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Pythium damping-off of lucerne in South Africa
Thesis (M. Sc.) -- University of Stellenbosch, 1992.
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| Format: | Thesis |
| Language: | English |
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Stellenbosch : Stellenbosch University
2012
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| _version_ | 1867613875698925569 |
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| access_status_str | Open Access |
| author | Denman, Sandra |
| author2 | Knox-Davies, P. S. |
| author_browse | Denman, Sandra Knox-Davies, P. S. |
| author_facet | Knox-Davies, P. S. Denman, Sandra |
| author_sort | Denman, Sandra |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (M. Sc.) -- University of Stellenbosch, 1992. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/69752 |
| institution | Stellenbosch University (South Africa) |
| language | English |
| last_indexed | 2026-06-10T12:43:06.129Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2012 |
| publishDateRange | 2012 |
| publishDateSort | 2012 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/69752 Pythium damping-off of lucerne in South Africa Denman, Sandra Knox-Davies, P. S. Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology. Pythium Phytopathogenic microorganisms Alfalfa -- Diseases and pests Damping-off diseases Dissertations -- Plant pathology Thesis (M. Sc.) -- University of Stellenbosch, 1992. In response to an international move towards consolidating current work on the genus Pythium, and a need for such work in South Africa, I have compiled an overview of Pythium species, the diseases they cause and their hosts in South Africa from 1926 to 1989. Contributions from the various authors who have published information on Pythium diseases in South Africa, are summarised and briefly discussed. Special emphasis is placed on the pioneering work of V.A. Wager. It is concluded that Pythium species have caused many disease problems in South Africa in the past, and will continue to do so in the future. Various media and methods were used to isolate Pythium spp. from lucerne seedlings. Seven basal media (CMA, LBA, OA, PCA, PDA, V8, WA), three selective media (GASM, MPVM, VP3), and basal media amended with antibiotics (penicillin, pimaricin, streptomycin, vancomycin) were screened. Selective media yielded the largest proportion of pure Pythium spp. Significantly (P < 0,001) more Pythium was recovered from basal media than from the other media. Basal media, especially water agar (WA), together with selective media, are recommended for isolating Pythium spp. from lucerne seedlings. The effect of surface disinfestation with 1% sodium hypochlorite for 1 min on the recovery of Pythium spp. was also studied. Significantly (P < 0,001) less Pythium spp. were recovered from surface-disinfested tissue than from non-surface-disinfested tissue. Surface disinfestation is therefore not recommended when isolating Pythium spp. from lucerne seedlings. Glass rings were used for reducing bacterial contamination of cultures. It is concluded that bacterium-free, single-hyphal-tip Pythium cultures can be obtained when isolates are transferred to glass rings embedded in WA with 250 ppm vancomycin. Pythium isolates obtained from damped-off lucerne (Medicago sativa cv. SA Standard) seedlings were identified to species level using morphological features. Three species were identified: P. irregulare, P. sylvaticum and P. ultimum var. ultimum. Intraspecific differences of oogonial and oospore diameters occurred and sometimes exceeded the limits defined by the ranges in the sources consulted. This might be reduced if the methods of preparation of material for identification were standardised and if a statistically-valid number of measurements per structure were used. I therefore suggest that the universal adoption of a single system of identification and standardisation of the methods used might help reduce incorrect identification. Pythium irregulare, P. sylvaticum and P. ultimum var. ultimum were isolated from damped-off lucerne (Medicago sativa) cv. SA Standard and their pathogenicity tested on the same cultivar. Three levels of sand-bran inoculum were used: 0.5%, 2.5% and 5% (mass of inoculum to mass of planting medium). All three species were pathogenic to lucerne. They caused over 80% pre-emergence damping-off. There were differences in virulence among the species and isolates tested. P. sylvaticum was the most virulent and P. irregulare the least virulent. At the higher inoculum levels (2.5 and 5%) survival was so low in some cases, that inter- and intra-species differences could not be detected. This is the first report of the pathogenicity of P. sylvaticum to lucerne in South Africa. Field trials were conducted in naturally-infested soil at George in an attempt to control Pythium damping-off of lucerne (Medicago sativa). The following were tested: seed treatments with fungicides (captab/metalaxyl, captab/metalaxyl + a (acetone imbibition application), diazoxolon and thiram), in combination with planting dates (monthly from March to August), cultivars (Baronette, Cuf 101, Diamond, Meteor and SA Standard) and sowing densities (2.5, 5, 10, 15 and 20 kg/ha). Preliminary results indicated that both the captab/metalaxyl and captab/metalaxyl + a treatments were effective in increasing seedling survival. SA Standard sown at low densities (2.5 or 5 kg/ha) had a high survival rate. The effect of fungicidal treatments on root nodulation was evaluated but the results could not be analysed statistically. It is therefore suggested that an alternative method of rating nodulation be adopted. Masters 2012-08-27T12:27:14Z 2012-08-27T12:27:14Z 1992 Thesis http://hdl.handle.net/10019.1/69752 en Stellenbosch University 91 pages : ill. application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Pythium Phytopathogenic microorganisms Alfalfa -- Diseases and pests Damping-off diseases Dissertations -- Plant pathology Denman, Sandra Pythium damping-off of lucerne in South Africa |
| title | Pythium damping-off of lucerne in South Africa |
| title_full | Pythium damping-off of lucerne in South Africa |
| title_fullStr | Pythium damping-off of lucerne in South Africa |
| title_full_unstemmed | Pythium damping-off of lucerne in South Africa |
| title_short | Pythium damping-off of lucerne in South Africa |
| title_sort | pythium damping off of lucerne in south africa |
| topic | Pythium Phytopathogenic microorganisms Alfalfa -- Diseases and pests Damping-off diseases Dissertations -- Plant pathology |
| url | http://hdl.handle.net/10019.1/69752 |
| work_keys_str_mv | AT denmansandra pythiumdampingoffoflucerneinsouthafrica |