Characterization and pathogenicity of South African isolates of Fusarium oxysporum f. sp. melonis

[摘要] ENGLISH ABSTRACT:The purpose of this study was to characterize the race and vegetativecompatibility of Fusarium oxysporum f. sp. melonis (FOM) isolates collected in themajor melon producing areas, to report on their geographical distribution, and theirpossible relatedness to isolates from other countries.Seventy two FOM isolates obtained from 30 fields in 17 melon producing regionswere race-typed using the differential cultivars Topmark (susceptible to all races),Doublon (Fomi), CM 17187 (Fom2) and Perlita (Fom3) and grouped by means ofvegetative compatibility. All isolates belonged to vegetative compatibility group 0134,indicating a high degree of genetic homogeneity among the South African FOMpopulation. Fifty four isolates were identified as race 0, eight as race 1, and 10 as race 2.Race 0 occurred in 15 of the regions whereas race 1 was sporadically recovered. Race 2,on the other hand, was obtained only from four fields located in one geographical region.Perlita plants (carrying the gene Fom3) inoculated with local isolates ofrace 0 and race 2and reference isolates of race 0 became stunted, their leaves turned yellow, and becamethickened and brittle. These results suggested that Fom3 in Perlita confers a tolerantreaction compared to the resistant reaction of gene FornI in Doublon. The diseasereaction of cultivar Perlita to FOM was therefore reinvestigated. Twenty isolates,including the four FOM races (0, 1, 2, and 1,2) obtained from different countries, wereused. The differential cultivars were included to verify virulence of the isolates. Perlitaplants inoculated with three isolates of race 2 remained asymptomatic. The remainingrace 2 and 0 isolates, induced severe stunting of Perlita plants, but mean percentagestunting values did not differ significantly (P = 0.05) and ranged between 25.1 and 50.0.Leaves of stunted plants were chlorotic, thickened and brittle. Disease reaction of Perlitawas verified at a lower inoculum concentration with two race 2 (pipette method) and tworace 0 isolates (root dip method). Results proved that Fom3 does not confer similarresistance towards race 0 and some race 2 isolates as FornI in Doublon. Cultivarspossessing Fom3, should therefore be considered tolerant to FOM races 0 and 2.The ability of a nit mutant isolate, generated from FOM race 0 which belongs toVCG 0134, to change its virulence during infection of melon plants, was investigatedunder quarantine. Seedlings of melon cultivars Imperial 45 and Early Sweet (noresistance genes), Amber (Fom2) and Fiata (FomI, Fom2) were consecutively grown intwo cement troughs in a gauzehouse. Each planting was terminated when plants hadadvanced Fusarium wilt or after the fruit were harvested. In the first planting, Imperial45 seedlings were transplanted and artificially inoculated with the nil mutant isolate. Inthe consecutive plantings, seeds were sown in the infested soil to enable natural infection.For each crop, representative plants showing Fusarium wilt were selected for isolation.All F. oxysporum isolates recovered were single-spored and their nit mutant and VCGstatus verified. Virulence of the labelled isolates was determined using differentialcultivars. In trough A, all plants of the susceptible cultivars Imperial 45 and Early Sweetcrops showed Fusarium wilt. The labelled isolates recovered from the selected plantswere all designated race O. In the first crop (planting No.5) of the resistant cultivarAmber, 6.7% of the plants developed Fusarium wilt. In the second Amber crop thedisease incidence increased to 56.6%, and to 81.8% in the final crop. Contrary to thesusceptible cultivars, only race 2 isolates were obtained from the symptomatic Amberplants. Similar data were found with the susceptible cultivar Imperial 45 and the resistantcultivar Amber in trough B. Planting of Fiata caused a dramatic reduction in Fusariumwilt incidence in trough B. However, 1.2% of plants were affected by Fusarium wilt inthe first Fiata crop (planting No.6), whereas 4% of the plants were symptomatic in thefinal planting. From these symptomatic Fiata plants only race 1,2 isolates were obtained.These findings, and the fact that the symptomatic plants represented a substantialproportion of the first Amber (approximately 7-15%) and Fiata (approximately 2%)crops, provedthat changes in the race structure of this fungal pathogen occurred rapidlywhen confronted with a resistant cultivar. The potential of RAPD analysis to differentiate between the isolates displayingvirulence changes was evaluated. Four F. oxysporum f. sp. niveum isolates were includedas an outgroup. A histopathological study was conducted to verify whether these isolatesretain their ability to behave as true vascular pathogens. The three primers used clearlydistinguished the 12 FOM isolates from the four F. oxysporum f. sp. niveum isolates.However, the primers showed a highly conserved and characteristic banding pattern forthe FOM isolates which represented three physiological races (race 0, race 2, race 1,2),indicating that RAPD analysis cannot detect race-specific groupings in FOM. Diseasereactions on the three differential cultivars confirmed the virulence of FOM isolates. Thehistopathological data furthermore proved that the two FOM races (race 2, race 1,2),which derived from the race 0 parent isolate, retained their ability to behave as truevascular pathogens.

[发布机构] Stellenbosch University