Die rol van endofitiese fungusse by enkele siektes van Sorghum bicolor
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The aim of this study was to determine the potential of endophytic fungi as biological control agents. Consequently, in vitro tests were initially conducted to determine the measure of antagonism which these fungi displayed towards the pathogens, Macrophomona phaseolina and Phoma sorghina. Endophytic fungi were isolated from sorghum plants and after initial tests a few endophytes, namely, Cladosporium cladosporioides, Epicoccum purpurascens, Fusarium equiseti, Paecilomyces lilacinus, Penicillium chrysogenum en Phoma sp.4 were selected for further experiments. In order to determine the measure of antagonism the pathogens and endophytes were cultured separately and in certain combinations on three growth media, namely potato-dextrose-agar, sorghumagar and wheatstraw. C.cladosporioides, E.purpurascens, F.equiseti, Paecilomyces lilacinus, Penicillium chrysogenum en Phoma sp.4 are, to a certain extent, capable of retarding the growth of the pathogens on all three growth media. Paecilomyces lilacinus and E.purpurascens proved to have the highest antagonistic potential when compared with M.phaseolina and Phoma sorghina. Tha potensial of the endophytes to colonise sorghum seeds in the presence of pathogens was also determined. All the endophytes obtained a high percentage of colonisation of sorghum seeds in the presence of M.phaseolina. However, in the presence of Phoma sorghina only E.purpurascens, F.equiseti, Paecilomyces lilacinus and Penicillium chrysogenum showed a high percentage of colonisation. As this aspect was also studied in vivo, methods of infecting sorghum plants artificially with the endophyte were considered. The influence of the endophyte on the pathogen and on the plant was determined by means of chlorophyl.; fluorescence measurements. This measure is sensitive to any detrimental or advantageous effects that the fungi might have on the phisiology of the plant. In the presence of M.phaseolina only E.purpurascens had an advantageous effect on photosynthesis, while C.cladosporioides, E.purpurascens, F.equiseti, Paecilomyces lilacinus and Penicillium chrysogenum in combination with Phoma sorghina made a positive contribution. C.cladosporioides, E.purpurascens, F.equiseti, Penicillium chrysogenum and Phoma sp.4 show a tendency to favour fluorescence quenching (r2) in the presence of M.phaseolina. In the presence of Phoma sorghina only Paecilomyces lilacinus made a positive contribution. Fe-energy distribution during photosynthesis is favoured by F.equiseti in combination with M.phaseolina while energy distribution is favoured by F.equiseti and Penicillium chrysogenum in the presence of Phoma sorghina. Preservation of thylakoid membrane-integrity in the presence of M.phaseolina was favoured by F.equiseti and Phoma sp.4. The same was observed with Phoma sorghina in combination with E.purpurasccens, F.equiseti and Phoma sp.4. Although in vitro tests indicated antagonism between Paecilomyces lilacinus and M.phaseolina it was not confirmed by fluorescence determinations. A positive effect on different aspects of photosynthesis reaction was indicated in all cases where Phoma sp.4 was present together with M.phaseolina in the plant. The results suggested that the endophytes were physiologically, probable not adapted to the plant. This could explain why in vivo the endophytes did not have such a large effect on the pathogens as they had in vitro.