In vivo control andin vitro antifungal activity of rhamnolipid B, a glycolipid antibiotic, againstPhytophthora capsici andColletotrichum orbiculare

Abstract

The glycolipid antibiotic rhamnolipid B isolated from Pseudomonas aeruginosa strain B5 was evaluated for in vitro antifungal activity and in vivo control against phytophthora blight and anthracnose under glasshouse conditions. Rhamnolipid B showed antifungal activity against Cercospora kikuchii, Cladosporium cucumerinum, Colletotrichum orbiculare, Cylindrocarpon destructans, Magnaporthe grisea and Phytophthora capsici. Microscopic observation revealed that the high level of antifungal activity (10 µg ml −1) against P capsici was mainly due to a lytic effect on zoospores. Zoospore lysis began in the presence of 10 µg ml −1 of rhamnolipid B and most of the zoospores were collapsed at 25 µg ml −1. Rhamnolipid B showed inhibitory activity against the germination of zoospores and hyphal growth of P capsici at concentrations of 50 µg ml −1. Spore germination of the anthracnose plant pathogen C orbiculare was also inhibited in the presence of 50 µg ml −1 of rhamnolipid B, although hyphal growth was not affected at this concentration. In the glasshouse, the efficacy of rhamnolipid B against phytophthora blight was similar to that of metalaxyl on pepper plants when treated just before inoculation with P capsici. Treatment with either at 500 µg ml −1 completely protected pepper plants from phytophthora blight. Rhamnolipid B also suppressed the development of C orbiculare infection on leaves of cucumber plants. © 2000 Society of Chemical Industry