Agricultural By-products as Substrates for Growth, Conidiation and Chlamydospore Formation by a Potential Mycoherbicide, Fusarium oxysporum Strain EN4

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Abstract
Characteristics, such as mycelial growth, conidiation and chlamydospore formation, of three distinct variants (EN4-FT, EN4-FF and EN4-S), of the potential mycoherbicide, Fusarium oxysporum, strain EN4 were studied in liquid cultures. Agricultural by-products, such as cornmeal, corn cob, soya bean hull fiber (Dietfiber-Soyfiber), cotton seed embryo flour (Pharmamedia), cane molasses yeast extract and potato dextrose medium were used as substrates. The mycelial growth and conidia and chlamydospore formation were different for the three strains and varied with the substrate used. The quantity of conidia and chlamydospores produced depended on the concentration of the substrates. Irrespective of carbon:nitrogen ratios, chlamydospores were formed readily in liquid media and in greater amount on substrates with low utilizable carbon content (aqueous extracts of soya bean hull fiber and corn cob) than on those with higher utilizable carbon content (potato dextrose broth, aqueous extracts of cotton seeds and molasses yeast extract broth). In liquid cultures, increases in the concentration of substrates high in utilizable carbon resulted in reduced chlamydospore formation; however, this was far less in those with a lower carbon content. The effect on fungal growth characteristics due to the presence or absence of plant fibers in various concentrations of unfiltered and filtered extracts of soya bean hull fiber depended on the fungal variant tested. However, it had a greater effect on viable and microconidial counts than on chlamydospore and macroconidial counts. The viable and microconidial counts were significantly higher in unfiltered than in filtered extracts. The final pH of the liquid cultures (4.1-8.5), after 10 days of growth, was affected by the fungal variant, the substrate used and by the substrate concentration. In liquid cultures with 1% soya bean hull fiber, chlamydospore formation corresponded to an increase in the pH of the medium from 6.0 to 7.7. The interaction between fungal variants, the substrate used and the substrate concentration was highly significant for all the variables tested, indicating that optimization of the production of fungal propagules is complex. Overall, for all three variants, the best substrates for optimum conidiation and chlamydospore formation were aqueous extracts of soya bean hull fiber and corn cob at concentrations of 1-2.5%.