Identification of Dominant Microbial Community and Diversity in Continuously Cropped Pepper Fields

Pepper blight is the most significant soil-borne disease affecting the continuous cropping of peppers. To identify the effect of Phytophthora capsici infection on microbial flora, we isolated and counted the microorganisms collected from the rhizosphere soil of P. capsici-affected farms that continuously cropped pepper for 3, 6, and 9 years in Liaoning Province, China. The colony and cell morphology, physiological and biochemical characteristics, and 16S rDNA sequence of bacteria and actinomycetes were documented. In addition, colony and microscopic morphology of fungi and the rDNA-ITS sequence were analysed for classification. We observed that healthy and diseased peppers had the largest number of bacteria in the rhizosphere followed by actinomycetes and fungi. After infection, the number of bacteria and actinomycetes decreased with a corresponding increase in the number of fungi, leading to a reduction in the ratio of bacteria/fungi to actinomycetes/fungi. We identified 15 dominant bacterial strains, of which Bacillus represented the most abundant genus consisting of 7 strains followed by Flavobacterium and Staphylococcus. Furthermore, 15 of the 17 actinomycetes strains belonged to the genus Streptomyces. Among the six fungal strains, we found P. infestans, Fusarium, and Penicillium consisting of two strains each. This study elucidated the impact of pathogenic P. capsici on the composition of soil microbes over time and characterized several cultivatable dominant bacterial groups, which can provide a basis for practical intervention strategies to improve soil conditions for continuous cropping. Additional key words: continuous cropping, dominant microorganism, microbial community, pepper, Phytophthora capsica