In an effort to characterize the relationship between fungal pathogenicity and host
genetic resistance, sorghum panicles of 11 genotypes were inoculated with five fungi
frequently isolated from sorghum grain. Panicles were inoculated at anthesis with
Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and
Alternaria alternata spores. There were highly significant differences in the levels of
fungal pathogenicity on different sorghum genotypes. These differences accounted for
58.4% of observed variation in ergosterol concentration. Genotype by pathogen (G x P)
interactions accounted for 33.5% of the observed ergosterol concentration variation. The
implication is that different genotypes reacted differently to different fungi. The
genotypic reactions of the hosts accounted for 8.1% of the observed ergosterol
concentration variation. Overall, fungal pathogenicity is the most important factor to
consider in the evaluation of germplasm for grain mould resistance. Possible sources of
resistance could be identified by use of biplot analysis of G x P interactions. Visual
scoring for grain mould has limited value without identifying causal fungi. Fusarium
thapsinum and Phoma sorghina were the most abundant fungi across all genotypes.
Gene action and heritability for grain mould resistance in sorghum were
investigated using a selection of 9 random pollen parents with varying levels of grain
mould resistance to a different set of three random seed parents. Differences in ergosterol
concentration were used as a measure of level of grain mould resistance among all
genotypes. Use of ergosterol concentration as a measure of grain mould severity did not
correlate with visual field scoring. Other traits measured include plant yield, plant height,
kernel hardness, field grade score, days to flowering, glume color and seed color. The combined analysis of variance showed no genotypic variance for grain mould resistance.
The expression of grain mould resistance was also not stable with significant genotype x
location interaction. The analysis from Potchefstroom showed significant differences
among genotypes whereas Cedara-1 and 2 showed no differences. Additive genetic
variance was greater than dominance variance for all traits except grain mould resistance.
A significant heterosis of -20.15% was observed for grain mould resistance indicating the
importance of use of hybrid seed. LM124 (white seeded female) and LM130 (brown
seeded female) produced resistant hybrids. Due to very high environmental variance,
grain mould heritability could not be detected. Yield indicated the highest heritability of
0.41 at Cedara-1, plant height 0.39 at Cedara-2 and kernel hardness 0.38 at
Potchefstroom.
The significance of genotype by environment interaction was assessed over three
environments. Differences in ergosterol concentration were used as a measure of level of
grain mould resistance among all genotypes. Significant G x E interaction was detected
after analysis of variance across all three locations. Single site analysis was then done to
better explain the nature of the G x E interaction. Potchefstroom is the only location that
showed significant genotypic responses to grain mould infection. Low grain mould
pressure at Cedara-1 and Cedara-2 caused very low genotypic responses. A biplot was
then used to indicate all genotypic performances across the three locations in a graphical
design. Fungal species infecting sorghum kernels were isolated, counted and identified.
Low genotypic responses at the Cedara locations seems to have been caused by
prevalence of a less aggressive fungal species, Mucor spp. Weather variables did not
correlate significantly with ergosterol concentration. Mycotoxin levels of aflatoxin, deoxyvalenol (DON), and zearalenone together
with ergosterol concentration levels across 39 sorghum genotypes were measured.
Varying levels of ergosterol and mycotoxins were observed across the three locations.
There was no correlation between ergosterol concentration and any of the mycotoxins
which indicates that mycotoxins concentration is not related to total fungal biomass.
Mycotoxin concentration must therefore be related only to the biomass of the fungal
species that is producing the mycotoxin among all fungal species infesting the grain.
Farmers in Potchefstroom should be wary of aflatoxins and zearalenone while farmers in
Cedara should choose varieties that tend to be low in DON and aflatoxins while not
ignoring zearalenone.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-02032010-091704 |
| Date | 03 February 2010 |
| Creators | Mpofu, Leo Thokoza |
| Contributors | Prof CS van Deventer, Prof NW McLaren |
| 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-02032010-091704/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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