Internal fruit rot of netted melon caused by <Emphasis Type="Italic">Pantoea ananatis</Emphasis> (=<Emphasis Type="Italic">Erwinia ananas</Emphasis>) in Japan

An internal fruit rot with a malodor was found in netted melons (Cucumis melo L.) in commercial greenhouses in Kochi Prefecture, Japan, in 1998, despite their healthy appearance and lack of water-soaking or brown spots on the surface. A yellow bacterium was consistently isolated from the affected fruits. To confirm the pathogenicity of eight representative isolates of the yellow bacterium, we stub-inoculated ovaries (immature-fruits) 5–7 days after artificial pollination, with a pin smeared with bacteria. After the melon fruits had grown for 60 more days, an internal fruit rot resembling the natural infection appeared, and the inoculated bacterium was reisolated. The melon isolates had properties identical with Pantoea ananatis, such as gram-negative staining, facultative anaerobic growth, indole production, phenylalanine deaminase absence, and acid production from melibiose, sorbitol, glycerol, and inositol. Phylogenetic analysis based on 16S rDNA sequences showed that the melon bacterium positioned closely with known P. ananatis strains. The melon bacterium had indole acetic acid (IAA) biosynthesis genes (iaaM and iaaH) and a cytokinin biosynthesis gene (etz). The bacterium could be distinguished from the other ‘Pantoea’ group strains by rep-PCR genomic fingerprinting. From these results, the causal agent of internal fruit rot was identified as a strain of P.ananatis [Serrano in (Philipp J Sci 36:271–305, 1928); Mergaert et al. in (Int J Syst Bacteriol 43:162–173, 1993)]. The nucleotide sequence data reported are available in the DDBJ database under accessions AB297969, AB373739, AB373740, AB373741, AB373742, AB373743 and AB373744.     

Virus transmission by host-specific strains ofOlpidium bornovanus andOlpidium brassicae

Zoospores of 12 isolatesO. bornovanus from geographically diverse sites and representing the three host specific cucurbit strains were tested as vectors for seven viruses using watermelon bait plants and the in vitro acquisition method. All isolates of the cucumber, melon, and squash strains transmitted melon necrotic spot carmovirus (MNSV) and cucumber necrosis tombusvirus (CNV) but none transmitted petunia asteroid mosaic tombusvirus (PAMV) or tobacco necrosis necrovirus (TNV). The isolates varied as vectors of three other carmoviruses: cucumber leaf spot virus (CLSV); cucumber soil borne virus (CSBV); and squash necrosis virus (SqNV). All cucumber isolates transmitted CLSV and SqNV but not CSBV. Some of the melon isolates transmitted CLSV and SqNV but none transmitted CSBV. Two squash isolates transmitted CSBV and SqNV but not CLSV. Two isolates ofO. brassicae transmitted only TNV and a third did not transmit any of the viruses. The species of bait plant sometimes affected transmission. The most efficient vector strains ofO. bornovanus, as determined by reducing zoospores and virus in the inoculum, were the cucumber strain for CLSV; the cucumber strain for CNV if cucumber was the bait plant or melon strain if watermelon was the bait plant; and the squash strain for SqNV. The plurivorous strain ofO. brassicae was the most efficient vector of TNV.Olpidium bornovanus is the first vector reported for CSBV and is confirmed as a vector of SqNV. It is proposed that all carmoviruses may have fungal vectors.Ligniera sp. did not transmit any of the viruses in one attempt.Abbreviations CLSV cucumber leaf spot virus - CNV cucumber necrosis virus - CSBV cucumber soil borne virus - MNSV melon necrotic spot virus - PAMV petunia asteroid mosaic virus - SqNV squash necrosis virus - TNV tobacco necrosis virus - TBSV tomato bushy stunt virus     

Phenotypic diversity,host range and molecular phylogeny of <Emphasis Type="Italic">Dickeya</Emphasis> isolates from Spain

