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Sclerotinia stem rot caused by Sclerotinia sclerotiorum ((Lib.) de Bary) attacks a wide host range of broad-leafed plants which includes soybeans, Glycine max L., an economically and strategically important crop worldwide. SSR epidemiology is a complex process that remains poorly understood and treatment is quite difficult thus has attracted lots of attention. For this study, laboratory investigation was carried out to determine the distribution and intensity of S. sclerotiorum using radial growth assay method on soil samples of different agro-ecological soybean growing regions of Kenya, with each sample replicated thrice. Rift Valley (Nakuru) registered the highest inoculum density while Nyanza (Homabay) had the lowest. The isolate, in the form of mycelia, was used for the duration of the study. The second experiment examined the impact of culture media representing disparate carbon to nitrogen (C:N) ratios and sources on mycelial growth, oxalate accumulation, and culture pH. The three parameters exhibited variations with respect to the differing preference for the nutrient sources and ratios; most oxalate accumulated on C:N (75:1) nutrient media, C:N (35:1) nutrient media exhibited the best growth potential and C:N (3.6:1) nutrient media was most oxalogenic. C:N (35:1) and C:N (3.6:1) were identified as the most commendable media for growth and fungal tolerance assay, respectively. HPLC analysis of metabolites present in the culture filtrate as potential regulators for oxalate metabolism revealed 6–17 peaks. Nine peaks were identified as acetate, citrate, succinate, malate, oxalate, oxaloacetate, succinate, glycolate, and indole-3-acetic acids (IAA). Taken together, these results indicate that; 1) oxalate production did not correlate with growth; 2) oxalate accumulation and regulation is dependent on nutritional conditions and; 3) the decrease in culture pH was independent of oxalate accumulation. Cultivation of resistance cultivar is the preferred means of controlling this disease, however resistant cultivars are not known. Using a detached-leaf bioassay technique, fifteen soybean varieties grown in Kenya were screened for their reaction to S. sclerotiorum. SB-19 exhibited the lowest rate of pathogen development 0.94 mm per day while BRS-MG 68 exhibited the highest rate of pathogen development 4.66 mm per day. Variety displaying the lowest rate of pathogen development under the disease pressure provided by mycelium disc would be expected to perform well under natural field infections. The fourth experiment examined the ability of metal ions (Cd2+, Co2+, Cr3+, La3+, Ag+, Mo6+, Sn2+, Sr2+, Cu2+, Hg2+, Ca2+, V5+, and Mn2+) and metal oxides (MnO2, V2O5, Cu2O, Pb2O3, and La2O3) to bind with oxalic acid which was verified by FTIR analysis. Thereafter, the in vitro effect of these metal ions on growth of S. sclerotiorum on CN (35:1) nutrient media were assesed. The growth was nature and dose dependent, while growth rate was enhanced by Ca2+, Co2+, Cu2+ and Mn2+, but inhibited in presence of La3+, Ag+, Hg2+, Sn2+, Cd2+ and Murtano. Growth inhibition was accompanied by oxalate accumulation as confirmed by IR analysis. In this study, silver NP were prepared by green synthesis process from AgNO3 solution through the extract of S. sclerotiorum and characterised by UV and IR spectroscopy. Synthesized silver Np exhibited inhibitory effect against S. sclerotiorum, from which potential fungicide formulation (AgNP, Ca2+, Mo6+ and B3-) was fabricated. The formulation elicited better activity than positive control (Murtano) thus potential substitute fungicide against S. sclerotiorum. |
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