No description available.

The research is focused on soil factors which can affect <i>Pythium ultimum</i> and the bio control properties of <i>Pseudomonas fluorescens </i>and <i>Bacillus subtilis.  </i>Similar studies have previously been carried out with results that were not always consistent.  One possible explanation was the occurrence of water flow which can influence the location of antagonist bacteria in the rhizosphere, and thus their competitive action.  The innovative point of the work was to design a watering method which minimised the possibility of unwanted water flow.  Three soil factors were considered:  pH (from 5 to 8), temperature (several regimes between 3 and 25°C) and matric potentials (-10, -100, -300 kPa).  Their effect on microbial survival and activity was studied by measuring root colonisation, activity and biocontrol of the antagonistic bacteria.  <i>Bacillus subtilis</i> did not show antagonistic activity against <i>Pythium ultimum</i> and its colonisation was not affected by any soil factors investigated.  No soil pH effect was found on the biocontrol activity of <i>Pseudomonas fluorescens.  </i>The root colonisation of <i>P. fluorescens </i>decreased with increasing temperature and at extreme low matric potential (<-300 kPa).  However, between -120 and -140 kPa this bacterium reached its highest level of root colonisation and its greatest antagonistic activity.  Under such conditions colonisation data showed that there was downward movement of <i>P. fluorescens </i>which could be explained most likely by nematodes as possible vectors.  These results were further investigated during a field test in which using minor doses of antagonistic bacteria no effect of biocontrol treatment against damping off was measured.  A distinct study was carried out on forecasting the time to crop emergence from a model of sugar beet emergence with a measured and/or simulated set of data.

Thesis (Ph. D.)--University of Adelaide, Dept. of Crop Protection, 1997. / Includes bibliographical references (leaves 162-198).

No description available.

In-vitro and greenhouse screening of 78 bacterial isolates from sorghum rhizosphere in Ethiopia and 86 isolates from the rhizosphere of grasses at Nylsvlei Nature Reserve in South Africa was conducted in terms of inhibition of Fusarium oxysporum that causes root rot in sorghum. Among the Ethiopian isolates KBE5-7, KBE5-1, KBE2-5 and NAE5-5 resulted in 100% disease suppression while disease suppressions ranging from 85.6% - 95.8% were rendered by South African isolates KBS9-H, KBS9-B, KFP9-A, NAS6-B and KBS5-F. According to identification by means of API and 16S rDNA sequencing, the majority of the effective isolates belong to members of the genus Bacillus. Other Gram negative isolates effective in this study have been identified as Serratia marcescens, Chryseomonas luteola, Stenotrophomonas maltophilia and Enterobacter sakazaki. Screening of rhizobacterial isolates was also conducted in terms ofin-vitro and in-vivo antagonistic activity against Pythium ultimum Trow, a common soilborne pathogen causing yield reductions in a wide variety of crops including sorghum. Statistically significant disease suppression was achieved by a number of isolates both from Ethiopia and South Africa. Most of the effective isolates maintained themselves in the rhizosphere at a level of ≥ 105 cfu/g four weeks after inoculation. While Bacillus cereus was the predominant isolates from both sites, Brevilbacterium laterosporus, Serratia marcescens and Pseudomonas fluorescens were among the most effective isolates with the potential to suppress Pythium ultimum in-vitro and in-vivo. Modes of action studies assessing production of antibiotics, siderophores, chitinolytic activity and induction of systemic resistance in sorghum were conducted for rhizobacterial isolates effective against F. oxysporum and P. ultimum. The antibiotic substances produced in the culture filtrates of many of these effective bacteria resulted in strong antifungal activity against both pathogens. The antibiotics from Bacillus cereus (KBS5-H) and Bacillus subtilis (KBS6-3) resulted in an efficient antagonistic activity against F. oxysporum and Pythium ultimum respectively. Siderophore production was evident in the Gram-negative strains Serratia marcescens (KBS9-R), C. violaceum (KBE9-1) and E. sakazaki (NAS6-B) with prominent yellow/orange halo development on CAS-agar plates demonstrating the potential by these isolates to produce siderophores under iron stressed conditions. Chitinolytic activity on chitin-agar plates was shown by isolates which mostly (83%) belonged to strains of B. cereus The split root system has also demonstrated that B. cereus (KBS5-H), C. violaceum (KBE9-1) and S. marcescens (KBS9-R) were capable of rendering significant induction of systemic resistance against F. oxysporum in sorghum. The successful in-vitro and in-vivo suppression of F. oxysporum and P. ultimum by the effective rhizobacterial isolates and the presence of various modes of action provide useful information on the potential of these isolates as biocontrol agents against soilborne fungal pathogens. The isolation and screening of rhizobacteria for growth promotion of sorghum has also been conducted under greenhouse condition in pathogen free soils. Three isolates from Ethiopia and 10 isolates from South Africa have been identified as the most effective growth promoting isolates in these studies. The isolates also tested positive for the production of siderophores, production of indoleacetic acid and phosphate solubilization, the direct modes of actions through which bacteria promote plant growth in the rhizosphere of several plants. Of the most effective isolates 44 % were identified as Bacillus cereus, 19 % as Chryseomonas luteola, 13 % as Serratia marcescens, 13 % as Sphingomonas paucimobilis, and 6% each as Stenotrophomonas maltophilia and Brevibacterium laterosporus respectively. The best biocontrol agents were selected out of a total of 24 isolates both from Ethiopia and South Africa. The selection procedure was conducted by using criteria such as the in-vitro and in-vivo suppression of Fusarium oxysporum and Pythium ultimum, the root colonization ability of the bacterial isolates and selected modes of action including production of antibiotic substances and siderophores, chitinolytic activity and induction of systemic resistance in sorghum. According to this procedure five isolates from Ethiopia (KBE5-7, KBE5-1, KBE9-1, NAE1-7 and NAE5-7) and six isolates from South Africa (KBS5-F, KBS9-R, KBS6-H, KBS5-H, KFP9-K and KBE6-17) have been selected as the most efficient biocontrol isolates. The selection of the best performing growth promoting isolates was conducted out of 12 efficient isolates using the following criteria: root colonization, siderophores and indoleacetic acid (IAA) production, phosphate solubilization and bacterial growth profiles in liquid cultures. Two isolates from Ethiopia (KBE7-8 and KBE9-1) and five isolates from South Africa (KBS5-H, KBS5-F, KBS6-H, KBS9-B and NAS4-3) have been selected as the best growth promoting isolates. As the screening and selection of this study are based on laboratory and greenhouse studies, further evaluation of the best isolates under field conditions and additional modes of action studies are warranted to ascertain their full potential as biocontrol and growth promoting agents. / Thesis (PhD (Plant Pathology))--University of Pretoria, 2009. / Microbiology and Plant Pathology / unrestricted

