Seed transmission of<Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f.sp.<Emphasis Type="Italic">lactucae</Emphasis>

Twenty-seven seed samples belonging to the lettuce cultivars most frequently grown in Lombardy (northwestern Italy), in an area severely affected by Fusarium wilt of lettuce, were assayed for the presence ofFusarium oxysporum on a Fusarium-selective medium. Isolations were carried out on subsamples of seeds (500 to 1500) belonging to the same seed lots used for sowing, and either unwashed or disinfected in 1% sodium hypochloride. The pathogenicity of the isolates ofF. oxysporum obtained was tested in four trials carried out on lettuce cultivars of the butterhead type, very susceptible to Fusarium wilt. Nine of the 27 samples of seeds obtained from commercial seed lots used for sowing in fields affected by Fusarium wilt were contaminated byF. oxysporum. Among the 16 isolates ofF. oxysporum obtained, only one was isolated from disinfected seeds. Three of the isolates were pathogenic on the tested cultivars of lettuce, exhibiting a level of pathogenicity similar to that of the isolates ofF. oxysporum f.sp.lactucae obtained from infected wilted plants in Italy, USA and Taiwan, used as comparison. The results obtained indicate that lettuce seeds are a potential source of inoculum for Fusarium wilt of lettuce. The possibility of isolatingF. oxysporum f.sp.lactucae, although from a low percent of seeds, supports the hypothesis that the rapid spread of Fusarium wilt of lettuce observed recently in Italy is due to the use of infected propagation material. Measures for prevention and control of the disease are discussed. http://www.phytoparasitica.org posting Dec. 16, 2003.     

Characterization of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melongenae</Emphasis> isolates from eggplant in Turkey by pathogenicity,VCG and RAPD analysis

Fusarium wilt is an economically important fungal disease of common eggplant (Solanum melongena) cultivated in the eastern Mediterranean region of Turkey. Seventy-four isolates of Fusarium oxysporum isolated from diseased eggplant displaying typical Fusarium wilt symptoms were screened for pathogenicity on the highly susceptible cv. ‘Pala’. All the isolates tested were pathogenic to eggplant and designated as Fusarium oxysporum f. sp. melongenae (Fomg). Genetic diversity among a core set of 20 Fomg isolates that were selected based upon geographic locations, were characterized by using pathogenicity, vegetative compatibility grouping (VCG), and random amplified polymorphic DNA (RAPD) analysis. The area under the disease progress curve (AUDPC) was calculated for each Fomg isolate until 21 days after inoculation (DAI). The most virulent isolate was identified as Fomg10 based on AUDPC, disease severity and vascular discoloration measurements at 21 DAI. At this date, a good correlation was observed between disease severity and AUDPC values for all isolates (r = 0.73). UPGMA (unweighted pair group method with arithmetic average) cluster analysis of RAPD data using Dice’s coefficient of similarity differentiated all the Fomg isolates tested, and indicated considerable genetic variation among Fomg isolates, but isolates from the same geographic region were grouped together. There was no direct correlation between clustering in the RAPD dendrogram and pathogenicity testing of Fomg isolates. Twenty isolates of Fomg were assigned to VCG 0320.     

Cloning of the pathogenicity-related gene <Emphasis Type="Italic">FPD1</Emphasis> in <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">lycopersici</Emphasis>

We selected a reduced-pathogenicity mutant of Fusarium oxysporum f. sp. lycopersici, a tomato wilt pathogen, from the transformants generated by restriction enzyme-mediated integration (REMI) transformation. The gene tagged with the plasmid in the mutant was predicted to encode a protein of 321 amino acids and was designated FPD1. Homology search showed its partial similarity to a chloride conductance regulatory protein of Xenopus, suggesting that FPD1 is a transmembrane protein. Although the function of FPD1 has not been identified, it does participate in the pathogenicity of F. oxysporum f. sp. lycopersici because FPD1-deficient mutants reproduced the reduced pathogenicity on tomato.The nucleotide sequence data reported are available in the DDBJ/EMBL/GenBank databases under the accession number AB110097     

Genetic Diversity of Fusarium oxysporum Isolates from Cucumber: Differentiation by Pathogenicity, Vegetative Compatibility, and RAPD Fingerprinting

