Studies of integrated control of selected root diseases of sunflowers using Trichoderma harzianum (ECO-T®) and silicon

Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.The soil-borne fungi Rhizoctonia solani Kuhn and Sclerotinia sclerotiorum De Bary are ubiquitous plant pathogens with a wide host range. They are among the most widespread and destructive diseases of many crops, including sunflowers. Although in many cases, the use of chemicals appears to be the most economical and efficient means of controlling plant pathogens, their environmental concerns and the development of tolerance in pathogen populations have led to drastic reduction in their usage and increased the need to find alternative means of disease control. The potential benefits of applying Trichoderma harzianum Rifai and silicon (Si) nutrition to plants have been extensively reviewed. In this study, the ability of T. harzianum (Eco-T®), soluble silicon, and their combination was evaluated on sunflower (Helianthus annuus L.), for their potential to suppress pathogenic strains of R. solani and S. sclerotiorum. The ability of this crop to take up and accumulate Si in different plant parts was also investigated. In vitro assessment of fungal responses to Si in PDA showed that both R. solani and S. sclerotiorum were inhibited in the presence of Si. More inhibition was observed as the Si concentration increased with a relative increase in pH. Maximum growth inhibition was observed at 3000 mg ;-1 – 6000 mg ;-1 of PDA. No difference in inhibition between the two pathogens was observed, thus confirming the fungitoxic/suppressive ability of high Si concentrations to fungal growth. In addition, in vivo trials showed that the Si concentration of 200 mg ;-1 applied weekly significantly increased the dry weight of plants inoculated with R. solani and S. sclerotiorum and was therefore considered the optimum concentration. Assessments on in vitro antifungal activities of Eco-T® on R. solani and S. sclerotiorum, showed that Eco-T® significantly inhibited mycelial growth, in both dual culture methods and volatile and non-volatile compounds produced by Eco-T®. In addition, the combination of Eco-T® and Si was most effective in suppressing damping-off and increasing plant dry weight of sunflower seedlings in the greenhouse. The combination of Si and Eco-T® significantly increased percentage germination, number of leaves and head dry weight of the sunflower cultivars tested. Silicon alone increased growth but was unable to control R. solani and S. sclerotiorum effectively. Rhizotron studies showed that S. sclerotiorum infected the host through the roots and the stem, whereas R. solani only infected the host through the roots. A study on Si uptake and distribution showed that sunflower accumulates Si in various plant tissues. Analysis of plant tissues revealed that more Si was accumulated in leaves > stems > roots, with the Si levels in leaves being significantly higher than in stems and roots. In conclusion, Si alone could be used to increase growth but was unable to control R. solani and S. sclerotiorum. However, Si together with Eco-T® provides an environmentally friendly alternative for the control of R. solani and S. sclerotiorum, and enhanced plant growth and yield

Optimum gamma radiation doses to enhance genetic diversity in selected cowpea (Vigna unguiculata) genotypes

Cowpea (Vigna unguiculata L.) improvement is necessary to increase its contribution to food and nutrition security in the impoverished regions such as the Sub-Saharan African Region. Genetic variability is fundamental to selecting parental genetic resources with agronomic and economic important traits for recombination. Gamma radiation is one of the valuable mutagens for creating novel mutant populations with allelic combinations needed to develop varieties with the desired traits. In this study, we determined the optimum dosage of gamma irradiation to use in breeding programs to enhance high yield and tolerance to field pests, including Maruca pod borers (Maruca vitrata) in locally adapted cowpea varieties. Seeds of two locally adapted cowpea genotypes (NkR1P3 and NamCp201) were treated with six gamma irradiation doses – 0 (un-irradiated), 75, 150, 300, 450, and 600 Gy. Consequently, the radio-sensitivity test was conducted in seed trays under net house conditions at the experimental field of Ogongo Campus, University of Namibia, Namibia. The study was arranged in a completely randomized design with three replications. Significant interactions (p < 0.001) of genotype and gamma irradiation dosage were found on percentage emergence (%E) and seedling survival percentage (%SS). This revealed that optimum gamma irradiation doses for tested cowpea genotypes where genotypic and dose depended. The significant effect (p < 0.001) of dose on all tested traits revealed the need to identify highly responsive traits for large-scale mutagenesis. Seedling survival rate and shoot length were the most responsive and were consequently used to determine the optimum dose for mutagenesis. The optimum dosage of gamma radiation for genotypes NkR1P3 and NamCp201 ranged between 382 and 427 Gy and 324 and 335 Gy, respectively. These optimal doses can be used to generate genetic variation to improve yield and tolerance to local stresses including tolerance to M. vitrata pod borers