Characterisation Of Bean Root Rot Pathogens From Soils In Western Kenya And Their Management With Biochar And Vermicompost

Abstract

The importance of common bean (Phaseolus vulgaris L.) has continued to be emphasised in the world and as a key source of dietary protein both in the rural and urban livelihoods in Kenya. Reduction in its productivity has been attributed to variability in climatic conditions, declining soil fertility, insect pests and diseases. In Western Kenya losses due to soil borne diseases have been recorded as high as 70%. The objective of this study was to determine the prevalence of bean root rot and to contribute to improved bean productivity through management of the root rots with biochar and vermicompost soil amendments. This study evaluated the effect of biochar and vermicompost in suppression of fungal root rot of bean in farmer fields and in the greenhouse. The study also identified the mechanisms by which the disease suppression occurs. A survey was conducted to establish the prevalence of bean root rot pathogens in four AEZ’s across Kakamega, Bungoma and Busia counties of Western Kenya at the onset of long rains in 2013. Different species of Fusarium, Pythium, Rhizoctonia and Macrophomina were isolated from soils sampled from farmer fields in Western Kenya. Identification was undertaken by use of morphological and molecular means. All soils were infected with root rot pathogens including Fusarium species, Pythium ultimum, Pythium irregular, Rhizoctonia solani Macrophomina phaseolina. Fusarium spp was the most abundant across all AEZ’s. Pythium spp. and Rhizoctonia spp. were most abundant in LM2 and UM3 respectively. Species of Fusarium were also the most abundant in different soil textures with the loamy fine sand having the highest populations of the pathogens. Quantification of root rot pathogen DNA in soils using real time PCR was recorded highest for Rhizoctonia solani from UM3 at 2.23pg μL-1. Treatment combinations of biochar and vermicompost had a positive impact on plant emergence in all the four seasons under field conditions. However, in the greenhouse experiments, vermicompost had phytotoxic effect on plant emergence while soils amended with sugarcane bagasse and rice husks biochar recorded 92% plant emergence. Plant growth was enhanced in soils amended with biochars and challenged with root rot pathogens than in non-challenged soils that were amended with both biochars. Incidence and severity of bean root rot were significantly (p<0.05) reduced in plots amended with a combination of biochar and vermicompost as well as standalone treatments. Similar observations were made in the greenhouse experiments where the root rot severity was reduced by 27% in soils treated with sugarcane bagasse and rice husks biochar treatments. Root rot xxii pathogen populations were significantly (p<0.05) reduced in the field following application of sugarcane bagasse biochar and vermicompost. Populations of Trichoderma spp, Paecilomyces spp, Athrobotrys spp and Penicillium spp were also increased in fields where the soils were amended. Significant higher grain yields were recorded in the amended plots in the two long rain seasons and the short rain season of 2013. Plots amended with vermicompost and fertiliser had the highest yields at 565.2 kg Ha-1 while the lowest yields were in control plots at 311.7 kg Ha-1. The highest 100 seed weight was in vermicompost treated plots which was significantly different from the control plots but not biochar treatments. Highest pH of 6.06 was recorded in biochar amended plots in LM2. Highest levels of nitrogen and phosphorus were recorded in vermicompost amended soils in LM1 and combination of biochar and vermicompost amended soils in UM3 respectively. Sporangial and spore germination of Pythium ultimum s and Fusarium solani respectively were significantly inhibited (p<0.05) by biochar and vermicompost water extracts. Length of exposure period of biochar to air after pyrolysis significantly reduced its ability to suppress pathogen growth. Biochar was also found to adsorb phytochemicals from root and seed exudates in turn disrupting their ability to induce sporangial and conidial germination of root rot pathogens. In conclusion, biochar and vermicompost amendments are effective in reduction of incidence and severity of bean root rot. This study identified the mechanisms involved in control of root rot by biochar and vermicompost to include adsorption of germination trigger molecules for pathogens and inhibition of spore germination by the water extractible substances.