| A number of vegetables such as tomato, pepper, eggplants and cucumber have frequently suffered continuous cropping obstacle. Continuous mono cropping often caused these plants weak growth, inferior fruit quality and specific root diseases and/or insects. To explore the origin of its occurrence mechanisms, agricultural researchers had contributed much work to many single factors including rhizosphere nutrition, soil borne plant pathogens and related soil physical-chemical characteristics. However, the exact causes have never been fully explained. The most limitation of previous studies was that much effort was put on the single factor of cropping obstacle, the interaction among factors should have, but not emphasized ever since.Regarding allelophathic substances in root exudates as the ligament of many factors of continuous cropping obstacles, we first detected the variation of rhizosphere microbial population in response to continuous cucumber cropping, then collected and identified a few allelophathic substances in root exudates. In addition, we also trialed the effect of phenolic acid on cucumber seedlings and wilt hypha growth, investigated the root disease severity and cucumber seedlings growth after phenolic acid degradation in situ by Phanerochaete chrysosporium in this article. The main conclusions were as following:Plate count analysis revealed a reduction trend in the number of culturable microorganism both in rhizosphere and bulk soil as cucumber cropping continued. Bacteria showed more sensitive than actinomyces did, and had an apparent decrease extent in CFU number. In a fourth cropping season, the number of culturable bacteria in rhizosphere and non-rhizosphere decreased 31.3% and 25% respectively compared with that in non-replant cucumber soil, and both of the decrease extent showed significance at p=0.05. The change tendency of bacterial number on poor nutrition R2A media was similar with that on rich nutrition media TSBA.
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