnosis of strains was limited, although the procedure was good for delineation of Xcc to the strain level. These findings indicate the existence of Xcc strains in Tanzania that are distinct from those included in Biolog and MIS databases. The limitations noticed warrant continued improvement of databases and inclusion of pathogenicity testing, using universally susceptible cultivars, as an integral part of strain identification.     

Genetic relationships among strains of <Emphasis Type="Italic">Xanthomonas campestris</Emphasis>pv. <Emphasis Type="Italic">campestris</Emphasis>revealed by novel rep-PCR primers

Novel primers for rep-PCR were developed with the original software and based on `ancient diverged periodical sequences'. Rep-PCR with these primers was applied to study genetic relationships among 51 Xanthomonas campestris strains. The strains were collected from different countries including Russia, Japan, UK, Germany and Hungary. Reference strains of three X. campestrispathovars and five other Xanthomonas species were included. Based on qualitative differences in amplification profiles, the strains were divided into four major groups. Two subgroups recognised within X. campestrispopulation were similar to RFLP haplotypes. The third subgroup included strains of two other pathovariants and Japanese isolates of X. campestris pv. campestriswhile the fourth group comprised the other species of Xanthomonas. The analysis of the diversity within X. campestris resulted in the conclusion that isolates belong to distinct clonal populations (subgroups). The differences between the subgroups of X. campestris were only slightly smaller than between species of Xanthomonas. A PCR fragment about 600 bp amplified by primer KRPN2 was found in nearly all tested strains of X. campestris.SCAR primers designed for this marker produced a single specific band for strains of X. campestris, but not for other Xanthomonas, Pseudomonas and Erwiniastrains tested. Application of the new primer set for rep-PCR offers a rapid, simple and reproducible method for identification of bacterial strains. The X. campestris-specific SCAR primers may be used in diagnostics of this important plant pathogen.     

Identification of Resistance to Common Bacterial Blight Disease on Bean Genotypes Grown in Turkey

Common bacterial blight (CBB) in edible beans (Phaseolus vulgaris), incited Xanthomonas campestris pv. phaseoli, reduces bean yields and seed quality. The main objective of this study was to determine resistance to common bacterial blight in bean genotypes. Twenty-two bean genotypes grown in Turkey including common and snap bean cultivars/lines were collected from different parts of Turkey and tested for resistance against to Xanthomonas campestris pv. phaseoli strain MFD-11. All the common and snap bean lines/cultivars tested were moderately susceptible, susceptible or highly susceptible, except AG-7117 which was found resistant to Xanthomonas campestris pv. phaseoli. This is the first report of a resistance source in a common bean line (AG-7117) against Xanthomonas campestris pv. phaseoli.     

Phenotypic and Genotypic Characterization of Xanthomonas campestris pv. zinniae Strains

During 1997 and 1998, serious outbreaks of bacterial leaf spot disease were observed on zinnia plants grown in home and commercial gardens in Ohio, USA. Twenty-two strains of Xanthomonas campestris pv. zinniae, isolated from diseased zinnia plants and contaminated seeds, were identified based on morphological, physiological and biochemical tests, fatty acid methyl ester analyses and pathogenicity tests on zinnia cv. Scarlet. Host range studies indicated that all of the X. campestris pv. zinniae strains were pathogenic on zinnia and tomato, but not on cabbage, lettuce, pepper and radish. The phenotypic and genotypic relationships among the strains determined based on serological reaction pattern, fatty acid profiles, repetitive extragenic palindromic-polymerase chain reaction (rep-PCR) fingerprints and sequence analysis of the 16S–23S rDNA spacer region suggested that X. campestris pv. zinniae strains were closely related to each other, but clearly distinct from other Xanthomonas species including X. campestris pv. campestris, X. axonopodis pv. vesicatoria, X. vesicatoria and X. hortorum pv. vitians tested in this study. The results also demonstrated that rep-PCR fingerprinting is rapid, reliable and the most practical method for routine detection and identification of X. campestris pv. zinniae strains.     

