Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by browse.metadata.advisor "Lamprecht, Sandra, C."

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    Characterization and pathogenicity of South African isolates of Fusarium oxysporum f. sp. melonis
    (Stellenbosch : Stellenbosch University, 2000-03) Schreuder, Wouter; Holz, G.; Lamprecht, Sandra, C. ; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    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.
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    Identification and management of toxigenic fusarium species associated with fusarium head blight and fusarium crown rot of wheat in South Africa
    (Stellenbosch : Stellenbosch University, 2018-03) Van Coller, Gerhardus Johannes; Viljoen, Altus; Lamprecht, Sandra, C. ; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Fusarium head blight (FHB) and Fusarium crown rot (FCR) are important diseases of wheat. FHB, which is caused by members of the Fusarium graminearum species complex (FGSC), reduces yields and grain quality, and contaminates grain with mycotoxins (toxic secondary fungal metabolites) like deoxynivalenol (DON), nivalenol, and zearalenone (ZEA). FCR, caused by Fusarium pseudograminearum, infects the lower stem and causes stem rotting, which also results in yield losses. Fusarium pseudograminearum can also cause FHB. Management of FHB involves integrating agronomical practices (crop rotation, tillage), host resistance and chemical control. In the irrigation regions of South Africa, however, producers rotate wheat with summer crops like maize, which also hosts the FGSC, while crop rotation and minimum/no-till is common in the Western Cape. These practices necessitates the use of host resistance and chemical control to manage FHB in South Africa. Accurate identification of pathogens causing FHB and FCR in South Africa is needed to determine their distribution and to assist breeders in developing resistant wheat cultivars. Comprehensive surveys were thus conducted over 2 years to identify Fusarium species and their chemotypes in the country. The FGSC with the 15-cetyldeoxynivalenol chemotype predominated in the irrigation regions of South Africa, while F. pseudograminearum was the dominant species in the Western Cape region. Commercial irrigation and dryland wheat cultivars, as well as test lines with different quantitative trait loci (QTL), were evaluated for resistance to FHB by field-inoculation with F. graminearum s.s. and F. pseudograminearum. Among the irrigation cultivars, Krokodil, SST843, SST866, and SST884 performed the best. The Fhb1 QTL had a marked effect on the percentage Fusarium-damaged kernels, and reduced DON content in most test lines. One test line, BFUS2011-17, with no QTLs from Sumai 3, grouped among the top performers. Disease levels were low in dryland trials, but DON and ZEA levels were high in the first year, with the resistant control and SST027, SST056 and SST087 containing low mycotoxin levels. The efficacy of three commercial fungicides (Abacus, Amistar Xtra and Prosaro) and two chemical seed treatments (Galmano Plus and Vitavax Plus) to manage FHB and increase grain quality was determined in field trials over 2 years. All foliar treatments reduced disease and DON levels, while significantly improving grain quality and yield. Seed treatments had little effect on FHB, but Abacus combined with seed treatments reduced disease incidence more than Abacus alone. Prosaro combined with Galmano Plus reduced yields compared to Prosaro alone. This study provided the first comprehensive report of Fusarium species and their type B trichothecene chemotype associated with FHB and FCR in South Africa. Irrigation cultivars, test lines and dryland cultivars with improved resistance to FHB and DON contamination were identified, while fungicides reduced FHB and DON, and improved grain quality. Future studies should determine the natural occurrence of mycotoxins in wheat fields throughout South Africa, develop resistant irrigation and dryland cultivars, determine the efficacy of chemical control of FHB in different environments over multiple years, and optimise chemical control of FHB through timing of application, different nozzles and adjuvants.