A Comparative Study of XR-539 and other Oomycete Fungicides: Similarity to Dimethomorph and Ammo Acid Amides in its Mechanism of Action

摘要

XR-539 is an experimental Oomycete fungicide of unknown mechanism which shows some structural similarity to the amino acid amides iprovalicarb and the developmental compound benthiavalicarb-isopropyl (KIF-230). The purpose of this study was to evaluate whether XR-539 has a unique mechanism of action among Oomycete fungicides, and its potential for cross-resistance with existing products. XR-539, amino acid amides and dimethomorph could be clearly differentiated from other Oomycete fungicides on the basis of their morphological effects on Phytophthora capsici, the shape of their fungitoxicity dose-response curves, and their antifungal spectrum within the Oomycetes. These materials shared a number of characteristics including: (1) arrest of Phytophthora capsici development from a zoospore inoculum at the time of encystment, accompanied by lysis of the encysting zoospores; (2) very steep fungitoxicity dose-response curves relative to those of other Oomycete fungicides; and (3) highly selective fungitoxicity within the Oomycete class, exemplified by high fungitoxicity towards Phytophthora capsici and lack of activity towards Pythium ultimum. Cross-resistance studies with field isolates of Plasmopara viticola which show reduced sensitivity to dimethomorph revealed cross-resistance to XR-539 and amino acid amides, but no cross-resistance to the antitubulin fungicide zoxamide. These results suggest that XR-539, amino acid amides and dimethomorph have a similar mechanism of action which appears to involve disruption of cell wall synthesis. However, it remains to be established whether these compounds bind to the same target protein or act on different proteins in the same biochemical pathway or process.