Six Dickeya spp. strains representative of a larger group of bacteria isolated from potato, onion and irrigation water in Spain between years 2003–2005, were characterised by biochemical, serological, molecular and pathogenicity assays. Biochemical and serological differences, as well as pathogenic behaviour in host range and virulence levels, were observed among the strains. They were classified into biovars 3 and 6. Phylogenetic analysis and comparison of the isolates with type strains of Dickeya species characterised to date were performed using concatenated partial sequences of the housekeeping genes gapA and mdh. One of the Spanish strains was identified as D. dieffenbachiae, whereas the other ones did not fit clearly into the previously described six Dickeya species, and may therefore constitute novel species. Isolation of dissimilar pathogenic strains in different rivers and irrigation water sources supports the idea that Dickeya species is commonly present in such an environment, and contaminated water is a potential source of inoculum for the disease in different crops.     

Characterization of Acidovorax citrulli strains isolated from solanaceous plants

Acidovorax citrulli is the causal agent of bacterial fruit blotch disease of cucurbits. Strains of this pathogen are distributed into two major groups: Group I strains have been mainly isolated from melon and other non-watermelon cucurbits, while Group II strains have been mainly recovered from watermelon. Here we report the characterization of strains T1 and EP isolated from diseased tomato and eggplant plants, respectively, and further confirmed to belong to A. citrulli species. Based on PCR, PFGE, and rep-PCR, these strains showed high similarity to the Group II strain 7a1. Sequencing and comparative analyses revealed that the genomes of T1 and EP aligned with that of the Group II model strain AAC00-1, over 97.88% and 99.22%, respectively. The virulence of T1, EP, and 7a1 determined on tomato, eggplant, and watermelon was similar and significantly higher than that of Group I strain M6. In contrast, M6 was more virulent on melon. Expression levels of seven virulence genes measured 24 hr after inoculation of tomato, eggplant, watermelon, and melon showed that the expression pattern was generally similar in strains 7a1, T1, and EP, whereas for M6 the expression was high only on melon. Overall, our results indicate that the solanaceous strains belong to Group II. To the best of our knowledge, this is the first study that reports characterization of A. citrulli strains isolated from solanaceous species. The fact that A. citrulli is able to naturally colonize and cause disease in non-cucurbit crops poses additional challenges for management of this important pathogen.     

Genetic diversity and pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> isolated from wilted rocket plants in Italy

Thirty-six isolates of Fusarium oxysporum originated from Eruca vesicaria and Diplotaxis tenuifolia together with eight reference strains belonging to the formae speciales raphani, matthioli and conglutinans, typical on the Brassicaceae family, were tested for pathogenicity on two species of rocket plants (E. vesicaria L., syn. E. sativa, cv. ‘Rucola coltivata’; and D. tenuifolia cv. ‘Winter’) cultivated in the glasshouse. The results showed that different isolates were slightly, moderately or highly virulent. The strains were examined for differences in the nucleotide sequence of the ribosomal DNA (rDNA) intergenic spacer (IGS) region, about 2.5 kb long. The phylogenetic (neighbor-joining) analysis performed on the isolates enabled identification of four different groups, named I, II, III and IV. Thirty-one isolates out of 36 clustered in group I and were genetically similar to F. oxysporum f.sp. raphani. By considering the pathogenicity of the strains included in Group I, a partial host specialization could be observed: the average disease index of the isolates from D. tenuifolia was higher on wild rocket, whereas the average disease index of the isolates from E. vesicaria was higher on cultivated rocket. Moreover, isolates from cultivated rocket showed, on average, a higher degree of aggressiveness than the isolates from wild rocket. Concerning Group I, the sequence analysis confirmed the homogeneity of the population, with only five parsimony-informative SNPs and five haplotypes. Twenty-six out of 31 isolates belonged to haplotype 1. Groups II and III were genetically similar to strains of F. oxysporum f.sp. matthioli. Three other strains, not pathogenic or with a medium level of virulence, clustered together in Group 4, but their sequence was distant from that of other formae speciales. The pathogenicity and IGS analysis confirmed the presence of virulence variation and genetic diversity among the F. oxysporum isolates studied. To our knowledge, this is the first report of differentiation of formae speciales of F. oxysporum on rocket plants by IGS analysis.     