Differentiation of Xanthomonas species Pathogenic to Sugarcane by PCR-RFLP Analysis

The PCR-RFLP of the 16S-23S rDNA spacer region was used to differentiate Xanthomonas species pathogenic to sugarcane. Strains of X. albilineans, X. campestris pv. vasculorum Types A and B, X. sacchari and Xanthomonas sp. from Trinidad, South Africa and India were examined. The amplification products were digested with Alu I, Hae III, Hpa II and Mbo I and the results showed that the different groups of bacterial strains exhibited distinct RFLP patterns for each tested endonuclease, except X. albilineans and X. sacchari which could only be differentiated from each other by the digestion with Hpa II. The results also allowed the separation of X.c. pv. vasculorum Type A from X.c. pv. vasculorum Type B and strongly suggested that the analyzed Xanthomonas sp. strains belong to X. sacchari. Nine X. campestris (pv. not determined) strains included in this study showed identical profiles to X.c. pv. vasculorum Type A group and DNA–DNA hybridization experiments confirmed these results. PCR-RFLP of the 16S-23S rDNA spacer region could be applied as a reliable method for differentiating the xanthomonads pathogenic to sugarcane.     

Evaluation of glyphosate-tolerant soybean cultivars for resistance to bacterial pustule

Xanthomonas axonopodis pv. glycines causes bacterial pustule of soybean, which is a common disease in many soybean-growing areas of the world and is controlled by a single recessive gene (rxp gene) commonly found in many conventional glyphosate-sensitive soybean cultivars. Since glyphosate-tolerant cultivars are commonly planted today, there has been no information about whether these new cultivars have bacterial pustule resistance. The goal of this study was to screen glyphosate-tolerant soybean cultivars for resistance to X. axonopodis pv. glycines. Three experiments were completed to evaluate resistance. In experiment 1, 525 commercial glyphosate-tolerant cultivars from 2001 were inoculated with X. axonopodis pv. glycines strain UIUC-1. Following inoculation, many of the cultivars were resistant (developed no detectable pustule symptoms) although 152 (~29%) developed bacterial pustule. In experiment 2, the aggressiveness of three strains (UIUC-1, UIUC-2, and ATCC 17915) of X. axonopodis pv. glycines were compared on three bacterial pustule-susceptible, glyphosate-tolerant cultivars. One strain (UIUC-1) was less aggressive than the other two (UIUC-2 and ATCC 17915) on all three cultivars examined. In experiment 3, 45 cultivars from 2005 (all different from 2001) were inoculated with X. axonopodis pv. glycines ATCC 17915. A range of disease severities developed with five cultivars (11%) having disease severity ratings as high as or higher than those on a susceptible check cultivar. Overall, these results suggested that resistance to bacterial pustule occurs in glyphosate-tolerant soybean cultivars, but not at 100% frequency, which means bacterial pustule outbreaks could occur when a susceptible cultivar is planted and conditions are conducive for bacterial pustule development.     

Development of a specific molecular tool for detecting Xanthomonas campestris pv. musacearum

A specific and rapid diagnostic tool has been developed to detect Xanthomonas campestris pv. musacearum, the causal agent of bacterial wilt of banana. PCR primers were developed from intergenic regions of X. campestris pv. musacearum following its partial sequence. A total of 48 primers were tested for specificity to X. campestris pv. musacearum strains collected from various regions in Uganda. These were also tested for specificity against related Xanthomonas species from the vasicola group, Xanthomonas species pathogenic to other crops, and against those existing saprophytically on banana plants. Seven primer sets (Xcm12, Xcm35, Xcm36, Xcm38, Xcm44, Xcm47 and Xcm48) were found to be very specific to X. campestris pv. musacearum. These primer sets directed the amplification of the expected product for all 52 strains of X. campestris pv. musacearum collected from different locations in Uganda. No amplification products were obtained with unrelated phytopathogenic bacteria or endophytic/epiphytic bacteria from banana. A detection limit of 10³ CFU mL^?1 corresponding to about four cells per PCR reaction was observed when X. campestris pv. musacearum cells were used for all the seven primer sets. The DNA samples from symptomless plant tissues also tested positive with primer set Xcm38. The specific PCR method described here is a valuable diagnostic tool which can be used to detect the pathogen at early stages of infection and monitor disease.     

Characterization and PCR-based Typing of Xanthomonas campestris pv. vesicatoria from Peppers and Tomatoes in Serbia

