Thesis (PhD(Agric))--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: The purpose of this study was to characterize the race and vegetative
compatibility of Fusarium oxysporum f. sp. melonis (FOM) isolates collected in the
major melon producing areas, to report on their geographical distribution, and their
possible relatedness to isolates from other countries.
Seventy two FOM isolates obtained from 30 fields in 17 melon producing regions
were race-typed using the differential cultivars Topmark (susceptible to all races),
Doublon (Fomi), CM 17187 (Fom2) and Perlita (Fom3) and grouped by means of
vegetative compatibility. All isolates belonged to vegetative compatibility group 0134,
indicating a high degree of genetic homogeneity among the South African FOM
population. 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 2
and reference isolates of race 0 became stunted, their leaves turned yellow, and became
thickened and brittle. These results suggested that Fom3 in Perlita confers a tolerant
reaction compared to the resistant reaction of gene FornI in Doublon. The disease
reaction 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, were
used. The differential cultivars were included to verify virulence of the isolates. Perlita
plants inoculated with three isolates of race 2 remained asymptomatic. The remaining
race 2 and 0 isolates, induced severe stunting of Perlita plants, but mean percentage
stunting 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 Perlita
was verified at a lower inoculum concentration with two race 2 (pipette method) and two race 0 isolates (root dip method). Results proved that Fom3 does not confer similar
resistance towards race 0 and some race 2 isolates as FornI in Doublon. Cultivars
possessing 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 to
VCG 0134, to change its virulence during infection of melon plants, was investigated
under quarantine. Seedlings of melon cultivars Imperial 45 and Early Sweet (no
resistance genes), Amber (Fom2) and Fiata (FomI, Fom2) were consecutively grown in
two cement troughs in a gauzehouse. Each planting was terminated when plants had
advanced Fusarium wilt or after the fruit were harvested. In the first planting, Imperial
45 seedlings were transplanted and artificially inoculated with the nil mutant isolate. In
the 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 VCG
status verified. Virulence of the labelled isolates was determined using differential
cultivars. In trough A, all plants of the susceptible cultivars Imperial 45 and Early Sweet
crops showed Fusarium wilt. The labelled isolates recovered from the selected plants
were all designated race O. In the first crop (planting No.5) of the resistant cultivar
Amber, 6.7% of the plants developed Fusarium wilt. In the second Amber crop the
disease incidence increased to 56.6%, and to 81.8% in the final crop. Contrary to the
susceptible cultivars, only race 2 isolates were obtained from the symptomatic Amber
plants. Similar data were found with the susceptible cultivar Imperial 45 and the resistant
cultivar Amber in trough B. Planting of Fiata caused a dramatic reduction in Fusarium
wilt incidence in trough B. However, 1.2% of plants were affected by Fusarium wilt in
the first Fiata crop (planting No.6), whereas 4% of the plants were symptomatic in the
final planting. From these symptomatic Fiata plants only race 1,2 isolates were obtained.
These findings, and the fact that the symptomatic plants represented a substantial
proportion of the first Amber (approximately 7-15%) and Fiata (approximately 2%)
crops, provedthat changes in the race structure of this fungal pathogen occurred rapidly
when confronted with a resistant cultivar. The potential of RAPD analysis to differentiate between the isolates displaying
virulence changes was evaluated. Four F. oxysporum f. sp. niveum isolates were included
as an outgroup. A histopathological study was conducted to verify whether these isolates
retain their ability to behave as true vascular pathogens. The three primers used clearly
distinguished the 12 FOM isolates from the four F. oxysporum f. sp. niveum isolates.
However, the primers showed a highly conserved and characteristic banding pattern for
the 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. Disease
reactions on the three differential cultivars confirmed the virulence of FOM isolates. The
histopathological 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 true
vascular pathogens. / AFRIKAANSE OPSOMMING: DIE KARAKTERISERING EN PATOGENESITEIT VAN SUID-AFRIKAANSE
ISOLATE VAN FUSARIUMOXYSPORUMF. SP.MELONIS
Die doel van die studie was om Fusarium oxysporum f. sp. melonis (FOM) isolate
wat in die hoof spanspekproduserende gebiede versamel is, volgens ras en vegetatiewe
verenigbaarheid te karakteriseer, en hul geografiese verspreiding en verwantskap met
isolate van ander lande aan te dui.
