Ascherxanthone B from Aschersonia luteola, a new antifungal compound active against rice blast pathogen Magnaporthe grisea

Aims: Fungicide resistance now exists in the rice blast fungus, Magnaporthe grisea, necessitating the need for new active agents. Fungi isolated from habitats in Thailand were screened with reference to this problem. Methods and Results: A new, reliable in vitro screening system based on a microdilution plate format was set up using a virulent strain of M. grisea THL 16. Culture broth extracts from approximately 800 fungal strains were investigated, one of these, Aschersonia luteola BCC 8774, was found to produce an active fungicidal compound, ascherxanthone B, with an IC90 value of 0 center dot 58 mu g ml-1 (0 center dot 95 mu mol l-1). An in vivo study of anti-blast efficacy of ascherxanthone B showed a positive effect in disease reduction. Conclusions: Previous report has shown that a species of Aschersonia produces ascherxanthone A. Research on the species, A. luteola BCC 8774, led to the discovery of related novel metabolite, ascherxanthone B with fungicidal properties. Significance and Impact of the Study: Current methods of rice blast control seem to fail leading to increase in crop losses. Our discovery of the anti-blast activity shown by ascherxanthone B is the first step in the development of a potentially novel fungicide.