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1.
ABSTRACT The presence of the European (EU) race of Gremmeniella abietina var. abietina, the causal agent of Scleroderris canker of conifers, was first reported in North America in 1975 in the northeastern United States and subsequently in southern Quebec and Newfoundland during the late 1970s, where it quickly became established. We analyzed DNA profiles in samples from a historic collection of G. abietina var. abietina that included some of the first isolates of the EU race reported in the United States to test hypotheses concerning the G. abietina var. abietina epidemic in North America. Genetic diversity was partitioned by an analysis of molecular variance with haplotype frequencies and distances. Genetic differentiation was high between populations in continental North America and Newfoundland (between region differentiation, Phi(ct) = 0.665, P < 0.001). This result was not consistent with the hypothesis of a single introduction of the pathogen into the northeastern United States followed by secondary spread into northeastern Canada. In contrast, small levels of genetic differentiation were observed among continental North American populations (Phi(ct) = 0.047, P = 0.079), suggesting gene flow among these populations. A single haplotype of G. abietina var. abietina dominated the continental populations (80% of the isolates) but was absent from Newfoundland and Europe. Five haplotypes were found in the New-foundland population, all of which were either absent or very rare on the continent. Populations from continental North America clustered together and were distinct from a second cluster composed of European and Newfoundland populations. A phylogenetic analysis of the haplotypes indicated that some of the rare haplotypes may have derived from somatic mutations, whereas others probably occurred as the result of new introductions. The results are consistent with a scenario of distinct primary introductions of this pathogen into Newfoundland and continental eastern North America followed by secondary asexual propagation.  相似文献   

2.
ABSTRACT In spring 2002, an unusual disease outburst was recorded on Norway spruce seedlings in southeast Norway. Extensive damage was recorded on 1- and 2-year-old Norway spruce seedlings that either had wintered in nursery cold storage or had been planted out in autumn 2001. The damage was characterized by leader shoot dieback and stem necroses on the upper or lower part of the shoot from 2001. Gremmeniella abietina and a Phomopsis sp. frequently were isolated from the diseased seedlings. Internal transcribed spacer (ITS) ribosomal (r)DNA sequence analysis and random amplified microsatellites profiling indicated that the G. abietina strains associated with diseased nursery seedlings belonged to the large-tree type (LTT) ecotype of the European race of G. abietina var. abietina, and inoculation tests confirmed their pathogenicity on Norway spruce. Based on ITS rDNA sequence analysis, the Phomopsis strains associated with diseased seedlings did not represent any characterized Phomopsis spp. associated with conifers. The Phomopsis sp. was not pathogenic in inoculation tests, indicating that it may be a secondary colonizer. ITS-based real-time polymerase chain reaction assays were developed in order to detect and quantify G. abietina and Phomopsis in the nursery stock. We describe here the G. abietina-associated shoot dieback symptoms on Norway spruce seedlings and conclude that the unusual disease outburst likely was related to the G. abietina var. abietina epidemic caused by the LTT on large Scots pines in 2001.  相似文献   

3.
The taxonomic placement of Spanish isolates of Gremmeniella abietina var. abietina (Gaa) (race and biotype) was determined, for the first time, using RAPD markers and comparison of RAPD patterns for Spanish isolates and those originating from different regions of Europe and North America. Fifteen Spanish isolates of Gaa were isolated from recently dead shoots of 10 Pinus halepensis trees. Additionally, 17 isolates of Gaa from Canada, Finland, Switzerland and the USA, previously characterized according to race and biotype, were used. Five primers, previously reported to yield readable, reproducible and polymorphic fragments, were used in PCR reactions. The results showed that the Spanish isolates represent the European race and not the Alpine or northern biotypes. Spanish isolates appear to be clearly separated from all other biotypes within the EU race and preliminary data suggest that Spanish isolates have low genetic variability.  相似文献   