ABSTRACT A total of 106 isolates of Fusarium oxysporum obtained from diseased cucumber plants showing typical root and stem rot or Fusarium wilt symptoms were characterized by pathogenicity, vegetative compatibility, and random amplified polymorphic DNA (RAPD). Twelve isolates of other formae speciales and races of F. oxysporum from cucurbit hosts, three avirulent isolates of F. oxysporum, and four isolates of Fusarium spp. obtained from cucumber were included for comparison. Of the 106 isolates of F. oxysporum from cucumber, 68 were identified by pathogenicity as F. oxysporum f. sp. radicis-cucumerinum, 32 as F. oxysporum f. sp. cucumerinum, and 6 were avirulent on cucumber. Isolates of F. oxysporum f. sp. radicis-cucumerinum were vegetatively incompatible with F. oxysporum f. sp. cucumerinum and the other Fusarium isolates tested. A total of 60 isolates of F. oxysporum f. sp. radicis-cucumerinum was assigned to vegetative compatibility group (VCG) 0260 and 5 to VCG 0261, while 3 were vegetatively compatible with isolates in both VCGs 0260 and 0261 (bridging isolates). All 68 isolates of F. oxysporum f. sp. radicis-cucumerinum belonged to a single RAPD group. A total of 32 isolates of F. oxysporum f. sp. cucumerinum was assigned to eight different VCGs and two different RAPD groups, while 2 isolates were vegetatively self-incompatible. Pathogenicity, vegetative compatibility, and RAPD were effective in distinguishing isolates of F. oxysporum f. sp. radicis-cucumerinum from those of F. oxysporum f. sp. cucumerinum. Parsimony and bootstrap analysis of the RAPD data placed each of the two formae speciales into a different phylogenetic branch.     

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.     

Genetic variation among <Emphasis Type="Italic">Fusarium</Emphasis> isolates from onion,and resistance to <Emphasis Type="Italic">Fusarium</Emphasis> basal rot in related <Emphasis Type="Italic">Allium</Emphasis> species

The aim of this research was to study levels of resistance to Fusarium basal rot in onion cultivars and related Allium species, by using genetically different Fusarium isolates. In order to select genetically different isolates for disease testing, a collection of 61 Fusarium isolates, 43 of them from onion (Allium cepa), was analysed using amplified fragment length polymorphism (AFLP) markers. Onion isolates were collected in The Netherlands (15 isolates) and Uruguay (9 isolates), and received from other countries and fungal collections (19 isolates). From these isolates, 29 were identified as F. oxysporum, 10 as F. proliferatum, whereas the remaining four isolates belonged to F. avenaceum and F. culmorum. The taxonomic status of the species was confirmed by morphological examination, by DNA sequencing of the elongation factor 1-α gene, and by the use of species-specific primers for Fusarium oxysporum, F. proliferatum, and F. culmorum. Within F. oxysporum, isolates clustered in two clades suggesting different origins of F. oxysporum forms pathogenic to onion. These clades were present in each sampled region. Onion and six related Allium species were screened for resistance to Fusarium basal rot using one F. oxysporum isolate from each clade, and one F. proliferatum isolate. High levels of resistance to each isolate were found in Allium fistulosum and A. schoenoprasum accessions, whereas A. pskemense, A. roylei and A. galanthum showed intermediate levels of resistance. Among five A. cepa cultivars, ‘Rossa Savonese’ was also intermediately resistant. Regarding the current feasibility for introgression, A. fistulosum, A. roylei and A. galanthum were identified as potential sources for the transfer of resistance to Fusarium into onion.     

Impaired purine biosynthesis affects pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melonis</Emphasis>

The vascular wilt pathogen Fusarium oxysporum f. sp. melonis causes worldwide yield losses of muskmelon. In this study, we characterized a UV-induced non-pathogenic mutant (strain 4/4) of F. oxysporum f. sp. melonis, previously identified as a potential biological control agent. During comparative analysis of vegetative growth parameters using different carbon sources, mutant strain 4/4 showed a delay in development and secretion of extracellular enzymes, compared to the wild type strain. Amendments of the growth medium with yeast extract, adenine or hypoxanthine, but not guanine, complemented the growth defect of strain 4/4, as well as secretion and partial activity of cellulases and endopolygalacturonases, indicating that the strain is an adenine auxotroph. Incubation of strain 4/4 conidia in adenine solution, prior to inoculation of muskmelon plants, partially restored pathogenicity to the mutant strain.     

