Abstract

Cucumber wilt by Fusarium oxysporium f sp. cucumarinum is one of the most important soil-borne diseases. Among control strategies for plant soil-borne pathogen, biocontrol systems eliminate neither pathogen nor disease but bring them into natural balance. Polyketides form the largest part of the known natural products, and most of them come from actinomycetes especially streptomyces. Analysis for the ketosynthase genes in streptomyces can implicate new polyketides. In the course of the screening for producers of polyketides with antifungal activities, 117 strains were isolated. The screening program was performed by means of PCR amplification using degenerated primers corresponding to type II ketosynthase (KS) genes. Among 117 isolates, 33 isolates showed antifungal activities and 17 isolates showed positive amplification signal. Antagonism of the 17 isolates against Fusarium oxysporium f sp. cucumarinum, Bacillus subtilis and Erwinia carotovora subsp. carotovora in vitro were analyzed. The diversities of KS-II gene from the 17 isolates were abundant based on phylogenetic tree analysis. The 17 isolates were divided into 6 clades based on KS-II gene sequence. The results showed that different isolates which belong to the same species present different antagonism activities and also the different streptomyces species showed different bioactivities. Among 17 isolates, isolates DQ1, DQ23, GAN1, HVG60 and HVG71 have the potential ability to produce new type polyketides. This method may not only prevent reinvestigation to find bioactive molecules described previously, but also alleviate some of the biases introduced by using conventional cultivation techniques.

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