During the last two decades bacterial strains associated with necrotic leaf spots of pepper and tomato fruit spots were collected in Serbia. Twenty-eight strains isolated from pepper and six from tomato were characterized. A study of their physiological and pathological characteristics, and fatty acid composition analysis revealed that all of the strains belong to Xanthomonas campestris pv. vesicatoria. Being non-amylolytic and non-pectolytic, pathogenic on pepper but not on tomato, containing lower amounts of fatty acid 15 : 0 ante–iso, the pepper strains were designated as members of the A group of X. campestris pv. vesicatoria. However, the tomato strains hydrolyzed starch and pectate, caused compatible reactions on tomato but not on pepper, had higher percent of 15 : 0 ante–iso fatty acid, and were classified into B phenotypic group and identified as X. vesicatoria. PCR primers were developed which amplified conserved DNA regions related to the hrp genes of different strains of X. campestris pv. vesicatoria associated with pepper and tomato. Restriction analysis of the PCR product resulted in different patterns and enabled grouping of the strains into four groups. When xanthomonads isolated from pepper and tomato in Serbia were analyzed, they clustered into two groups corresponding to the grouping based on their physiological and pathological characteristics. According to the reaction of pepper and tomato differential varieties, the strains from pepper belong to races P7 and P8 and tomato strains belong to the race T2. All strains were sensitive to copper and streptomycin. Advantages and disadvantages of various bacterial spot management practices are discussed.     

Leaf blight and defoliation caused by two new pathovars of Xanthomonas axonopodis on Schinus terebinthifolius and Mabea fistulifera

Schinus terebinthifolius and Mabea fistulifera have been used for forest repositioning and urban forestry in Brazil. In October 2012, in a routine inspection at the research nursery of the Forestry Department of the Universidade Federal de Viçosa, in Minas Gerais, Brazil, a mortality of approximately 40% of the seedlings was observed as a result of diseases characterized by leaf blight and intense defoliation, which culminated in the death of the plants. Microscopy observations revealed oozing from the infected tissue and isolations revealed a bacterial aetiology for both diseases. Bacterial cells that formed bright yellow mucoid colonies with round edges were routinely isolated from lesion margins. Inoculation of isolated strains into healthy seedlings reproduced the symptoms observed under natural conditions. Bacterial cells showing the same morphological, biochemical and molecular characteristics as those originally isolated from naturally infected plants were reisolated from inoculated plants. Morphological, physiological and biochemical tests as well as 16S rDNA sequencing and multilocus sequence analysis using four housekeeping genes, dnaK, fyuA, gyrB and rpoD, confirmed the newly isolated strains belong to Xanthomonas axonopodis. Plant cross‐inoculations showed the strains did not belong to any known phylogenetically related pathovar. Pathovars X. axonopodis pv. schini pv. nov. and X. axonopodis pv. mabeae pv. nov. are proposed as the causal agents of bacterial leaf blight on S. terebinthifolius and M. fistulifera, respectively.     

Xanthomonas campestris pv. populi,the causal agent of bark necrosis in poplar

A bacterium was isolated from superficial bark necroses on young poplars and its pathogenicity demonstrated by inoculation experiments. The organism was identified asXanthomonas campestris. Cross-inoculations showed that a previously undescribed pathovar was involved. It is suggested to designate this organismX. campestris pv.populi.Samenvatting Uit een oppervlakkige bastnecrose bij jonge populieren werd massaal een bepaalde bacterie geïsoleerd. Met deze bacterie werden gezonde populieren in het veld geïnoculeerd via verwonding van de bast. Als gevolg van de inoculaties ontwikkelden zich bij ongeveer 40% van de geïnoculeerde bomen hetzelfde type bastnecrosen, terwijl bij de controleplanten geen enkele reactie optrad. Uit de kunstmatig verkregen necrosen werd dezelfde bacterie geïsoleerd.Identificatie met biochemische en serologische methoden toonde aan dat de bacterieXanthomonas campestris was.Vervolgens werden in de kas kruisinoculaties uitgevoerd met verschillende xanthomonaden op populier, wilg, kool en geranium. DeX. campestris isolaten uit populier tastten behalve populier ook wilg aan. De andere gebruikte stammen waren waardplant-specifiek, al bleven sommigen ervan minstens acht maanden in leven in een niet-waardplant, evenwel zonder symptomen te veroorzaken. Geconcludeerd wordt, dat de bastnecrosen zijn veroorzaakt door een nog niet beschreven pathovar vanX. campestris. Voorgesteld wordt om deze bacterieXanthomonas campestris pv.populi te noemen.     