Host resistance mediated inter-genotype competition and temporal variation in <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">vasinfectum</Emphasis>

Temporal variation in Fusarium oxysporum f. sp. vasinfectum (Fov) populations was determined by comparing the genetic diversity of pathogen isolates recovered from three consecutive cotton crops (2002, 2004 and 2006) in the Boggabilla area of New South Wales, Australia. A total of 288 isolates were collected, among which 25 distinct AFLP genotypes were identified. These genotypes were classified into two main groups corresponding to known vegetative compatibility groups (VCG)—01111 and 01112. The Fov populations were dominated by four genotypes (I-A, I-B, II-A, II-B) that accounted for 87.5% of the isolates. Significant temporal variation was observed in both sampled fields with 6.8% and 10.7% of total genetic variation being attributed to differences among collections in different years. Genetic diversity based on Nei’s gene diversity and the Shannon-Wiener index increased over time. Significant changes in the frequency of the dominant Fov genotypes were observed in one field, where genotype I-A declined from 84.8% to 40.0% over the study period (2002–2006), while genotype I-B increased from 7.6% to 35.4%. Strong inter-genotype competition was detected in glasshouse bioassays with 93.4% of symptomatic plants sampled from dual inoculation trials being infected by single genotypes. Competition was differentially mediated by cotton cultivars as the competitive ability of pathogen genotype I-B was enhanced on the resistant cultivar Sicot 189 relative to the susceptible cultivar Siokra 1–4. This suggests that host-mediated inter-genotype competition may play an important role in temporal variation in Fov populations in the field.     

Weed hosts of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> in cotton fields in Turkey and characterization of <Emphasis Type="Italic">V. dahliae</Emphasis> isolates from weeds

A weed survey conducted in 2004 and 2005 in Aydin province of Turkey showed that Solanum nigrum, Xanthium strumarium, Amaranthus retroflexus, Portulaca oleracea, Sonchus oleraceus and Datura stramonium were the most prevalent weeds in the cotton fields exhibiting Verticillium wilt. Verticillium dahliae Kleb. was recovered from A. retroflexus and X. strumarium in those cotton fields. This is the first report of V. dahliae occurring naturally in A. retroflexus in Turkey. Pathogenicity tests on cotton and weeds showed that the virulence of V. dahliae isolates from weeds was higher on cotton plants than on weeds, with the disease severity ranging from 31.7% to 98.0%. Disease severity of V. dahliae isolates was 54.7–93.9% on eggplant, 23.7–51.6% on cucumber and 11.0–16.4% on tomato, whereas it did not cause any disease symptoms, or only low levels, on pepper and bell pepper. Two vegetative compatibility groups (VCGs) were identified among seven tested weed isolates: VCG2A (two isolates) and VCG2B (three isolates) using international reference strains.     

Genetic and pathogenic characterization of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> isolates from eggplant in Turkey

During 2005 to 2007, eggplant fields in 19 provinces from three different regions (western, southern and southeastern Anatolia regions) of Turkey were surveyed for Verticillium wilt. Sixty-seven isolates of Verticillium dahliae from wilted eggplants were collected and used for vegetative compatibility analysis using nitrate non-utilizing mutants and reference tester strains of vegetative compatibility groups (VCGs) 1A, 2A, 2B, 3, 4A and 4B. Among all isolates, 33 (12 from western, 15 from southern and six from southeastern Anatolia) were assigned to VCG2B, 23 (four from western, eight from southern and 11 from southeastern Anatolia) to VCG2A, six (four from southern, one from western, and one from southeastern Anatolia) to VCG4B and five (one from western, one from southern and three from southeastern Anatolia) to VCG1A, whereas VCG3 and VCG4A were not defined among isolates. In order to test if there is a correlation between VCG and pathogenicity in V. dahliae, pathogenicity of 30 isolates, representing the four multimember VCGs, were tested on Solanum melongena cvs. ‘Kemer’ and ‘Aydın Siyahı’ in an unheated greenhouse. All isolates were found to be pathogenic on both cultivars and there was no difference in susceptibility between the two cultivars. VCG4B isolates collectively led to higher vascular discoloration index (VDI) on both cultivars and higher disease severity index (DSI) on ‘Kemer’ compared with other VCGs. Similarly, VCG1A caused lower VDI on both cultivars and lower DSI on ‘Kemer’. Isolates within each of VCGs 1A, 2A and 4B caused similar VDI on both cultivars. Isolates of VCG2B were found to vary in their VDI values on both cultivars. To the best of our knowledge, the present study is the first report of natural infections of eggplant by VCG1A.     