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Bibliography: leaves 162-198. This thesis investigates the mechanism(s) involved in the protection of Capsicum seedlings by two isolates of BNR against the damping-off fungus, P.u. sporangiiferum. Emphasis is placed on ecological attributes of the antagonists in relation to the pathogen, in conditions resembling those of the nursery environment for which biological control is intended. It is proposed that competition for resources, such as the host tissues with exudate-rich infection sites and the organic residue in the potting mix, is the principal factor influencing the interactions between P.u. sporangiiferum and BNR. It is also suggested that BNR isolates employ at least three strategies to exert competitive advantage over P.U. sporangiiferum. It is postulated that the ability of BNR to capture and utilise resources, in the presence of a potential competitor, is the principal attribute of these biocontrol agents that brings about a sucessful control of P.u. sporangiiferum in nursery potting mix.

Tubers of Russet Burbank were surface disinfested and 3 mm diameter by 3 mm deep wounds were made with cork borer. The holes were inoculated with 20muL of 104 macroconidia/ml suspension of (Fusarium sambucinum) or 20muL of 104 sporangia/ml suspension (Pythium ultimum) and incubated under mist. For infection studies, the inoculated tubers were exposed to 0--48 h of mist at 4--20ºC, dried and stored at 16ºC and 95% RH in growth chamber with forced air for 15 d (F. sambucinum), whereas stored at 12ºC and 95% RH for 30 d (P. ultimum). For lesion expansion studies tubers exposed to 24 h wet at 16ºC were stored in growth chambers at 4, 8, 12, and 16ºC for 15--90 d. At the end of storage tubers were cut and the volume of diseased area was measured. Models explained 94.2% of the variation in infection and 99.7% in lesion expansion for F. sambucinum . Whereas, models explained 96.7% of the variation in infection and 99.6% lesion expansion for P. ultimum. (Abstract shortened by UMI.)

Bibliography: leaves 162-198. / x, 198 leaves, [12] leaves of plates : ill. [some col.] ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis investigates the mechanism(s) involved in the protection of Capsicum seedlings by two isolates of BNR against the damping-off fungus, P.u. sporangiiferum. Emphasis is placed on ecological attributes of the antagonists in relation to the pathogen, in conditions resembling those of the nursery environment for which biological control is intended. It is proposed that competition for resources, such as the host tissues with exudate-rich infection sites and the organic residue in the potting mix, is the principal factor influencing the interactions between P.u. sporangiiferum and BNR. It is also suggested that BNR isolates employ at least three strategies to exert competitive advantage over P.U. sporangiiferum. It is postulated that the ability of BNR to capture and utilise resources, in the presence of a potential competitor, is the principal attribute of these biocontrol agents that brings about a sucessful control of P.u. sporangiiferum in nursery potting mix. / Thesis (Ph.D.)--University of Adelaide, Dept. of Crop Protection, 1997

Thèse (Ph. D.)--Université Laval, 2007. / Bibliogr.: f. 120-138. Publié aussi en version électronique dans la Collection Mémoires et thèses électroniques.