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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Basis for the biocontrol of Pythium by fluorescent pseudomonads

Ellis, Richard John January 1997 (has links)
The aim of this thesis was to gain an understanding of the molecular and ecological basis for the biological control of Pythium by fluorescent pseudomonads. A fluorescent pseudomonad biocontrol agent (BCA), Pseudomonas fluorescens 54/96, identified as a potential candidate for commercial development, was analysed together with transposon induced mutants in a variety of assays for anti-fungal activity (Chapter 2). It was revealed that 54/96 had a fungistatic effect generated by a number of different mechanisms, which included nutrient competition and antibiosis. The synecology of this organism with Pythium was then compared to a similar organism (P. fluorescens SBW25) demonstrating a similar degree of anti-fungal activity (Chapter 3). The similarity of the population dynamics of these two strains prompted an examination of the genetic basis for the anti-fungal activity of the second strain, with the intention of comparing with 54/96 (Chapter 4). Again this revealed a multifactorial mode of action of SBW25 against Pythium. Whilst some mutants with reduced anti-fungal activity were deficient in growth on seed exudate others were unaffected, but the mechanisms appeared to be different to those utilized by 54/96. The comparison of strains was expanded to a larger collection of pseudomonad BCAs which were contrasted by a number of phenotypic and genotypic methods (Chapter 5). Various markers were identified which showed commonality within the different classes of BCA, the most useful of which was cyclopropanated fatty acids. These may prove to be a useful marker when screening for new pseudomonad BCAs. It was concluded that a greater understanding of the molecular, physiological and ecological basis of anti-fungal activity of bacterial will lead to the development of biocontrol strategies with improved efficacy.
2

Implications of green manure amendments on soil seed bank dynamics /

Short, Nicolyn. January 2006 (has links)
Thesis(Ph.D.)--University of Western Australia, 2006.
3

Evaluation of Field Pea Varieties for Resistance to Fusarium Root Rot Pathogens

Odom, Jennifer Lorraine January 2017 (has links)
Fusarium root rot is one of the most important diseases of pulse crops, with numerous Fusarium spp. comprising the disease complex. Fusarium solani and F. avenaceum have been reported to be major pathogens in the pea root rot complex, and all commonly grown varieties are susceptible. Greenhouse methods to evaluate peas for resistance to Fusarium root rot resulted in inconsistent disease severity across varieties. In 2015, F. avenaceum infested field plots were more heavily damaged based on emergence and yield than F. solani infested plots, and opposite trends were observed in 2016. Differences in root rot severity between years could be due to F. solani infestation causing more damage under warmer temperatures, while plots infested with F. avenaceum caused more damage under cooler temperatures. These results highlight the difficulties observed when screening for soil-borne pathogens, and the increased difficulties when a pathogen complex and changing environmental conditions are involved.
4

Characterisation of Rhizoctonia in cropping systems in the Western Cape Province

Tewoldemedhin, Yared Tesfai 03 1900 (has links)
Thesis (MScAgric (Plant Pathology))--University of Stellenbosch, 2005. / The genus Rhizoctonia includes important soilborne plant pathogens that can cause severe economic losses on a wide range of crops including cereal, canola, pasture and grain legume crops. Limited information is available on specific anastomosis groups (AGs), nuclear status, pathogenicity and cross-pathogenicity of Rhizoctonia isolates associated with each of these crops in the Western Cape province. Therefore, these aspects were investigated in this study, since information in this regard is crucial for the development of Rhizoctonia disease management practices.
5

Evaluation of seed and drench treatments for management of damping-off and seedling blight pathogens of spinach for organic production

Cummings, Jaime Anne, January 2007 (has links) (PDF)
Thesis (M.S. in plant pathology)--Washington State University, December 2007. / Includes bibliographical references.
6

Biology, epidemiology, and biological and chemical control of Phytophthora vignae