Characterization of Fusarium oxysporum f. sp. radicis-cucumerinum attacking melon under natural conditions in Greece

A severe root and stem rot disease of melon was observed during the 2001 growing season on four glasshouse crops in Heraklio, Greece. A total of 43 isolates of F. oxysporum , obtained in Crete from glasshouse-grown melon and showing fusarium wilt or root and stem rot symptoms, were characterized by pathogenicity and vegetative compatibility. The majority of these isolates was also fingerprinted via amplified fragment length polymorphic (AFLP) analysis. Of the total number of isolates, 22 were identified by pathogenicity tests as F. oxysporum f. sp. melonis , 20 as F. oxysporum f. sp. radicis-cucumerinum , while one isolate was nonpathogenic on cucumber, melon, sponge gourd and pumpkin. All 22 isolates of F. oxysporum f. sp. melonis were assigned to vegetative compatibility group (VCG) 0134, and all 20 isolates of F. oxysporum f. sp. radicis-cucumerinum to VCG 0260. Isolates of F. oxysporum f. sp. radicis-cucumerinum were incompatible with isolates of F. oxysporum f. sp. melonis. AFLP fingerprinting allowed for the clustering of the isolates of the two formae speciales of F. oxysporum along two separate phenetic groups: f. sp. melonis to AFLP major haplotype I, and f. sp. radicis-cucumerinum to AFLP major haplotype II. Overall, pathogenicity, vegetative compatibility grouping and AFLP analysis were correlated and effectively distinguished isolates of F. oxysporum from melon. This appears to be the first report of natural infection of melon by F. oxysporum f. sp. radicis-cucumerinum worldwide.     

Involvement of antifungal compounds from rockmelon fruit rind (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.) in resistance against the fruit rot pathogen <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melonis</Emphasis>.

Fusarium rot caused by Fusarium oxysporum f. sp. melonis, causes significant postharvest losses in rockmelon crops. Although latent infection is often present in the field, symptoms of the disease may not appear until fruit maturity. The susceptibility of different-aged rockmelon fruit cv. “Colorado” was determined by inoculating fruit at different stages of development with a spore suspension of F. oxysporum f. sp. melonis. Disease symptoms appeared first and were more severe in older fruit compared to younger fruit. Disease symptoms on fruit 35 DAA (Days After Anthesis) and 42 DAA appeared within 3 days of inoculation and rapidly covered the fruit within 5 days. In contrast, disease symptoms on fruit 7 DAA appeared 6 days after inoculation and grew slowly. Extraction of antifungal compounds without involving acid hydrolysis from 7 DAA fruit rind did not show antifungal activity on TLC plates. However, hydrolysis of the ethyl acetate fraction resulted in a strong fungal inhibitory zone on agar plates against colonies of F. oxysporum f. sp. melonis. Separation of the hydrolysed crude extracts on TLC plates indicated the presence of two distinct antifungal zones with R_f 0.36 and 0.13 in young fruit 7, 14 and 21 DAA. The area of fungal inhibition of compound R_f 0.36 was greater than that of R_f 0.13 on the TLC plate. Extracts from mature fruit of 35 and 42 DAA did not have detectable levels of antifungal compounds. The decrease in the susceptibility of rockmelon fruit during maturity may be correlated to a decrease in the antifungal compounds in the fruit with maturity.     

Cell interactions between a nonpathogenic <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> strain and root tissues of <Emphasis Type="Italic">Eucalyptus viminalis</Emphasis>

Nonpathogenic isolates of Fusarium oxysporum can be successful antagonists of pathogenic forms of the same fungal species that commonly attacks crop plants. The characteristics that distinguish nonpathogenic from pathogenic forms are not well understood. In this study, the mode of root colonization of Eucalyptus viminalis seedlings by a nonpathogenic F. oxysporum strain is described at the ultrastructural level. Root systems of E. viminalis plants were inoculated with nonpathogenic F. oxysporum strain Fo47 in an in vitro model system. Changes in the occurrence of nonesterified and methyl-esterified pectins in colonized E. viminalis roots were evaluated by in situ immunolabeling using two monoclonal antibodies, JIM 5 and JIM 7. Modes of penetration and root colonization patterns in E. viminalis seedlings by the nonpathogenic fungus were similar to those described for pathogenic forms of F. oxysporum. However, root interactions differed in that the nonpathogenic fungus did not induce host tissue damage. No papilla-like appositions were observed in host cells in response to invading hyphae, which did not disrupt the host plasma membrane in many cases, suggesting that a biotrophic relationship was established. Root colonization by the nonpathogenic strain did not induce alteration in JIM 7 labeling of methyl-esterified pectin in E. viminalis cell walls, whereas nonesterified pectin was detected to a significantly greater extent in cell walls of roots colonized by the fungus. Pectin components decreased slightly only at points of hyphal contact with host cells. Because nonpathogenic strains utilize pectin in pure culture, host control over enzyme activity or production by the fungi may at least partly explain their compatible interactions with host tissues.     