Relationships between aflatoxin production and sclerotia formation among isolates of <Emphasis Type="Italic">Aspergillus</Emphasis> section <Emphasis Type="Italic">Flavi</Emphasis> from the Mississippi Delta

Aspergillus section Flavi isolates, predominately A. flavus, from different crops and soils differed significantly in production of aflatoxin and sclerotia. About 50% of the isolates from corn, soil and peanut produced large sclerotia, while only 20% of the rice isolates produced large sclerotia. There was a higher frequency of small sclerotia-producing isolates from rice compared to the other sources and isolates that did not produce sclerotia were significantly less likely to be toxigenic than strains that produced large sclerotia.     

Suppressive effect of bacteriophage on bacterial palea browning of rice caused by Pantoea ananatis

Bacterial palea browning of rice, caused by Pantoea ananatis from infection during flowering, occurs widely in Japan and degrades the quality of rice. In a search for environmentally friendly control measures, effects of bacteriophages on the incidence of the disease were examined. Phages lytic to both pathogenic and nonpathogenic P. ananatis were isolated from an inflorescence of eulalia (Miscanthus sinensis), a gramineous weed, and one of the phages was sprayed with and without a nonpathogenic isolate of P. ananatis on rice plants at the flowering stage. Coapplication of the phage and nonpathogenic P. ananatis suppressed the disease in sunlight. Surprisingly, application of the light-labile phage by itself was suppressive. The phage retarded the growth of the pathogen on rice plants and on LB medium. Because nonpathogenic Pantoea strains are abundant on rice panicles at the flowering stage and could be hosts of the phage and the optimum infection period of rice with P. ananatis is during the flowering stage, disease suppression by the phage is thought to be due to the combined effects of the phage, naturally inhabiting nonpathogenic bacteria, and the limited susceptible period.     

Pathogenic variation and molecular characterization of <Emphasis Type="Italic">Fusarium</Emphasis> species isolated from wilted Welsh onion in Japan

Thirty-two isolates of Fusarium species were obtained from wilted Welsh onion (Allium fistulosum) grown on nine farms from six regions in Japan and identified as F. oxysporum (18 isolates), F. verticillioides (7 isolates), and F. solani (7 isolates). The pathogenicity of 32 isolates was tested on five commercial cultivars of Welsh onion and two cultivars of bulb onion in a seedling assay in a greenhouse. The Fusarium isolates varied in the degree of disease severity on the cultivars. Five F. oxysporum isolates (08, 15, 17, 22, and 30) had a higher virulence on the cultivars than the other isolates. The host range of these five isolates was limited to Allium species. Molecular characterization of Fusarium isolates was performed using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of the internal transcribed spacer (ITS) regions of ribosomal DNA. The 32 isolates were grouped into eight types (four types for F. oxysporum, one for F. verticillioides, and three for F. solani). Restriction patterns of the ITS region were not related to pathogenicity. However, the haplotypes obtained with five enzymes (RsaI, HinfI, HaeIII, ScrFI, and MspI) and the phylogenetic analysis permitted the discernment of the three Fusarium species. The PCR-RFLP analysis should provide a rapid, simple method for differentiating Fusaruim species isolated from wilted Welsh onion in Japan.     

Assessing the phenotypic and genotypic diversity of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in France

