Effect of Lablab (Lablab Purpureus L.) Green Manure on Population of Pathogenic and Non-pathogenic Soil Microorganisms and Bean (Phaseolus Vulgaris L.) Crop Establishment

Abstract

The use green manure as soil amendments stimulates the growth and activity of soil microorganisms, with subsequent mineralization of nutrients and increase in fertility. However, undecomposed plant residues have been reported to reduce crop emergence and establishment. The study was carried out to determine the effect of lablab green manure on population of soil microorganisms and to investigate the mechanisms contributing to poor crop establishment. A survey was conducted in Nandi South to determine the usage of green manures as soil amendment and field experiments where two bean varieties, KK8 and GLP2 were planted on plots treated with lablab green manure incorporated over the whole plots, and between rows, diamonium phosphate fertilizer and lime. The effect of time after incorporation of lablab green manure on bean root rot and establishment was determined by incorporating lablab green manure at 0, 7, 14 and 28 days before planting. Data was collected on soil nutrient content, soil microorganisms, crop emergence, plant stand, incidence and severity of root rot, biomass and grain yield. Mechanisms contributing to poor crop establishment incorporation were determined by evaluating the effect of fresh and compost extracts of lablab and also in comparison with other legumes on bean germination, mycelial growth of root rot fungal pathogens, spore germination and germ tube elongation. Potential antagonistic activity between saprophytes and the root rot fungal pathogens was also determined Most farmers applied green manure as a soil amendment with 62% evenly distributing green manure in soil before planting and majority (48%) of the farmers planted crops two weeks after incorporation of green manure. Lablab green manure improved soil organic carbon, nitrogen, phosphorus and potassium but reduced germination percentage by 35% and increased incidences of root rot by 30% compared to the plots without green manure. Root rot pathogen population xvii was significantly high in green manure treated plots while the population of saprophytic fungi was low. Plots treated with lablab green manure had reduced grain and biomass yields by up to 25%. Green manure incorporated 28 days before planting resulted in improved germination by 90% with corresponding reduction in root rot incidence and severity by 8% and 36%, respectively compared to plots incorporated with lablab green manure at planting. Green manure incorporated at planting stimulated population of root rot pathogens. However, it reduced the population of saprophytic fungi such as Trichoderma, Aspergillus, and Penicillium and also significantly reduced grain yield by up to 70%. Fresh legume extracts were found to inhibit spore germination, germ tube elongation and mycelial growth of root rot fungi while lablab extracts was stimulated. Ethanol extracts from lablab and soybean significantly inhibited mycelial growth by up to 62% but fresh aqueous extracts from lablab had minimal antifungal activity. The ethanol extracts from groundnut, lablab and soybean increased mean germination time by 120%, with corresponding decrease in final germination percentage. Aqueous extract of lablab and soybean resulted in significantly increased spore germination by over 70% and increasing germ tube length and number of germ tubes by 8.0% and 13%, respectively. The saprophytic fungi were antagonistic against the root rot pathogens with Trichoderma showing the highest inhibition of up to 64%. The results of the study showed that lablab green manures stimulated an increase in root rot pathogens resulting in low emergence and establishment of bean crop probably due to pre-and post-emergence damping off. However, allowing decomposition for four weeks increases nutrients release and causes decline in the population of root pathogens in the soil. Therefore, timing and method of application of green manure is important in attaining the full crop yield. A period of 28 days after incorporation of lablab green manure is recommended for planting common beans