4.
ABSTRACT The biochemical basis of tolerance in banana to Fusarium wilt, caused by the pathogen Fusarium oxysporum f. sp. cubense race four, was investigated. Tissue culture banana plants from tolerant cv. Goldfinger and susceptible cv. Williams were maintained in a hydroponic system and inoculated with conidial suspensions to evaluate the degree of tolerance to susceptibility between the two clones and to investigate the effectiveness of this technique as a potential tool for early screening for resistance in breeding programs. Similarly, defense responses were induced by treatment of the plants with an elicitor preparation from the mycelial cell walls of the pathogen. Differences in the induction of lignin and callose deposition, phenolics, and the enzymes involved in cell wall strengthening; phenylalanine ammonia lyase, cinnamyl alcohol dehydrogenase, peroxidase, and polyphenol oxidase were determined. Root tissue of the tolerant cv. Goldfinger responded to the fungal elicitor through the strong deposition of lignin, preceded by the induction or activation of the enzyme activities involved in the synthesis and polymerization thereof, whereas only slight increases were observed for the susceptible cv. Williams. No increase in callose content was observed for either clone. These results indicate an important role for cell wall strengthening due to the deposition of lignin as an inducible defense mechanism of banana roots against F. oxysporum f. sp. cubense race four.  相似文献   

5.
ABSTRACT Tomato (Lycopersicon esculentum) plants of 'Lorena' were induced with a conidial suspension (10(7) conidia per ml) of Penicillium oxalicum before inoculation with Fusarium oxysporum f. sp. lycopersici, the wilt pathogen. Histological changes occurred in plants under both growth chamber and glasshouse culture conditions and there was a reduction of disease severity. In noninduced plants, the pathogen produced almost a complete loss of cambium (75 to 100% reduction), an increase in the number of bundles, and a decrease in the number of xylem vessels (20% reduction), in which the diameter also was reduced by 20 to 30% in hypocotyls and epicotyls. The percentage of vessels colonized by F. oxysporum f. sp. lycopersici was positively correlated to the area under the disease progress curve (AUDPC). However, plants induced with P. oxalicum showed less disease, did not lose the cambium, had a lower number of bundles, and had less vascular colonization by F. oxysporum f. sp. lycopersici (35 to 99%). These effects also were observed in 'Precodor', which is susceptible to races 1 and 2 of F. oxysporum f. sp. lycopersici, and partially in 'Ramón', which is resistant to both races. Renewed or prolonged cambial activity that led to the formation of additional secondary xylem could be one of the reasons for disease reduction in P. oxalicum-induced tomato plants.  相似文献   

6.
ABSTRACT Agrobacterium vitis is the causal agent of crown gall disease in grapevine, which can be severe in many regions worldwide. Vitis vinifera cultivars are highly susceptible to freeze injury, providing the wounds necessary for infection by A. vitis. Wound position in relation to the uppermost bud of cuttings was determined to be important in tumor development. Inoculated wounds below buds developed tumors, whereas wounds opposite the bud did not, implying that indole-3-aectic acid flow contributes to tumor formation. If auxin was applied to wounds prior to inoculation with a tumorigenic A. vitis strain, all sites of inoculation developed tumors, accompanied by an increased amount of callus in the cambium. Wounds inoculated with an A. vitis biological control strain F2/5 prior to application of the pathogen did not develop galls. A closer examination of these wounds determined that callus cells formed in the cambium during wound healing are susceptible to transformation by the pathogen. Although the mechanism by which F2/5 prevents transformation is unknown, our observations suggest that F2/5 inhibits normal wound healing by inducing necrosis in the cambium.  相似文献   

7.
Pseudomonas syringae pv.pisi, pea bacterial blight agent, is seed-transmitted. Some aspects of its life cycle and its biology were investigated. The colonization of pea plants obtained from naturally infected seeds was studied in natural conditions while high populations of bacteria developed on plants showing no symptoms. Two streptomycin-resistant mutants were used to study the epiphytic life of the pathogen. Populations were monitored in different host-parasite compatibilities. When race 2 or race 6 of the pathogen was surface-inoculated on susceptible cultivars, a decrease of population size was observed during the following one to three days but was followed by an increase to levels 1000 times greater than the initial number detected, without symptoms for most of the plants. When race 2 was surface-inoculated on resistant genotypes or race 6 on non-host plants, bacteria did not multiply but population levels slightly decreased.Pseudomonas syringae pv.pisi shows a resident phase and its development is race-specific. Weeds collected in naturally contaminated pea fields, diseased or not, often harboured the pathogen but with levels smaller than those observed on peas. Pea crop debris and volunteers kept high levels of bacteria for at least eight months after the harvest of a diseased crop. As long as two pea crops are not grown one after the other in the same field, it is unlikely that debris and volunteers will act as an important inoculum source. The development of this pathogen during the growing season is considered as an important parameter to take into account for controlling the disease through seed health testing.  相似文献   