White mould caused by the ascomycete Sclerotinia sclerotiorum affects the production of many economically important crops. The incidence of this disease has recently increased in France, especially in melon crops, which were not affected much in the past. One possible explanation for this situation is the emergence of strains with particular characteristics, including increased aggressiveness to melon. To test this hypothesis, 200 isolates of S. sclerotiorum were collected from six host crops (bean, brassica oilseed rape, carrot, lettuce, melon, witloof chicory) in different regions. They were genotyped with 16 microsatellites markers. A subsample of 96 isolates were assessed for their aggressiveness on melon leaves. Overall, the isolates from melon did not show higher aggressiveness on melon leaves than those which originated from other host plants. Moreover, the melon isolates did not present distinctive genetic characteristics in comparison with those from other crops and shared several of the 128 identified multilocus haplotypes with isolates collected from carrot, witloof chicory and oilseed rape. Furthermore the Bayesian analysis of the genetic structure indicated that the host plant is not a structuring factor of the three genetic clusters identified, and it suggested instead the occurrence of an isolation-by-distance process. Possible consequences of these results for the management of white mould and alternative hypotheses to explain the recent changes in disease incidence are presented.     

Pathogenicity and mycotoxin production by <Emphasis Type="Italic">Fusarium proliferatum</Emphasis> isolated from onion and garlic in Serbia

Fusarium proliferatum can occur on a wide range of economically important vegetable plants but its role in disease is not always well established. In 2000 and 2001, from forty-one field samples of wilting onion and garlic plants in Serbia, F. proliferatum as the predominant fungal species was isolated from root and bulbs. Seventy isolates were firstly characterized for their sexual fertility and were shown to be mostly members of Gibberella intermedia (sixty-seven of seventy isolates, the remaining three isolates were unfertile), the sexual stage of F. proliferatum (syn. mating population D of G. fujikuroi complex). A selected set of eleven F. proliferatum isolates from both hosts were also tested for their pathogenicity and toxigenicity. Although onion and garlic plants were susceptible to all isolates, onion plants showed a significantly higher disease severity index. Six of the eleven isolates of F. proliferatum produced fumonisin B₁ from 25 to 3000 μg g⁻¹, and beauvericin from 400 to 550 μg g⁻¹; ten isolates produced fusaric acid from 80 to 950 μg g⁻¹ and moniliformin from 50 to 520 μg g⁻¹. Finally, all isolates produced fusaproliferin up to 400 μg g⁻¹. These results confirm F. proliferatum as an important pathogen of garlic and onion in Europe and that there is a potential mycotoxin accumulation risk in contaminated plants of both garlic and onion.     

Virulence and pCM1 plasmid carriage are related to BOX-PCR fingerprint type in strains of Clavibacter michiganensis subsp. michiganensis that cause bacterial wilt and canker of tomato in Argentina

Clavibacter michiganensis subsp. michiganensis (Cmm) causes bacterial wilt and canker in tomato, producing important economic losses worldwide. Its virulence has been related to several putative virulence factors present on a chromosomal pathogenicity island and on plasmids pCM1 and pCM2, in strain NCPPB382. We genotypically characterized a collection of Cmm isolates from the main greenhouse tomato-producing areas of Argentina by BOX-PCR fingerprinting and screened for the presence of genes and plasmids involved in pathogenicity by PCR. In addition, we evaluated in vitro cellulolytic activity and virulence in planta of selected strains. BOX-PCR fingerprinting clustered strains into four groups. Group II was dominant and included the most virulent strains, while Group III was the smallest and had the least virulent strains. All local strains exhibited similar cellulolytic activity. Most of the examined strains carry two plasmids of similar size to those of NCPPB382, although there were strains with one or three plasmids. By PCR amplification of repA, pCM1 was detected only in strains belonging to Group III, which includes local strains closely related to reference strain NCPPB382. All analysed pathogenicity genes were widespread among strains, and so in strains belonging to Groups I and II, celA found on pCM1 in NCPPB382 could be found in the chromosome or in plasmids other than pCM1. This study contributes to a better understanding of the diversity of Cmm genetic profiles and virulence of strains present in Argentina. Such information could be useful for the selection of strains for screening of host resistance and development of resistant tomato varieties.     