Fernando, W. Gerard Dilantha 04 October 1990 (has links)
Phytophthora vignae, causal agent of stem and root rot of cowpea (Vigna unguiculata), was reported for the first time in Sri Lanka. The pathogen was found in cowpea field soils from 3 of 5 geographic regions sampled. Only one site however, had plants exhibiting disease symptoms. Of the eight cowpea varieties grown in Sri Lanka, four were shown to be relatively resistant; all other legumes inoculated were completely resistant. Two morphologic and physiologic races of P. vignae were identified among the 24 isolates recovered, based on differential pathogenicity on cowpea varieties. Bacteria isolated from field soils, and other known bacterial biocontrol agents, inhibited P. vignae in culture, but only three Sri Lankan isolates considerably suppressed the disease in greenhouse tests. Volatile substances produced by most bacteria inhibited mycelial growth and sporangial production by P. vignae. The increased pH of the exposed medium suggested the involvement of ammonia. Volatile inhibitors were produced by these bacteria in soil, but only with added substrate; Strain DF-3101 also reduced oospore germination in soil. Cowpea plants inoculated with the VA mycorrhizal (VAM) fungus Glomus intraradices in P. vignae-infested soil were larger than non-mycorrhizal plants, but only at low levels of the pathogen. VAM colonization was reduced at high levels of the pathogen, and root infection by the pathogen was reduced by VAM. The fungicides metalaxyl, fosetyl-Al, Banrot, and Manzate-200DF reduced in vitro mycelial growth, but at different concentrations. Sporangia formation and germination, and oogonia formation by P. vignae, was reduced significantly by metalaxyl and fosetyl-Al. In greenhouse tests, metalaxyl, even at low concentrations, reduced disease; Fosetyl-Al was effective at high concentrations; Manzate-200DF was effective as a soil drench but not as a foliar spray; Banrot effectively reduced disease at 50 mg a.i./L. Exposure of a bacterial biocontrol agent to these fungicides in vitro did not affect its capacity to subsequently produce volatile inhibitors, but exposure to 10 ug/ml of metalaxyl and 50 ug/ml of Manzate-200DF reduced its capacity to subsequently inhibit mycelial growth of P. vignae. / Graduation date: 1991
7

In vitro and in vivo screening of Bacillus spp. for biological control of Rhizoctonia solani.

Kubheka, Bongani Petros. January 2003 (has links)
The increasing concerns about chemical pesticides that are environmentally hazardous and the continuous development of resistance by palhogens to chemical pesticides have led to this study. Many studies have shown that some Gram-negative bacteria, such as Pseudomonas flouresens, control plant diseases and promote plant growth. In this study Gram positive bacteria, Bacillus sp., were chosen because of their ability to produce endospores. Endospores can be used in stable, dry formulations. The advantage of using endospores is their ability to survive harsh conditions such as droughts and high temperatures, which give a long shelf life to the biological control agent. Bacillus isolates were recovered from the rhizosphere of 12 different crops, and were subsequently screened in vitro for their antimicrobial activity. Of 130 isolates, 87 exhibited antimicrobial activity against the test organisms: Rhizoctonia solani, Pythium sp., Phytophthora cinnamoni, Fusarium sp., and single representatives of Gram negative and Gram positive bacteria, namely, Erwinia carotovora and Staphylococcus aureus respectively. The Bacillus isolates B77, B81 and B69 inhibited all the test organisms investigated, which suggests that they produced broad spectrum antimicrobial compounds or more than one antimicrobial compound. Of the isolates that showed antimicrobial activity, 78 of them did not inhibit Trichoderma harzianum K D, which is a registered biological control agent; indicating their potential for combined application. Selected Bacillus isolates were tested for the biological control of R. solani under greenhouse conditions in wheat, cabbage, tomato, maize, and cucumber seedlings. Bacillus isolates were applied as seed treatments, and the inoculated seeds were planted in R. solani infested speedling trays. Shoot dry weight measurement of seedlings indicated that 12 out of 19 Bacillus isolates showed significantly different shoot dry weight in wheat whereas all the isolates tested in tomato and cucumber gave significantly different shoot dry weight. No significantly different shoot dry weight was obtained for maize or cabbage. Seed emergence findings indicated that none of the Bacillus isolates gave significantly different emergence percentage on wheat, cabbage, tomato, and maize but all of them showed significantly different emergence percentage on cucumber. The results indicate that both the pathogen and the biological control agents exhibited varying levels of specificity on each crop tested. The biological control potential of the best Bacillus isolates was tested on bean and maize crops in the field. Green bean and maize seeds were coated with the selected Bacillus isolates and then sown under field conditions. For each isolate, four replicate treatment plots were established, with and without a R. solani inoculum. Percentage emergence, plant survival levels to harvesting and yield of maize cobs and green beans pods were measured. For all parameters measured the positive and negative controls were not significantly different thereby rendering the results for the entire field study inconclusive. However, Bacillus isolates B77, BII, R5 and R7 improved green bean pod yield and Bacillus Isolate B8I increased maize yield, indicating their potentials as plant growth promoting rhizobacteria (PGPR). / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.
8

