Doctoral Degrees (Plant Pathology)
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Browsing Doctoral Degrees (Plant Pathology) by browse.metadata.advisor "Holz, G."
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- ItemCharacterization 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.
- ItemEpidemiology of Monilinia laxa on nectarine and plum : infection of fruits by conidia(Stellenbosch : Stellenbosch University, 2001-03) Fourie, Paul H. (Paul Hendrik); Holz, G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.ENGLISH ABSTRACT: Postharvest decay of stone fruit in the Western Cape province of South Africa is caused primarily by Botrytis cinerea (grey mould) and Monilinia laxa (brown rot). Little is known about the relative importance and seasonal occurrence of the two pathogens in nectarine and plum orchards, the mode of penetration of fruits by M laxa, latency and subsequent disease expression by the latter pathogen. These aspects were investigated in this study. By sampling from the Unifruco Quality Evaluation Scheme and from 11 stone fruit orchards, observations were made over a 3-year period of the occurrence of grey mould and brown rot in the major stone fruit regions. Botrytis cinerea was found to be the most important pathogen causing blossom blight and postharvest decay on stone fruit. The pathogen was most prominent on early- and mid-season culti~ars. Brown rot was exclusively caused by M laxa and no evidence was found that M fructicoZa had been introduced into the region. Monilina laxa was most prominent on the later maturing cultivars. Botrytis cinerea blossom infection did not contribute directly to postharvest decay. Both surface inoculum and latent infection consistently occurred on fruit in each orchard, although at fluctuating levels. Disease expression on developing fruit was not governed by the amount of B. cinerea occurring on fruit surfaces, but by the ability of fruit to resist disease expression. The amount of B. cinerea on fruits was generally higher during spring than during summer. Monilinia laxa occurred sporadically on the blossoms of late-maturing cultivars. Immature fruit were generally pathogen-free and disease expression occurred on maturing fruit only. These findings suggest that conidia of M laxa are generally produced in orchards when fruits are approaching maturity and can penetrate and infect maturing fruit only. The behaviour of airborne M laxa conidia was subsequently studied on nectarine (cultivar Flamekist) and plum (cultivar Laetitia) fruit. For these studies, an inoculation method that simulates natural infection by airborne conidia was used. Fruit at pit hardening, 2 wk before harvest, harvest stage and after cold storage (nectarines 4 wk at -o.soC followed by 1 wk at 23°C at ±56% RH; plums 10 days at .....().5°C,18 days at 7.5°C followed by 1 wk at 23°C at ±56% RH) were dusted with dry conidia of M laxa in a settling tower. The fruits were incubated for periods ranging from 3 to 48 h at high relative humidity (2':93%, humid fruit) or covered with a film of water (wet fruit). Behaviour of the solitary conidia was examined with an epifluorescence microscope on skin segments stained in a differential stain containing fluorescein diacetate, aniline blue and blankophor. The ability of solitary conidia to colonise the fruit surface, penetrate fruit skins and to induce disease expression was determined by using a differential set of tests. For these tests, fruit were surface-sterilised (30 s in 70% ethanol) or left Unsterile. From each group, fruit were selected for isolation (skin segment test), immersed in a 3% paraquat solution (paraquat-treated fruit test) or left untreated (sound fruit test). 1be findings demonstrated that solitary conidia of M laxa behaved consistently on plum and nectarine fruit surfaces: appressorium formation and direct penetration was not observed on any of the fruit surfaces and germ tubes penetrated fruit predominantly through stomata, lenticels and microfissures in the fruit skin. The monitoring of airborne conidia revealed subtle effects of the fruits on the behaviour of solitary germlings, which could not be seen when using conidial suspensions. On both fruit types, no deleterious effect was seen on conidial and germling survival when fruit were kept humid at pit hardening, 2 wk before harvest and harvest. However, conidial and germling survival were drastically reduced by prolonged wet incubation of fruits. The findings on disease expression in the skin segment, paraquat-treated fruit and sound fruit tests clearly showed that the skin of both nectarine and plum fruits were not penetrated at the pit hardening stage, latent infections were not established and fruitsreacted resistant to disease expression. These facets on both fruit types were furthermore unaffected by wetness. The barrier capacity of the fruit skin of the two stone fruit types however differed drastically later in the season. On nectarine, fruit skins were more readily penetrated and disease expression became more pronounced when fruit approached maturity. Penetration and disease expression on ripening nectarine fruit were furthermore greatly influenced by wetness. Maturing plum fruit, on the other hand, did not display the drastic change in the barrier capacity of fruit skins as observed on nectarine. The influence of wetness on infection and disease expression was also less pronounced than on nectarine. In fact, plum fruit remained asymptomatic in the sound fruit test after inoculation and humid incubation at the 2 wk before harvest stage, harvest stage and after cold storage. Plum fruit at these stages only developed disease after a prolonged period (~12 h) of wet incubation. The paraquat fruit test revealed that these fruits became more susceptible to latent infection, but they were not as susceptible as nectarine. Collectively, these findings indicate that M. laxa fruit rot epidemics on plum and nectarine are driven by inoculum levels on fruit approaching maturity and by weather conditions prevailing during the preharvest and harvest period. However, the barrier capacity of plum skins is considerably more effective than that of nectarine fruit. Wounds would therefore play an important role in the epidemiology of M. laxa on plum fruit. Infection of fresh wounds by airborne M. laxa conidia, and by conidia and germlings that have established on fruits, was therefore investigated. Plum fruit (cultivar Laetitia) at pit hardening, 2 wk before harvest, harvest stage and after cold storage were dusted with dry conidia of M. laxa in a settling tower.- Infection of rionwounded fruit and of fresh wounds by \ the airborne conidia on dry, humid and wet plum fruit surfaces, and by conidia and germlings that have been established on fruits under the wetness regimes was then investigated. Nonwounded immature and mature fruit remained mostly asymptomatic, whereas nonwounded cold stored fruit decayed readily. Wounding drastically increased infection by airborne conidia. Immature fruits were less susceptible to wound infection by the airborne conidia than mature fruits. Conidia dispersed freshly were more successful in infecting fresh wounds than conidia that were deposited, or germlings that established, on fruit surfaces 4 days prior to wounding. This decrease in infectivity was especially pronounced on humid and even more on wet incubated fruit. This study clearly showed that in order to reduce. the incidence of brown rot, inoculum levels on fruit approaching maturity should be reduced by sanitation practices and fungicide applications. Furthermore, it is essential to protect fruits, especially. near-mature fruits, from being wounded.