A novel type of Potato virus Y recombinant genome,determined for the genetic strain PVYE

Two Potato virus Y (PVY) isolates collected in Brazil, PVY‐AGA and PVY‐MON, were identified as recombinants between two parent genomes, PVY^NTN and PVY‐NE‐11, with a novel type of genomic pattern. The new recombinants had an ordinary PVY^NTN genome structure for approximately 6·7‐kb from the 5′‐end of the genome whereas the 3′‐terminal 3·0‐kb segment had two fragments of NE‐11‐like sequence separated by another small PVY^NTN‐like fragment. PVY strains are defined based on the hypersensitive resistance (HR) response in potato indicators. Both PVY‐AGA and PVY‐MON isolates did not induce the HR in potato cultivars carrying Ny, Nc, or (putative) Nz genes and thus were able to overcome all known resistance genes to PVY. Only one of the two isolates, PVY‐AGA, induced a vein necrosis reaction in tobacco. The biological responses of the potato indicators and tobacco defined PVY‐MON as an isolate of the PVY^E strain. To distinguish PVY‐AGA and PVY‐MON from other PVY^NTN isolates, an RT‐PCR test was developed utilizing new specific primers from the capsid protein gene area and producing a characteristic 955‐bp band. Serological profiling of these PVY isolates with three monoclonal antibodies revealed an unusual reactivity, where one of the two commercial PVY^N‐specific monoclonal antibodies did not recognize PVY‐AGA. The ability of these new PVY recombinants to overcome resistance genes in potato producing mild or no symptoms, combined with the lack of serological reactivity towards at least one PVY^N‐specific antibody may present a significant threat posed by these isolates to seed potato production areas.     

Distribution of avirulence genes <Emphasis Type="Italic">avrA</Emphasis> and <Emphasis Type="Italic">popP1</Emphasis> in 22 Japanese phylotype I strains of <Emphasis Type="Italic">Ralstonia solanacearum</Emphasis>

Sequence analysis of hrp loci and effector genes in the flanking regions showed significantly high similarities between two phylotype I strains of Ralstonia solanacearum, GMI1000 and Japanese strain OE1-1. Further sequence analysis of the distribution of avrA and popP1, known as determinants of a hypersensitive response (HR) induction on Nicotiana tabacum (tobacco), in 22 Japanese phylotype I strains revealed that all strains had one of the two distinct avrA alleles and that 10 strains had an identical popP1 but the other 12 did not. After infiltration of tobacco leaves, more than half of these 22 strains elicited HR. In combination with the ability to induce HR, avrA and popP1 are thus not likely to be the sole determinants of HR in Japanese phylotype I strains.     

Effect of Figaren, an Antiviral Protein from Cucumis figarei, on Cucumber mosaic virus Infection

Local lesion formation on cowpea leaves was more than 50% inhibited by treatment with a 23 kDa RNase-like glycoprotein from Cucumis figarei, figaren, from 24 hr before to 1 hr after inoculation with Cucumber mosaic virus (CMV). CMV accumulation detected by ELISA in tobacco leaves treated with figaren 6 or 0 hr before inoculation with CMV was suppressed. When upper leaves of tobacco plants were treated with figaren and inoculated 10 min later with CMV, mosaic symptoms were delayed for 5–7 days on most of the tobacco plants, and some plants remained asymptomatic. From fluorescence in situ hybridization, infection sites were present in figaren-treated cowpea or melon leaves after inoculation with CMV, though the sites were reduced in number and size compared with those in water-treated control leaves. The amount of CMV RNAs and CMV antigen in melon protoplasts inoculated with CMV and subsequently incubated with figaren similarly increased with time as did that in the control. ELISA and local lesion assays indicated that CMV infection on the upper surfaces of the leaves of tobacco, melon, cowpea and C. amaranticolor whose lower surfaces had been treated with figaren 5–10 min before CMV inoculation was almost completely inhibited. Figaren did not inhibit CMV infection on the opposite untreated leaf halves of melon, cowpea and C. amaranticolor, whereas it almost completely inhibited CMV infection on the untreated halves of leaves of tobacco. CMV infection was not inhibited in the untreated upper or lower leaves of the four plants. These data suggest that figaren does not completely prevent CMV invasion but does inhibit the initial infection processes. It may also induce localized acquired resistance in host plants. Received 10 October 2000/ Accepted in revised form 6 February 2001     