8.
Arbuscular mycorrhiza (AM) colonization led to a decrease in the severity of fusarium wilt disease caused by Fusarium oxysporum f. sp. lycopersici in tomato plants. The involvement of two plant defense hormones, namely methyl jasmonate (MeJA) and salicylic acid (SA), in the expression of mycorrhiza induced resistance (MIR) against this vascular pathogen was studied in the AM colonized and non-colonized (controls) plants. Activity of lipoxygenase (LOX), which plays a role in jasmonic acid (JA) biosynthesis, as well as levels of methyl jasmonate (MeJA) increased in AM colonized plants as compared to controls, but did not show any further changes in response to F. oxysporum inoculation. On the other hand, activity of phenylalanine ammonia lyase (PAL), which is an enzyme from salicylic acid (SA) biosynthetic pathway, as well as SA levels, increased in both controls and AM colonized plants in response to application of F. oxysporum spores. Hence the JA and not the SA signalling pathway appeared to play a role in the expression of MIR against this vascular pathogen. The resistance observed in AM colonized plants was completely compromised when plants were treated with the JA biosynthesis inhibitor salicylhydroxamic acid (SHAM). This confirmed that the AM-induced increase in JA levels was involved in the expression of resistance toward F. oxysporum. The SA response gene pathogenesis-related 1 (PR1) showed an increased expression in response to F. oxysporum infection in SHAM treated AM colonized plants as compared to plants that were not treated with this JA inhibitor. This suggested the possibility that JA inhibited SA responses, at least in the roots. AM colonization therefore appeared to prime plants for improved tolerance against the vascular pathogen F. oxysporum, which was mediated through the JA signalling pathway.  相似文献   

9.
Vascular plants have various inducible resistance mechanisms as defense against pathogens. Mosses, small nonvascular plants (subkingdom Bryophyta), have been little studied in regard to their pathogens or modes of defense. Data here show that Erwinia carotovora, a bacterial plant pathogen that causes softrot in many dicotyledonous plants, can also cause soft rot symptoms in the moss Physcomitrella patens. Infection of moss by E. carotovora required pathogenicity factors similar to those required to infect vascular plants and, again as in vascular plants, salicylic acid (SA) induced moss to inhibit tissue maceration by Erwinia. These data reveal that SA-dependent defense pathways may have evolved before differentiation of vascular and nonvascular plants.  相似文献   

10.
ABSTRACT In western North America, Douglas-fir (Pseudotsuga menziesii) is the most economically important conifer species susceptible to laminated root rot caused by Phellinus weirii. While attempting to internally sequence an endochitinase found to be up-regulated in P. weirii-infected Douglas-fir roots, we obtained overlapping peptide fragments showing 28% similarity with a PR-5 thaumatin-like protein (TLP) designated PmTLP (Pm for Pseudotsuga menziesi). A rabbit polyclonal antibody was reared against a synthetic peptide composed of a 29-amino-acid-long, conserved, internal sequence of PmTLP and purified by immunoaffinity. Western immunoblot analysis of infected roots of 24-year-old coastalfir showed significantly higher amounts of PmTLP (P < 0.01) closest to the point of P. weirii inoculation and infection than in uninfected regions of the same root. The antibody was also used to screen for PmTLP in roots of 25-year-old interior Douglas-firs naturally infected with a related pathogen, Armillaria ostoyae, and results showed significantly higher levels of PmTLP in bark tissues adjacent to infection (P < 0.05) than in uninfected tissue. Using polymerase chain reaction (PCR)-based cloning, the cDNA of PmTLP was shown to have a 702-bp open reading frame with a signal peptide cleavage site at 155 bp corresponding to a 29-amino-acid-long residue prior to the start of the N-terminal. Based on the deduced amino acid sequence, the molecular mass of the putative PmTLP was calculated to be 21.0 kDa with an isoelectric point of 3.71. Alignment analysis of PmTLP cDNA with a representative genomic DNA PCR sequence showed presence of one intron of variable size, within the coding region. The induction of PmTLP at the site of root infection and its presence in needle tissue suggests a general role for this protein in adaptation to stress and may be part of an integrated defense response initiated by the host to impede further pathogen spread.  相似文献   

