The phase out of well-known chemical control products, especially soil
fumigants, is a reality and biological control strategies need to be explored.
Biological control rarely eliminates a pathogen from the soil, usually it reduces
its numbers or ability to induce disease. However combining complementary
biological control tactics may become the only tool in integrated control
management for soilborne diseases. The biggest challenge will be to determine
the most feasible sequence of techniques for optimal control of each pathogen,
farming system and geographic area. Other factors that may have an influence
on the efficacy of any bio-control strategy are host related (genetic resistance),
environment related (longevity. soil type & climate) and pathogen related
(virulence, inoculum density, survival).
There is considerable interest in biofumigation as an alternative to
synthetic soil fumigants in horticulture and agriculture. The term biofumigation
usually referrers to the biocidal effects of isothiocyanates (ITCâs) released from
Brassicaceous plants. It can and should however be extended to other volatile
bio-substances with the same qualities like thiophene, a heterocyclic,
sulphurous compound released by Tagetes spp.
The rhizosphere presents a complex integrated ecosystem. Biotic and
abiotic factors present, can determine the dynamics and structure of the
microbial population in a specific rhizosphere. Screening different crops for
rhizosphere suppression of soilborne pathogens can be a point of departure to
select crops for a specific crop rotation system. A series of summer- and winter
crops were evaluated for rhizosphere suppression of Ralstonia solanacearum (Race 1, Biovar III), causal agent of bacterial wilt in tobacco. Ocimum basilicum
(sweet basil), Tagetes erecta (giant marigold), T. patula (dwarf marigolds),
Coriandrum sativum L. (coriander) and Triticum aestivum L. (wheat cv. Inia)
resulted in a significant control of the pathogen. However all Tagetes spp.had
the lowest pathogen population count of all crops evaluated.
A greenhouse trial conducted in 0,5 m deep soil indicated that the T.
patula in combination with non-host winter crops, could reduce the pathogen
population of the soil to such an extent that the disease incidence on the followup
tobacco crop was 40% lower than that of the initial tobacco crop. A four-year
rotation trial on a heavily infested field showed that T. patula in combination with
wheat was responsible for a significantly higher tobacco yield (green leaf mass).
In both greenhouse and field trials, cabbage (Brassica oleracea L. var capitata.)
proved to be a latent host, suggesting that it should be avoided as a winter cash
crop in tobacco fields infested with R. solanacearum.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-07292008-094028 |
| Date | 29 July 2008 |
| Creators | Terblanche, Johanna Dina |
| Contributors | Prof WJ Swart |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-07292008-094028/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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