Genetic characterisation of <Emphasis Type="Italic">Pectobacterium wasabiae</Emphasis> causing soft rot disease of potato in New Zealand

Pectobacterium wasabiae has a narrow host range, having previously only been associated with Japanese horseradish. However, recent characterisation of Pectobacterium causing soft rotting in New Zealand has identified putative P. wasabiae isolates pathogenic to potato. In this study, phylogenetic reconstruction of acnA and mdh DNA sequences and fluorescent amplified fragment length polymorphisms (fAFLP) were used to confirm the identity of the putative P. wasabiae isolates. Both methods clustered the potato isolates closely with the type strain for P. wasabiae, ICMP9121, and also differentiated them from other plant pathogenic enterobacteria. PCR, DNA hybridisation and hypersensitive response (HR) assays were subsequently used to investigate the presence in P. wasabiae of the type III secretion system (T3SS) as well as other virulence factors known to be involved in development of disease by enterobacteria. Although all P. wasabiae strains appeared to elicit a type III-dependent HR in tobacco, genes associated with the T3SS and the putative virulence factors HecB and DspE could not be detected. Thus, genetic characterisation of P. wasabiae confirmed that it is a naturally occurring pathogen on potato, which does not possess the same suite of virulence factors that are involved in the pathogenicity of other enterobacteria on this host.     

非亲和性稻瘟病菌对水稻的诱导抗性

为研究稻瘟病菌Magnaporthe oryzae不同菌株间的相互作用,选择与单抗性基因系水稻IRBL5-M （携带抗性基因Pi5）表现为亲和性的菌株HN52与非亲和性的菌株HN119为研究对象,将其单独或混合接种到单抗性基因系水稻IRBL5-M中,并通过荧光显微镜观察接种后水稻叶鞘的发病情况及病斑面积,测定接种后水稻内相关抗性基因OsWRKY45、OsNPR1、OsPR10、OsMAPK2的表达量以及活性氧的变化。结果显示,相较于单独接种亲和性菌株,混合接种后单抗性基因系水稻IRBL5-M病斑发病面积减少;混合接种中亲和性菌株HN52菌丝侵染能力降低,侵染菌丝细胞间扩展率显著降低73.13%;同时单抗性基因系水稻IRBL5-M中OsWRKY45、OsNPR1、OsPR10和OsMAPK2抗性基因表达量显著增加,水稻叶片中活性氧含量增加,表明在菌株混合侵染过程中,非亲和性菌株可通过激发水稻的抗性反应来降低亲和性菌株对水稻的侵染程度。     

Pathogenicity of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis> from different hosts causing brown spot in pear

Stemphylium vesicarium (teleomorph: Pleospora herbarum) is the causal agent of brown spot disease in pear. The species is also able to cause disease in asparagus, onion and other crops. Saprophytic growth of the fungus on plant debris is common. The objective of this study was to investigate whether isolates of S. vesicarium from different hosts can be pathogenic to pear. More than hundred isolates of Stemphylium spp. were obtained from infected pear fruits, dead pear leaves, dead grass leaves present in pear orchard lawns as well as from necrotic leaf parts of asparagus and onion. Only isolates originating from pear orchards, including isolates from dead grass leaves, were pathogenic on pear leaves or fruits in bioassays. Non-pathogenic isolates were also present in pear orchards. Stemphylium vesicarium from asparagus or onion, with one exception, were not pathogenic to pear. Analysis of the genetic variation between isolates using Amplified Fragment Length Polymorphism (AFLP) showed significant concordance with host plants. Isolates from asparagus or onion belonged to clusters separate from the cluster with isolates from pear or grass leaves collected in pear orchards. Multilocus sequencing of a subset of isolates showed that such isolates were similar to S. vesicarium.     