11.
Simultaneous inoculation with races 1 and 2 of the vascular wilt pathogenFusarium oxysporumf.sp.lycopersiciprovided a high level of protection against race 2 in three tomato cultivars carrying resistance geneI, which confers resistance to race 1 but not race 2. However, simultaneous inoculation did not provide any protection in cultivars lacking this gene. Protection resulted in reduction and delay of wilt symptoms. Similarly, avirulent races ofF. oxysporumf.sp.melonisprotected muskmelon plants against virulent races of the sameforma specialis.A ratio 10:1 between spore concentrations of inducer and challenger organism gave the highest cross protection, but ratio 0.1:1 still provided significant disease reduction. Cross protection was also obtained when inoculation with the inducer organism was performed 6 or 12 h before inoculation with the challenger organism. Autoclaved spores of the inducer did not have any protective effect, indicating that living propagules were required to initiate protection. The results suggest the presence of a gene-for-gene interaction betweenF. oxysporumf.sp.lycopersici-tomato andF. oxysporumf.sp.melonis-muskmelon, in which cross protection against a virulent race is mediated by recognition of a specific elicitor from the avirulent race by the plant resistance gene product and by subsequent induction of the plant defense reaction.  相似文献   

12.
Fusarium oxysporum f. sp. melonis is an important vascular wilt pathogen of melon. Races 1, 2 and 1–2 of this fungus have been identified in Portugal by pathogenicity tests with appropriate hosts. The aim of this research was to examine the relationships between different races of F. o. melonis of Portuguese and French origin through analysis of random amplified polymorphic DNAs (RAPDs). DNA fingerprint profiles were developed for all the accessions. Each isolate showed 5–10 DNA bands with each of the 16 primers employed. A total of 126 bands was obtained. The size of amplified DNA fragments generated with these primers ranged from 0.5 to 3.2 kb. A phenogram based on the Jaccard coefficient of similarity was computed by the unweighed pair group method using arithmetic averages (UPGMA). It was found that Portuguese race 2 is very similar to French race 1, while French race 2 is the most dissimilar being clearly separated from all other races. The genetic diversity of these isolates is also being studied for vegetative compatibility by using the nit mutant system.  相似文献   

13.
 利用抑制差减杂交技术构建了条锈菌(Puccinia striiformis)诱导的小麦叶片cDNA文库。文库质量检测表明:差减杂交效率较高,质量较好。用DNAStar 5.0对172条高质量的ESTs进行聚类,共获得120个contigs。对功能已知的contigs分类,能量和初级代谢类占32.5%,其中与光合作用相关的基因出现频率最高;参与膜及转运、抗病与防御的基因分别位于第二、三位;次生代谢、蛋白质合成加工和细胞结构建成的基因较少。通过分析抗病相关基因,初步推测HR和SAR可能为小麦抗条锈病过程中的2种抗性形式。最后利用RT-PCR对4条感兴趣基因进行分析,明确其在抗病过程中的表达模式。  相似文献   

14.
The evolution of the genetic structure of a pathogen population was studied in a varietal mixture with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races able to develop on only one component of the mixture and a complex race which developed on all mixture components. The effects on the simple race–complex race competition of a cost of virulence, of density dependence and of differential adaptation were studied. The selection for simple or complex races in the pathogen population did not depend on initial race frequencies. For a given multiplication rate, complex race frequency increased faster when the spore dispersal gradient was shallow, when distribution of initial disease was generalized, when amount of initial disease was reduced and when the number of mixture components was increased. This was attributed to a better efficacy of the mixture in controlling simple races, resulting in a higher relative fitness of the complex race. For measured values of density dependence or differential adaptation effects, the complex race was at a higher frequency after a mean number of pathogen cycles between 2.5 and 5. The effect of the cost of virulence was stronger and, in certain situations, could result in selection for simple races. In the conditions of our simulations and with the effects tested, stabilization of the pathogen population in host mixtures was unlikely to occur. However, more information is needed concerning the rate at which complex races could evolve and how quickly mixture resistance could be eroded.  相似文献   

