Impact of land use on distribution and diversity of fusarium species in ngangao forest, Taita Taveta District in Kenya and the adjacent farmlands
Kariuki, Maina Peter
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The effect of current Land Use Types (LUTs) and soil depth on distribution and diversity of soil Fusarium species was carried out in Taita Taveta district, Kenya. Sixty sampling points were chosen along a land use gradient to represent six different LUTs: maize, horticulture, napier, fallow/shrub farmlands, planted and indigenous forests. Soil samples were collected at the depths of 0 to 10 and 10 to 20 cm. Fusarium species were isolated from soil samples by dilution plate technique using Fusarium-selective media (PPA). A total of 1865 isolates were recovered which resulted into 26 Fusarium species. Fusarium oxysporum, F. solani and F. sporotrichioides were the dominant species in this study area. Fusarium oxysporum and F. solani isolates were recovered from all LUTs. Fusarium compactum had the lowest frequency of isolation with its isolates recovered from fallow/shrub LUT only. The study also revealed that there was significant difference in Fusarium species abundance and richness across the LUTs (P<0.001) and soil depths (P<0.05). There was significant difference in abundance (P=0.039) and richness (P=0.041) between the upper and the lower soil layers. It was also observed that Fusarium species diversity across the LUTs was significant (P<0.001) with horticulture being the most diverse while planted forests being the least diverse. Fusarium species diversity between the two soil depth levels was not significant (P=0.063). Regression analysis revealed that there was significant positive correlation (P<0.001) between abundance, richness and diversity of soil Fusaria with soil phosphorus and pH levels. Significant positive correlation (P<0.05) between soil Fusaria abundance(r=0.25) and diversity (x=0.0988) and soil Mg was also observed. Soil K was also noted to be positively correlated with soil Fusarium species richness (r=0.1688) and diversity (r=0.403). However, there was significant negative correlation between exchangeable soil acidity and soil Fusaria abundance (r--0.605), richness (r=-0.1317) and diversity (r=-0.16) (P<0.05). Principal Component Analysis based on the relative Fusarium species abundance differentiated the LUTs with 79.69 % variance explained by the first and second components. Genetic relatedness among the twenty six Fusarium species was assessed using Random Amplified Polymorphic DNA (RAPD) assay where six primers were used. Amplification products were examined and presence or absence of each size class of bands was scored as 1 and 0, respectively. The resulting matrix was used to compute Jaccard's similarity coefficients and UPGMA cluster analysis using computer package NTSYS-pc. Jaccard's similarity coefficients ranged from 0.257 to 0.583 among the Fusarium species studied indicating high genetic diversity. The UPGMA cluster analysis clearly grouped Fusarium species into two main clusters of 17 and 9 species, respectively. Fusarium avenaceum and F. nygamai depicted the highest genetic similarity of 58.3 %. The results indicate that there is a higher abundance and diversity of soil Fusarium in the more disturbed lands than in the forests, indicating that agricultural intensification increases soil Fusarium populations. This understanding can aid in devising ways of managing this potentially pathogenic fungus.