Association of <Emphasis Type="Italic">Pantoea ananatis and Pantoea agglomerans</Emphasis> with leaf spot disease on ornamental plants of Araceae Family

During a survey in 2011–2012, three ornamental plants of Araceae namely Aglaonema nitidum, Syngonium podophyllum and Dieffenbachia amoena showing foliar disease symptoms were collected from central region of Iran. Infected plants exhibited spots on their leaves which appeared as yellow and water-soaked with chlorotic haloes and necrotic center. To investigate the etiology of this disorder, symptomatic leaves were collected from affected plants and six bacterial strains (B2Y, J3Y, SY, E60Y, E68Y and E5MM) were isolated and identified as Pantoea ananatis or P. agglomerans based on morphological, physiological, biochemical and molecular characters. The pathogenicity tests of the isolates demonstrated that they were not host specific. Furthermore, 16S rRNA gene sequencing revealed that the strains were phylogenetically closely related to genus Pantoea. Multilocus sequence analysis (MLSA) of concatenated partial atpD, gyrB and rpoB gene sequences of the six isolates showed a high similarity of B2Y, J3Y, and SY strains to P. ananatis and also of E60Y, E5MM and E68Y strains to P. agglomerans. These results were confirmed by phylogenetic analysis. To the best of our knowledge, this is the first report of leaf spot and necrosis of A. nitidum, S. podophyllum and D. amoena caused by the genus Pantoea.     

大葱水提物对辣椒疫病的控制作用及其活性成分分析

为明确大葱水提物对辣椒疫病的控制效果及其主要活性成分,通过室内毒力、田间防效以及GC-MS联用测定发现,大葱茎叶与根水提物均对辣椒疫病菌有一定抑制作用,且浓度越高抑制作用越强。在含300 mg/mL根水提物的平板上培养2 d,辣椒疫病菌菌丝生长的抑制率高达81.50%;但随着时间的推移,抑制作用减弱,5 d后,200 mg/mL根和茎叶水提物对病菌的抑制率仅为34.18%和25.62%。用300 mg/mL大葱水提物灌根,对辣椒疫病的防效可达50%左右。将大葱与辣椒轮作或混栽可有效降低田间辣椒疫病的病情,防病效果分别为40.67%和41.21%。GC-MS测定结果表明,大葱根和茎叶水提物中分别含有14种和28种挥发性物质,且均以有机硫化物为主,分别占总挥发性物质的82.17%和99.40%,这些硫化物对辣椒疫病菌的生长均有较强的抑制作用。这些结果表明,大葱产生的挥发性物质在辣椒疫病的绿色防控上有很好的应用前景。     

Reaction of <Emphasis Type="Italic">Leersia</Emphasis> grasses to <Emphasis Type="Italic">Xanthomonas oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzae</Emphasis> collected from Japan and Asian countries

The present study was conducted to determine if there is specificity in the host-pathogen relationship between the isolates of Xanthomonas oryzae pv. oryzae, the causal bacterium for rice blight and Leersia grasses, the alternative weed hosts of the disease. Plants of three species of Leersia, namely, L. sayanuka, L. oryzoides and L. japonica, were collected from various parts of Japan and were inoculated with the X. oryzae pv. oryzae isolates obtained from various locations in Japan and from 11 Asian countries. Four L. sayanuka plants were found susceptible to all Race II isolates and some Race I isolates, but were resistant to all Race III isolates. Race III is known to have a wider range pathogenicity to rice cultivar groups compared with Race I and II. Although the reactions of two L. oryzoides plants to Race I and II isolates were similar to that of L. sayanuka, the L. oryzoides plant collected from Niigata Prefecture showed a susceptible reaction to some Race III isolates. On the other hand, L. japonica plants gave reactions different those of L. sayanuka and L. oryzoides, with two plants of L. japonica found to be resistant to all test isolates collected from Japan. The Asian isolates exhibited a wide host range against the international differential rice cultivars, but almost all of them were avirulent to Leersia plants. These results indicate that the relationship between the pathogenicity of the causal bacterium and the resistance of host plants is very complex, and suggest that pathogenic diversity of X. oryzae pv. oryzae might be related to the resistance of Leersia spp.