15.
ABSTRACT Genetic variation and variation in aggressiveness in Phialophora gregata f. sp. sojae, the cause of brown stem rot of soybean, was characterized in a sample of 209 isolates from the north-central region. The isolates were collected from soybean plants without regard to symptoms from randomly selected soybean fields. Seven genotypes (A1, A2, A4, A5, A6, M1, and M2) were distinguished based on DNA fingerprinting with microsatellite probes (CAT)(5) and (CAC)(5), with only minor genetic variation within the A or M genotypes. Only the A1, A2, and M1 genotypes were represented by more than one isolate. The A genotypes dominated in the eastern Iowa, Illinois, and Ohio samples, whereas the M genotypes were dominant in samples from western Iowa, Minnesota, and Missouri. In growth chamber experiments, isolates segregated into two pathogenicity groups based on their aggressiveness toward soybean cvs. Kenwood and BSR101, which are relatively susceptible and resistant, respectively, to brown stem rot. In both root dip inoculation and inoculation by injecting spores into the stem near the ground line (stab inoculations), isolates of the A genotypes caused greater foliar symptoms and more vascular discoloration than isolates of the M genotypes on both cultivars of soybean. All isolates caused foliar symptoms in both cultivars and in three additional cultivars of soybean with resistance to brown stem rot. Greater differences between the A and M genotypes were seen in foliar symptoms than in the linear extent of xylem discoloration, and greater differences were seen in Kenwood than in BSR101. Inoculation of these genotypes into five cultivars of soybean with different resistance genes to brown stem rot showed a genotype x cultivar interaction. A similar distinction was found in an earlier study of the adzuki bean pathogen, P. gregata f. sp. adzukicola, and consistent with the nomenclature of that pathogen, the soybean pathogens are named the aggressive race (race A) and the mild race (race M) of P. gregata f. sp. sojae.  相似文献   

16.
The ability of nonpathogenic Fusarium oxysporum, strain Fo47, to trigger plant defense reactions was investigated using Ri T-DNA-transformed pea roots. Cytological investigations of strain Fo47-inoculated roots showed that the fungus grew actively at the root surface and colonized a number of epidermal and cortical cells, inducing marked host cell metabolic changes. In roots inoculated with pathogenic F. oxysporum f. sp. pisi, the pathogen multiplied abundantly through much of the tissues, whereas in Fo47-inoculated roots, fungal growth was restricted to the epidermis and the outer cortex. Invading cells of strain Fo47 suffered from serious alterations, a phenomenon that was not observed in control roots in which F. oxysporum f. sp. pisi grew so actively that the vascular stele was invaded within a few days. Strain Fo47 establishment in the root tissues resulted in a massive elaboration of hemispherical wall appositions and in the deposition of an electron-opaque material frequently encircling pathogen hyphae and accumulating in the noninfected xylem vessels. This suggests that the host roots were signaled to defend themselves through the rapid stimulation of a general cascade of nonspecific defense responses. The specific relationship established between strain Fo47 and the root tissues is discussed in relation to other types of plant-fungus interactions, including pathogenic and symbiotic associations.  相似文献   

17.
Potato is one of the most important noncereal crops in the world today, and like other major crops, it is prone to substantial yield losses because of various factors including disease. Recent molecular advancements in plant–pathogen studies have led to the identification of various host genes involved in the plant’s defense against pathogen attack. These genes may encode antimicrobial peptides, enzymes for phytoalexin production, proteins involved in defense-signaling cascades, and hydrolytic enzymes or pathogenesis-related proteins that are directly or indirectly responsible for the plant’s defense responses following a pathogen attack. A plant’s disease-resistance (R) genes are another important group of genes that have been used with varying degrees of success in crop improvement programs. Cloning and characterization of these genes and the dissection of signal-transduction components of the defense response have greatly increased the scope for transgenic disease resistance. This article highlights the current scenario and potential of the molecular approaches to improve resistance against filamentous pathogens in potato.  相似文献   