Biological control and plant growth promotion by selected trichoderma and Bacillus species.

Yobo, Kwasi Sackey. January 2005 (has links)
Various Trichoderma and Bacillus spp. have been documented as being antagonistic to a wide range of soilborne plant pathogens, as well as being plant growth stimulants. Successes in biological control and plant growth promotion research has led to the development of various Trichoderma and Bacillus products, which are available commercially. This study was conducted to evaluate the effect of six Trichoderma spp. and three Bacillus spp. and their respective combinations, for the biological control of Rhizoctonia solani damping-off of cucumber and plant growth promotion of dry bean (Phaseolus vulgaris L.). In vivo biological control and growth promotion studies were carried out under greenhouse and shadehouse conditions with the use of seed treatment as the method of application. In vitro and in vivo screening was undertaken to select the best Trichoderma isolates from 20 Trichoderma isolated from composted soil. For in vitro screening, dual culture bioassays were undertaken and assessed for antagonisms/antibiosis using the Bell test ratings and a proposed Invasive Ability rating based on a scale of 1-4 for possible mycoparasitic/hyperparasitic activity. The isolates were further screened in vivo under greenhouse conditions for antagonistic activity against R. solani damping-off of cucumber (Cucumis sativus L.) cv. Ashley seedlings. The data generated from the in vivo greenhouse screening with cucumber plants were analysed and grouped according to performance of isolates using Ward‟s Cluster Analysis based on a four cluster solution to select the best isolates in vivo. Isolates exhibiting marked mycoparasitism of R. solani (during ultrastructural studies) viz, T. atroviride SY3A and T. harzianum SYN, were found to be the best biological control agents in vivo with 62.50 and 60.06% control of R. solani damping-off of cucumber respectively. The in vitro mode of action of the commercial Trichoderma product, Eco-T®, and Bacillus B69 and B81 suggested the production of antimicrobial substances active against R. solani. In vitro interaction studies on V8 tomato juice medium showed that the Trichoderma and Bacillus isolates did not antagonise each other, indicating the possibility of using the two organisms together for biological control and plant growth promotion studies. Greenhouse studies indicated that combined inoculation of T. atroviride SYN6 and Bacillus B69 gave the greatest plant growth promotion (43.0% over the uninoculated control) of bean seedlings in terms of seedling dry biomass. This was confirmed during in vivo rhizotron studies. However, results obtained from two successive bean yield trials in the greenhouse did not correlate with the seedling trials. Moreover, no increase in protein or fat content of bean seed for selected treatments was observed. In the biological control trials with cucumber seedlings, none of the Trichoderma and Bacillus combinations was better than single inoculations of Eco-T®, T. atroviride SY3A and T. harzianum SYN. Under nutrient limiting conditions, dry bean plants treated with single and dual inoculations of Trichoderma and Bacillus isolates exhibited a greater photosynthetic efficiency that the unfertilized control plants. Bacillus B77, under nutrient limiting conditions, caused 126.0% increase in dry biomass of bean seedlings after a 35-day period. Nitrogen concentrations significantly increased in leaves of plants treated with Trichoderma-Bacillus isolates. However, no significant differences in potassium and calcium concentrations were found. Integrated control (i.e. combining chemical and biological treatments) of R. solani damping-off of cucumber seedlings proved successful. In vitro bioassays with three Rizolex® concentrations, viz., 0.01g.l-1, 0.1g.l-1 and 0.25g.l-1 indicated that the selected Trichoderma isolates were partly sensitive to these concentrations whereas the Bacillus isolates were not at all affected. In a greenhouse trial, up to 86% control was achieved by integrating 0.1g.l-1 Rizolex® with T. harzianum SYN, which was comparable to the full strength Rizolex® (1g.l-1) application. Irrespective of either a single or dual inoculations of Trichoderma and/or Bacillus isolates used, improved percentage seedling survival as achieved with the integrated system, indicating a synergistic effect. The results presented in this thesis further reinforce the concept of biological control by Trichoderma and Bacillus spp. as an alternative disease control strategy. Furthermore, this thesis forms a basis for Trichoderma-Bacillus interaction studies and proposes that the two organisms could be used together to enhance biological control and plant growth promotion. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
9

