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Carpropamid {(JRS, 3SR)-2, 2-dichloro-N-[1-(4-chlorophenyl)ethyl]-1-ethyl-3-methylcyclopro-panecarboxamide} strongly inhibited pigmentation in shaking culture as well as in agar plate culture of Pyricularia oryzae and Verticillium dahliae. White crystals were isolated from the extract of the cultures of wild type strains of P, oryzae and V. dahliae treated with carpropamid. Analytical data of the crystals obtained from a spot at Rf 0.36 on the TLC plates coincided with those of scytalone. A large amount of scytalone and a small amount of vermelone accumulated in the culture treated with l0ug/ml of carpropamid, while 2-hydroxyjuglone was not detected in these analyses. When scytalone was added to the shaking culture of albino mutants of P. oryzae (P2-alb), it promptly disappeared from the culture and the culture was pigmented to light brown. But scytalone remained in the culture if treated with carpropamid. Scytalone and vermelone administered beside the colony of P2-alb on the agar plates were converted to black pigments but the pigmentation was inhibited by carpropamid. These results suggest that carpropamid inhibits dehydration of scytalone and vermelone in the fungal melanin biosynthesis.

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Carpropamid{(1 RS, 3 SR)-2,2-dichloro-N-[1-(4-chlorophenyl) ethyl]-1-ethyl-3-methyl cyclo-propanecarboxamide}, which is considered as a melanin biosynthesis inhibitor, enhanced the accumulation of the phytoalexins momilactone A and sakuranetin in the rice leaves subsequently inoculated with the blast pathogen (Pyricularia oryzae). Carpropamid and its two diastereo-isomers, A(1R,3S) and B(1S,3R), did not stimulate phytoalexin production directly, but they potentiated rice plants to produce the two phytoalexins more rapidly and much amounts in response to blast infection. This phenomenon was also observed in rice leaves treated with WL28325 (2,2-dichloro-3,3-dimethylcyclopropane carboxylic acid), a resistance inducer. The calculation data of the correlation among accumulation of phytoalexins, concentrations of chemicals and biological effects suggested that the correlation pattern of isomer A resembled that of WL28325. The main mode of action of isomer A appears to be an enhancement of phytoalexin synthesis during blast infection (EPAS mode) similar to that of WL28325, rather than the inhibition of melanin biosynthesis (MBI mode). In contrast, the main mode of action of isomer B was found to be MBI, although EPAS probably also contributes to its biological activity. Carpropamid is assumed to control rice blast through a combination of MBI and EPAS activities.

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