Identification and pathogenicity of<Emphasis Type="Italic">Fusarium</Emphasis> spp. from stem bases of winter wheat in Erzurum,Turkey

Four-hundred-sixty-eightFusarium andFusarium-like isolates were obtained from crowns and subcrown internodes of winter wheat grown in Erzurum, Turkey. Of these isolates, 34.8% wereFusarium acuminatum, 32.3% wereF. equiseti, 16.9% wereF. oxysporum, 15.0% wereMicrodochium nivale (formerlyFusarium nivale), 0.6% wereF. tabacinum and 0.4% wereF. solani. In pathogenicity tests on wheat, the highest disease severity was caused by isolates ofM. nivale, whereas isolates ofF. acuminatum, F. equiseti, F. oxysporum andF. solani were slightly virulent; isolates ofF. tabacinum were nonpathogenic. This is the first report ofM. nivale andF. tabacinum from wheat in Turkey. http://www.phytoparasitica.org posting Jan. 29, 2003.     

Genetic diversity and pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> isolated from wilted Welsh onion in Japan

Thirty isolates of Fusarium oxysporum from wilted Welsh onion plants were examined for their diversity in nucleotide sequences of the ribosomal DNA (rDNA) intergenic spacer (IGS) region and for pathogenicity with regard to five Welsh onion cultivars. Phylogenetic analysis based on the IGS sequences revealed polyphyletic origins of the isolates and a relationship between phylogeny and pathogenicity; low virulence isolates differed genetically from those with high and moderate virulence. Mating type analysis revealed that all F. oxysporum isolates were MAT1-1 idiomorphs, suggesting that the pathogens may be clonal in the fields examined.     

Genetic diversity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">spinaciae</Emphasis> in Japan based on phylogenetic analyses of rDNA-IGS and <Emphasis Type="Italic">MAT1</Emphasis> sequences

Twenty-eight isolates of Fusarium oxysporum f. sp. spinaciae (FOS; the causal agent of spinach wilt) collected from Japan were assessed for mating type and subjected to phylogenetic analysis. Mating type analysis revealed all isolates to be MAT1-2, suggesting that there is no sexual recombination within the population. Phylogenetic analyses based on nucleotide sequences of the ribosomal DNA intergenic spacer (IGS) and the mating type locus (MAT1) suggested that FOS is polyphyletic. The cluster analysis based on IGS showed four phylogenetic groups (S1–S4) among the isolates. Two distinct lineages, S1 and S3, included FOS isolates both of the vegetative compatibility group (VCG) types, 0330 and 0331, demonstrating that VCG differentiation in FOS may not necessarily reflect the phylogenetic relationships based on IGS and MAT1-2-1.     

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.     

A study on the susceptibility of the model legume plant <Emphasis Type="Italic">Medicago truncatula</Emphasis> to the soil-borne pathogen <Emphasis Type="Italic">Fusarium oxysporum</Emphasis>

Fusarium wilt is a soil-borne disease caused by formae specialis of Fusarium oxysporum on a large number of cultivated and wild plants. The susceptibility of the model legume plant Medicago truncatula to Fusarium oxysporum was studied by root-inoculating young plants in a miniaturised hydroponic culture. Among eight tested M. truncatula lines, all were susceptible to F. oxysporum f.sp. medicaginis, the causal agent of Fusarium wilt in alfalfa. However, a tolerant line, F83005.5, and a susceptible line, A17, could be distinguished by scoring the disease index. The fungus was transformed with the GFP marker gene and colonisation of the plant roots was analysed by epifluorescence and confocal microscopy. A slightly atypical pattern of root colonisation was observed, with massive fungal growth in the cortex. Although colonisation was not significantly different between susceptible and tolerant plants, the expression of some defence-related genes showed discrimination between both lines. A study with 10 strains from various host-plants indicated that M. truncatula was a permissive host to F. oxysporum.