Biological control of Phytophthora root rot of citrus seedlings and cuttings.

Abraham, Abraha Okbasillasie. January 2005 (has links)
With an increasing realization that many agrochemicals are hazardous to animals and humans, came the desire to replace these chemical agents with biological approaches that are more friendly to the environment and human health. Microorganisms play an important role in plant disease control, as naturally occurring antagonists. Microorganisms may also have beneficial effects on plant development when applied to plant roots. Research efforts worldwide have recorded successes in biological control and growth stimulation on many crops, particularly when using members of the genera Bacillus and Trichoderma. Their use on citrus rootstock could be advantageous to nurserymen and growers in reducing the incidence of seedling mortality and increasing production. To achieve these objectives, laboratory and tunnel experiments were conducted to develop effective biocontrol agents for citrus seedlings and cuttings. Nineteen 0 ut 0 f 23 Trichoderma isolates tested in vitro against Phytophthora p arasitica sp showed antagonistic activity by hyperparasitism and four out of eight Bacillus isolates resulted in antagonism by forming inhibition zones. The positive in vitro activity of Trichoderma and Bacillus isolates on Phytophthora provided motivation step for further trials in the greenhouse to evaluate their biological control activity on citrus seedlings and cuttings. A greenhouse trial was carried out to evaluate the biological control potential of 23 Trichoderma isolates (drenched at 5 x 105 spores / rnI) and two Bacillus isolates (drenched at 1 X 106 or 1 X 108 colony forming units (CFU) / rnI) to suppress Phytophthora parasitica sp. of rough lemon (Citrus jambhirini Lush.) seedlings. Five isolates ofTrichoderma (AA12, AA5, Trichoderma harzianum (AA16), SY3F and Eco-T~ were highly effective in suppressing Phytophthora root rot, with AA12 providing the best control. The Bacillus isolates also suppressed the pathogen but were not as effective as the Trichoderma isolates. This trial was used to test for growth stimulation activity by some of the biocontrol agents. To verify these results, a further trial was carried out to evaluate growth stimulation capabilities in the absence of any pathogen. Trichoderma Isolates AA13 and AA17 caused no 111 change in seedling growth, while other Trichoderma and Bacillus isolates had an inhibitory effect on the seedling growth. This trial indicated that the biocontrol activity was affected by inoculum densities, and as a result in vitro sporulation capacity was evaluated. TrichodermaIsolate AA16 was the largest spore producer, followed by Eco-T®. Spore production was lowest from Trichoderma isolates AA4 and AA12. Growth stimulation responses of Trichoderma Isolates AA4, AA16, Eco-TID and SYN6 were further studied at four different doses (1 X 103, 1 X 104, 5 X 105 or 1 X 106 spores / ml) on rough lemon and trifoliate orange seedlings. Trifoliate oranges responded positively to 1 X 104 and 5 X 105 spores / ml of Eco-TID, but rough lemon responded negatively to all dosages of the Trichoderma isolates applied. This indicates that the inoculum density responses may be host specific. Higher population density of 1 X 106 spores / ml of all tested Trichoderma isolates had a stunting effect on seedling growth of both species. Based on t he positive results 0 f individual applications of some Trichoderma and Bacillus isolates, of the biological control agents on rough lemon seedlings against Phytophthora parasitica in an earlier greenhouse trial, their combined effect in the control of the pathogen was performed. Before carrying out a greenhouse trial, activities of the isolates to be combined were evaluated in vitro. This trial showed that Trichoderma Isolates AA16 and Eco-T®were compatible. Trichoderma isolates AA16 and Eco-T®were also found to be compatible with Bacillus Isolates B77, B81 and PHP. As a result, further in vivo trials were conducted. The tunnel trials were carried out as two separate experiments: In the first experiment, a combination of two Trichoderma Isolates A A 16 and Eco-T®was conducted assayed at 5 X 105 or 1 X 106 spores / ml, on rough lemon seedling, and cuttings and trifoliate orange and sour orange seedlings. A combination of Trichoderma isolate AA16 and Eco-T®at 5 X 105 spore / ml increased significantly the new flush biomass of rough lemon cuttings compared to AA16 alone, but was not different from Eco-TID alone. The combination of AA16 and Eco-T® achieved no change of biomass of rough lemon and trifoliate orange seedlings. The combination of AA16 and Eco-TID did not increase the root biomass of sour orange compared to AA16 or Eco-r® alone. The combination of AA16 and Eco-r® at higher doses (1 x 106 spores / ml) showed significantly better suppression of Phytophthora root rot of rough lemon cuttings but did not show disease suppression in all seedling species verities tested. In a second experiment, individual and combined effects of Trichoderma isolates (drenched at 5 X 105 spores / ml) with Bacillus isolate (drenched at 1 X 106 colony forming units (CFU) / ml) for suppression of Phytophthora root rot on rough lemon and trifoliate orange seedlings was performed. The combination of Trichoderma Isolate AA16 and Bacillus Isolate B81 increased root biomass on rough lemon seedlings compared to the combination of Trichoderma AAI6 or Bacillus PHP but was not significantly different to Trichoderma AA16 alone. Bacillus PHP combined with Trichoderma AA16 or singly had no effect on rough lemon seedlings. Combining Trichoderma Eco--r® and with Bacillus B8I or PHP did not increase biomass of rough lemon seedlings compared to Trichoderma Isolate Eco--r® alone. There was no statistically significant differences in the effects of the combinations of the Trichoderma and Bacillus isolates compared to their individual applications on the biomass of trifoliate oranges. This study established the antagonistic potential of several South African isolates of Trichoderma and Bacillus as a viable alternative to agrochemicals for controlling Phytophthora parasitica. The growth stimulation capabilities of Trichoderma isolates in terms of seedling development was also demonstrated. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
10

