EFFECTS OF ORGANIC AND INORGANIC FERTILIZER ON THE POPULATION OF SOIL PATHOGENIC MICRO-ORGANISMS IN TEA RHIZOSPHERE

Abstract

The long-term cultivation of tea (Camellia sinensis) alters microorganism communities in the rhizosphere; it can increase saprotrophs, pathogenic microorganisms and reduce symbiotrophs. Fertilizers are sources of plant essential nutrients used regularly in tea production to replenish soil nutrition and can influence the activity and population of the soil microorganisms. This study aimed to determine the effect of fertilization regimes on the population dynamics of soil microorganisms in the tea rhizosphere. The study was carried out at the Tea Research Institute in Kericho, Kenya. Two main fertilizer types; organic (Phymix) and inorganic (Nitrogen, Phosphorus and Potassium) and foliar fertilizer Tecamin Max and Tecnokel Amino Mix as sub treatments application at the rates of 75 and 150 kg N ha-1 of inorganic fertilizer were applied in four replications. Sampling of soil was done before treatment, during the dry season (January-February), the wet season (JuneJuly) and short rainy season (September-October). The fungal and bacterial species for these seasons were identified. The data obtained was analyzed using Genstat Statistical Software, 15th Edition. The results demonstrated that the number of fungal colony units varied significantly (P≤0.05) between fertilizer type throughout the dry season. Similarly, in the rainy season the fungal population (32.9 CFU/g soil) varied significantly (P≤0.05), where organic fertilizer was applied in comparison with the fungal population where inorganic fertilizer was applied (19.2 CFU/g soil). The short rainy season indicated that fungal and bacterial colony units in the tea rhizosphere varied significantly (p<0.05) between organic and inorganic fertilizer form applied and all interactions of fertilizer and rate plus foliar varied significantly (p<0.05) for both fungal and bacterial populations. Both interactions of fertilizer type and rate varied significantly (P≤0.05) for fungal populations in all seasons. No significant variation was noted for the bacterial population (CFU/g soil) during the dry seasons regardless of fertilizer type and rates. The fungi identified included; Cylidrocarpon spp., Trichoderma spp., Penicillium spp., Aspergillus spp. Colletotrichum spp., Pestalotiopsis spp. and Fusarium spp. The bacteria species included; Pseudomonas spp., Bacillus spp., Rhizobium spp. and Xanthomonas spp. In regard to the dynamics of the pathogenic fungi, there was fluctuation and an increase of propagules recovered during the wet season than the other two seasons with Colletotrichum spp. (61 CFU/g soil) of soil followed by Pestalotia spp. and the least was Cylidrocarpon spp (29 CFU/g soil). Fields where organic Tecnokel amino mix fertilizer regimes were applied showed high increase in the fungal pathogens across the seasons except in the case of Fusarium spp. where the population increased due to inorganic Tecamin mix. The soil-borne pathogenic bacteria also varied between the seasons and the fertilizer form applied. The Pseudomonas sp. recorded the highest population across the seasons ranging between 54 CFU/g soil to 69 CFU/g soil during the dry seasons and the long rains respectively. Contrary to the fungal pathogens the organic Tecamin mix was the fertilizer form where a higher bacterial population was recorded between the two seasons. Generally, the organic fertilizer form increased fungal and bacterial populations significantly, except in Fusarium sp. an indication of its ability to increase propagules of both bacteria and fungi. Further it was noted that the microbial population was higher during the wet season, implying the need for integrated approaches to enhance soil health and suppress soil-borne diseases by promoting beneficial microbial communities that compete with or antagonize pathogenic fungi.