Consequences of the taxonomic revision of Xanthomonas axonopodis pv. dieffenbachiae and of the analysis of its associated pathogenicity for the recommendation of regulation (EPPO A2 List)

Xanthomonas axonopodis pv. dieffenbachiae, the causal agent of bacterial blight of Araceae (aroids), is a regulated pest in several countries and is included in the EPPO A2 List. Reference strains of Xanthomonas axonopodis pv. dieffenbachiae have recently been reclassified into the species Xanthomonas phaseoli, Xanthomonas citri and Xanthomonas euvesicatoria on the basis of different features, including multilocus sequence analysis, average nucleotide identity and homology in DNA–DNA hybridization analyses. Based on pathogenicity tests, Constantin et al. (2017) proposed naming the pathogens on aroids as X. phaseoli pv. dieffenbachiae, X. phaseoli pv. syngonii and X. citri pv. aracearum. Recommendations are made on how to deal with these changes for the group of pathogenic bacteria for Araceae. The name Xanthomonas axonopodis pv. dieffenbachiae on the EPPO List should be adjusted to the names proposed in the taxonomic study by Constantin et al. (2016). The current EPPO Diagnostic Standard is directed at strains pathogenic on Anthurium. They mainly belong to X. phaseoli pv. dieffenbachiae, but some also to X. citri pv. aracearum that are not detected by the EPPO Diagnostic Standard. Xanthomonas phaseoli pv. syngonii strains are also aggressive, but with a host range restricted to Syngonium. The pathogenicity specific to aroids of the bacterial isolates reclassified as Xanthomonas euvesicatoria was not confirmed and no pathovar epithet has been retained for these strains.     

Development of a semi-selective medium for isolation of <Emphasis Type="Italic">Xanthomonas campestris</Emphasis> pv<Emphasis Type="Italic">. musacearum</Emphasis> from banana plants

Banana Xanthomonas wilt, caused by Xanthomonas campestris pv. musacearum, is a new threat to banana cultivation in eastern Africa. The causal bacterium grows slowly in culture and is easily overgrown by contaminants. A selective culture medium for isolation of X. c. pv. musacearum will facilitate disease study. A medium that suppressed saprophytic growth and possessed diagnostic characters for the pathogen was developed. Various carbon sources were tested with two isolates of X. c. pv. musacearum, and sucrose was selected as main carbon source. The susceptibility of X. c. pv. musacearum and other bacterial strains was tested with 29 different antibiotics. Cephalexin and cycloheximide had no effect on X. c. pv. musacearum but cephalexin inhibited most of the saprophytes and cycloheximide inhibited the fungal contaminants. Based on these studies, we have developed a semi-selective medium YTSA-CC containing yeast extract (1%), tryptone (1%), sucrose (1%), agar (1.5%), cephalexin (50 mg l⁻¹) and cycloheximide (150 mg l⁻¹), pH 7.0. The pathogen X. c. pv. musacearum was easily identified as yellowish, mucoid and circular colonies on YTSA-CC medium. This simple semi-selective medium was effective for isolation of X. c. pv. musacearum from infected banana tissues and soil, and it should be a valuable tool in ecological and epidemiological studies.     

Bacterial blight of rice and its control in Nepal

Research on Xanthomonas oryzae pv. oryzae, the bacterial blight of rice pathogen, was initiated at the Institute of Agriculture and Animal Science (IAAS) with the main objective of assessing the population structure of X. o. pv. oryzae through the use of both conventional and molecular markers in combination with virulence typing. A high DNA polymorphism was detected in the pathogen populations using different DNA probes and rep-PCR primers. Most strains were avirulent to cultivars containing the bacterial blight resistance gene Xa-21, which suggested the strategy that targets gene deployment is feasible in Nepal.     

红掌拮抗内生菌Y-54的分离、鉴定及其生防作用

为筛选具有广谱拮抗作用的内生菌,采用平板对峙法从健康红掌组织中分离和筛选对多种病原菌有拮抗作用的内生菌株,选择拮抗作用较好的菌株进行抑菌谱和拮抗作用测定,通过形态学特征、生理生化特性和分子生物学特征对其进行鉴定,利用盆栽试验测定其对红掌的促生作用和对红掌细菌性疫病的防效。结果显示,在健康红掌中共分离得到237株内生细菌,其中菌株Y-54的拮抗作用最强,其50倍发酵液对红掌细菌性疫病菌Xanthomonas axonopodis pv. dieffenbachiae的抑制作用最强,抑制率达37.78%;同时对多种病原真菌具有较强的抑制作用,其中对番茄叶霉病菌Cladosporium fulvum的抑制率达86.42%。结合形态学特征、生理生化特性和分子生物学特征将菌株Y-54鉴定为暹罗芽胞杆菌Bacillus siamensis。菌株Y-54的50倍发酵液可显著提高红掌叶长、叶宽和叶绿素相对含量(soil and plant analyzer development,SPAD)值,施药14 d和28 d后对红掌细菌性疫病的防效分别为62.94%和59.56%,与对照药剂的防效相当,表明菌...