Impact of Continuous Cucumis sativus L. Cropping on the Incidence of Root Diseases and Bacterial Community Structure

Abstract

To investigate the change of cucumber root disease incidence and bacterial community shift in monocultrue soil, we established 4 treatments of greenhouse-pot cucumber, each representing various monoculture times, and monitored the bacterial variation in rhizosphere/bulk soil and root disease index. Results showed that culturable bacteria numbers gradually decreased as monoculture continued, a significant (P<0.05) difference was observed in rhizosphere samples of the fourth cropping (FC) and the third cropping (TC) soil in contrast to that of non-continuous cropping (NC). The Eco-Physiological diversity index also declined with consecutive cucumber cropping. Analysis by denaturing gradient gel electrophoresis (DGGE) showed that a few dominant bacterial species, including Pseudomonas sp. (95% similarity), Bacteriovorax sp. (93% similarity) and 2 uncultured soil bacteria, declined in population, while Sphingomonas sp. (100% similarity) and one uncultured bacterium increased. For all samples analyzed using DGGE or culture dependent approach, the bacterial population shift was more sensitive in the rhizosphere than in bulk soils, less bacterial diversity and higher root disease incidences were both detected in continuous monocultured soils.