S U M M A R Y2,2-Dichloro-~,~-dimethyl-cyclopropane carboxylic acid (WL 28325) is a systemic fungicide highly specific for the rice leaf 'blast' disease caused by Piricularia oryzae Cav. Antifungal activity in vitro of WL 28325 against P. oryzae was dependent on the composition of the growth medium, growth being unaffected in two complex non-defined media but inhibited in a medium of defined composition. The antifungal effects of WL 28325 are probably not related to its activity as a rice 'blast' fungicide. Its intrinsic fungitoxicity was low. The ability of various dichlorocyclopropanes of similar structure to WL 28325 to inhibit growth of P. orjzae iiz vitro did not correlate with their ability to prevent 'blast' on rice. Also, despite the high degree of pathogen specificity of WL 28325 as a disease protectant, WL 28325 inhibited the growth of several plant pathogens in culture.No evidence was obtained to suggest that WL 28325 is converted, after its application to the rice plant, to a derivative of higher intrinsic fungitoxicity. These compounds may act by stimulating the disease-resistance mechanisms of the rice plant.
I N T R O D U C T I O NThe dichlorocyclopropanes (DCPs) are a group of experimental systemic fungicides active against the rice (Oryza sativa L.) leaf-blast disease caused by Piricularia oryzae Cav. They are highly specific in their action, being inactive against any other host-parasite combination tested, including PeIlicuZaria sasakii on rice (sheath blight disease) or even P. oryzae on other hosts such as ryegrass (Lolium perenne L.) or barley (Hordeum vulgare L.).They appear to be more effective on Japonica varieties of rice as compared with Indica varieties. Their activity is also unusual in that they are most effective after application to the roots of the plant (e.g. by paddy-water application) and their fungicidal activity is distinctly prophylactic rather than curative. The most active of the compounds so far known is 2,2-dichloro-3,3-dimethyl cyclopropane carboxylic acid (code no. W L 28329, but a wide range of esters of this compound as well as several closely related compounds are also active. Activity of the DCPs appears to depend on the ease with which W L 2 8 3 2 5 , or a closely related dichlorocyclopropane carboxylic acid, can be generated in the rice plant, A more complete account of the fungicidal properties of the DCPs is in preparation.We investigated the effects of DCPs on P. oryzae in culture and the relevance of these effects to their mode of action as rice 'blast' fungicides. The term 'fungicidal' is used in this paper to denote compounds with the ability to prevent disease development, while the terms ' fungitoxic ' and ' antifungal ' apply to fungal-growth inhibitory effects in vitro.
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