Characterization of Antibiotic Substance Produced by Serratia plymuthica A21-4 and the Biological Control Activity against Pepper Phytophthora Blight

The biocontrol agent, Serratia plymuthica A21-4, has been developed for controlling pepper Phytophthora blight. Serratia plymuthica A21-4 strongly inhibits the mycelial growth, zoospore formation, and cyst germination of Phytophthora capsici in vitro. The application of a cell suspension of strain A21-4 to pepper plants in pot experiments and in greenhouse successfully controlled the disease. The bacteria produced a potent antifungal substance which was a key factor in the suppression of Phytophthora capsici. The most active chemical com-pound was isolated and purified by antifungal activity-guided fractionation. The chemical structure was identified as a chlorinated macrolide $(C_{23}H_{31}O_8Cl)$ by spectroscopic (UV, IR, MS, and NMR) data, and was named macrocyclic lactone A21-4. The active compound significantly inhibited the formation of zoosporangia and zoospore and germination of cyst of P. capsici at concentrations lower than $0.0625{\mu}g/ml$. The effective concentrations of the macrocyclic lactone A21-4 for $ED_{50}$ of mycelial growth inhibition were $0.25{\mu}g/ml,\;0.25{\mu}g/ml,\;0.30{\mu}g/ml \;and\;0.75{\mu}g/ml$ against P. capsici, Pythium ultimum, Sclerotinia sclerotiorum and Botrytis cinerea, respectively.