18.
Wang C  Roberts PA 《Phytopathology》2006,96(7):727-734
ABSTRACT Fusarium wilt, caused by the soilborne pathogen Fusarium oxysporum f. sp. vasinfectum race 1, is a vascular disease in cotton (Gossypium spp.), and is a component of a disease complex with root-knot nematodes (Meloidogyne incognita). Genetic analysis of two interspecific crosses (G. barbadense Pima S-7 x G. hirsutum Acala NemX and Pima S-7 x Acala SJ-2) showed that one major gene (designated Fov1) with allele dosage effect conferred resistance to F. oxysporum f. sp. vasinfectum race 1 in Pima S-7. Two amplified fragment length polymorphism (AFLP) markers were linked to Fov1 in Pima S-7, with genetic distance from the gene of 9.3 and 14.6 centimorgans. Less severe wilt symptoms in Acala NemX than Acala SJ-2 indicated that Acala NemX possesses one or more minor genes contributing to delay of wilt symptoms. Highly resistant plants in F(2) and F(3) (Pima S-7 x NemX) families indicated transgressive segregation effects of minor genes in Acala NemX combined with Fov1 from Pima S-7. The effects of wilt and nematode resistance on the nematode-wilt disease complex were assayed with two inoculation methods. In the presence of both pathogens, wilt damage measured as shoot and root weight reductions was greatest on wilt- and nematode-susceptible Acala SJ-2 and least in root-knot nematode-resistant and wilt-susceptible Acala NemX. Intermediate damage occurred in wilt-resistant and root-knot nematode-susceptible Pima S-7. The results indicated that nematode resistance was more effective than wilt resistance in suppressing wilt symptoms when either resistance was present alone. Nematode resistance combined with intermediate wilt resistance, as in the F(1) (Pima S-7 x NemX), was highly effective in protecting plants from root-knot nematodes and race 1 of Fusarium wilt as a disease complex.  相似文献   

19.
Plants recognize conserved molecular structures from microorganisms, which triggers active immune responses. Successful pathogens have to overcome this level of immunity; however, plants in turn can adapt their immune system, thus plants and pathogens are in an evolutionary arms race. As being sessile organisms, plants need to integrate and adapt to changing environmental conditions such as light, temperature, drought, or microorganisms. Plants protect themselves against diseases through sensitive recognition of potential pathogens and effective defense systems. The first level of the plant immune system provides recognition of a broad spectrum of microorganisms leading to defense activation (Bittel and Robatzek 2007). The second level of the plant immunity allows certain plant cultivars to detect of specific pathogen strains??a phenomenon also referred to as ??gene-for-gene resistance?? (Jones and Dangl 2006). The first level of immunity occurs rapidly and triggers active defenses normally without harm to the plant cell. The second level of plant immunity develops over days and deploys a local cell death, which prevents pathogens from further spread into tissues. In addition to these cell-autonomous defense systems, plants have also evolved strategies of systemic immunity.  相似文献   

20.
A total of 25 ornamental plant species representing 10 families were inoculated using three genotypes, each representing one of the genetic lineages NA1, NA2, and EU1 of the pathogen Phytophthora ramorum. Leaves were inoculated using suspensions with two zoospore concentrations and exposure at three temperatures, while stems were inoculated using agar plugs colonized by mycelia. Susceptibility was determined by measuring either the success of pathogen reisolation or lesion length caused by the pathogen. Infectivity was determined by counting sporangia in washes of inoculated leaves or stems. Results from all three pathogen genotypes combined were used to rank each of the 25 plant species for susceptibility and infectivity, while pooled results per genotype from all 25 hosts combined were employed for a preliminary comparison of pathogenicity and infectivity among genotypes. Statistical analyses showed that leaf results were affected by the concentration of zoospores, temperature, plant host, pathogen genotype, and by the interaction between host and pathogen genotype. Stem results were mostly affected by host and by the interaction between host and pathogen genotype. Hosts ranked differently when looking at the various parameters, and differences in rankings were also significant when comparing stem and leaf results. Differences were identified among the 25 hosts and the three pathogen genotypes for all parameters: results can be used for decision-making regarding regulations or selection of plants to be grown where infestations by P. ramorum are an issue.  相似文献   

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