Twee en sewentig FOM isolate afkomstig vanaf 30 landerye wat 17
spanspekproduserende areas verteenwoordig, is gebruik. Die differensiële kultivars
Topmark (vatbaar vir alle rasse), Doublon (Forni), CM 17187 (Fom2) en Perlita (Fond)
is gebruik om die rasbepalings te doen asook om die vegetatiewe verenigbare groepe
(VVG) te bepaal. Al die isolate is as VVG 0134 geklassifiseer, wat 'n hoë mate van
genetiese homogenesiteit binne die Suid-Afrikaanse populasie aandui. Vier en vyftig
isolate is as ras 0, agt as ras 1 en 10 as ras 2 geklassifiseer. Ras 0 is vanaf 15 gebiede
afkomstig, terwyl ras 1 sporadies voorgekom het. Ras 2 is vanuit vier landerye binne
dieselfde geografiese gebied verkry. Plante van die kultivar Perlita wat met plaaslike
isolate van ras 0 en 2, asook verwysings-isolate van ras 0 geïnokuleer is, het verdwerg
voorgekom. Die blare van die plante het vergeel, verdik en bros voorgekom. Hierdie
siekte reaksie het aangedui dat Fond in Perlita toleransie bewerkstellig in teenstelling
met die weerstandbiedende reaksie van geen Fomi in Doublon. Die siekte reaksie van
Perlita teenoor FOM is dus verder ondesoek. Hiervoor is 20 isolate wat al vier FOM
rasse insluit (0, 1, 2, en 1,2), en van verskillende wêrelddele afkomstig is, gebruik. Die
virulensie van die isolate is met die differensiële kultivars bevestig. Drie van die ras 2
isolate het geen siektesimptome op Perlita veroorsaak nie. Die ander ras 2 isolate, en al
die ras 0 isolate, het egter die Perlita plante aansienlik verdwerg en die blare vergeel en
verdik. Laasgenoemde groep isolate het 'n gemiddelde verdwergingsindeks van tussen
25.1% en 50.0% veroorsaak. Die siekte reaksie by Perlita is verder bevestig deur plante teen 'n laer inokulumdigtheid van twee ras 2 (pipet metode), en twee ras 0 (wortel-doop
metode) isolate, te inokuleer. Uit die resultate was dit duidelik dat die weerstand wat
Fom3 teenoor ras 0 en sommige ras 2 isolate verskaf, van FornI verskil. Kultivars wat
oor die weerstandsgeen Fom3 beskik moet dus as tolerant beskou word.
'n Ondersoek is geloods na die vermoë van 'n nil mutant isolaat, genereer vanaf
die wilde ras 0 isolaat van FOM (VVG 0134), om onder kwarantyn sy virulensie
gedurende infeksie van spanspekplante te verander. Saailinge van die spanspekkultivars
Early Sweet (geen weerstandsgene), Amber (Fom2) en Fiata (FornI, Fom2) is
opeenvolgens in twee sement trêe in 'n gaashuis verbou. Die afsonderlike aanplantings is
beëindig sodra gevorderde Fusarium-verwelksimptome verkry is, of nadat vrugte ge-oes
is. Vir die eerste aanplanting is oorgeplante Imperial 45 saailinge kunsmatig met die nil
mutant isolaat geïnokuleer. Tydens die opeenvolgende aanplantings is saad direk in die
besmette grond gesaai ten einde natuurlike infeksie te verkry. Met elke aanplanting is
isolasies gedoen vanaf verteenwoordigende plante wat Fusarium-verwelksimptome
getoon het. Alle F. oxysporum isolate wat verkry is, is ge-enkelspoor en hul nit mutant
status en VVG is bevestig. Virulensie van die gemerkte isolate is bepaal deur inokulasie
van die differensiële kultivars. Alle plante van die vatbare Imperial 45 en Early Sweet
kultivars wat in trog A geplant is, het Fusarium-verwelksimptome getoon. Die gemerkte
isolate wat vanaf die verteenwoordigende plante verkry is, is almal as ras 0 geklassifiseer.
Tydens die eerste aanplanting van die weerstandbiedende kultivar, Amber (aanplanting
No.5), het 6.7% van die plante Fusarium-verwe1ksimptome ontwikkel. Tydens die
tweede en derde aanplanting van Amber het die frekwensie van siektevoorkoms verhoog
na 56.6% en 81.8 %, onderskeidelik. In teenstelling met die vatbare kultivars, is slegs ras
2 vanuit die Amber plante met siektesimptome verkry. Soortgelyke resultate is met
Imperial 45 en Amber in trog B verkry. Aanplanting van kultivar Fiata het egter 'n
dramatiese verlaging in die voorkoms van Fusarium-verwelk bewerkstellig. Tydens die
eerste Fiata aanplanting (aanplanting No.6) het 1.2% plante Fusarium-verwelksimptome
ontwikkel, en 4% tydens die laaste aanplanting. Vanaf hierdie plante is slegs ras 1,2
isolate verkry. Hierdie bevindings, en die feit dat 'n aansienlike hoeveelheid van die
Amber (ongeveer 7-15%) en Fiata plante (ongeveer 2%) siektesimptome getoon het, bewys dat FOM vinnig van virulensie verander wanneer die patogeen 'n
weerstanbiedende kultivar infekteer.
Die vermoë van RAPD analise om tussen isolate wat in virulensie verander het, te
onderskei, is ondersoek. Vier isolate van F. oxysporum f. sp. niveum is as 'n buite-groep
ingesluit. Om te bevestig dat die isolate wat van ras verander het wel egte vaskulêre
patogene is, is 'n histopatologiese ondersoek gedoen. Die drie inleiers wat gebruik is, het
die 12 FOM isolate duidelik van die vier F. oxysporum f. sp. niveum isolate onderskei.
Die 12 FOM isolate wat drie fiosologiese rasse (ras 0, ras 2, ras 1,2) verteenwoordig het,
is egter saam gegroepeer, wat aandui dat hierdie metode nie tussen rasse van FOM kan
onderskei nie. Inokulasiestudies met die differensiële kultivars het die virulensie van die
isolate bevestig. Die histopatologiese ondersoek het verder bewys dat beide FOM rasse
(ras 2, ras 1,2) wat vanaf die wilde tipe ras ° isolaat ontstaan het, hul vermoë behou het
om as egte vaskulêre patogene op te tree.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/51651 |
| Date | 03 1900 |
| Creators | Schreuder, Wouter |
| Contributors | Holz, G., Lamprecht, S. C., Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 79 p. : ill. |
| Rights | Stellenbosch University |
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