Implications of green manure amendments on soil seed bank dynamics

Short, Nicolyn January 2006 (has links)
[Truncated abstract] Weeds are a major limitation to agricultural and horticultural production and the main method of control is the use of herbicides. In addition to the resulting chemical pollution of the environment, the wide spread and continues use of herbicides have resulted in many weeds developing resistance to commonly used herbicides. This study investigated the potential of using green manures as a cultural method of control of weed invasion in agricultural fields. To understand the general mechanisms involved in the suppression of seed germination in green manure amended soils, seeds of crop species with little or no dormancy requirements were used in certain studies. Lettuce (Lactuca sativa) and cress (Lepidium sativum) seeds were sown to a sandy soil amended with green manures of lupin (Lupinus angustifolius), Brassica juncea, or oats (Avena sativa) to determine if the amendments affected seed germination and/or decay. It was hypothesised that the addition of plant material would increase the microbial activity of the soil thereby increasing seed decay, under laboratory and greenhouse conditions. Initial experiments used lettuce, cress and lupin seeds. Lettuce and cress are commonly used as standard test species for seed viability studies. Subsequent experiments used seeds of annual ryegrass (Lolium rigidum), silver grass (Vulpia bromoides), wild radish (Raphanus raphanistrum) and wild oat (Avena fatua) as these weed species are commonly found throughout agricultural regions in Western Australia. Amending the soil with lupin or Brassica green manure was established as treatments capable of developing environments suppressive to seed germination. Lupin residues as green manure showed the strongest inhibition of seed germination and seed decay. The decay of certain seeds was enhanced with changes to soil microbial activity, dissolved organic carbon and carbon and nitrogen amounts in lupin amended soil. Seeds of weed species were decayed in lupin amended soil, but showed varied degree of decay. Annual ryegrass and silver grass were severely decayed and wild oat and wild radish were less decayed, in lupin amended soil.

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