Studies on the Mode of Action of the Dichlorocyclopropane Fungicides: Effects of 2,2-Dichloro-3,3-dimethyl Cyclopropane Carboxylic Acid on the Growth of Piricularia oryzae Cav

Abstract

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.

Synthetic plant defense elicitors

“…At the same time, 2,2-dichloro-3,3-dimethylcyclopropane-carboxylic acid (WL28325) was described as a compound suitable for controlling rice blast in rice. WL28325 affects the phenol metabolism of rice plants by enhancing peroxidase activities ( Langcake and Wickins, 1975a , b ). Two years later, 3-allyloxy-1,2-benzisothiazole-1,1-dioxide, widely called Probenazole (PBZ), was described.…”

Plant defense elicitor, 2, 4-dichloro-6-{(E)-[(3-methoxyphenyl) imino] methyl} phenol (DPMP) and its mode of action against fungal pathogen Alternaria solani in tomato (Solanum lycopersicum L.)

“…Polyacrylamide acid compounds were tested against Tobacco Mosaic Virus (TMV) as plant activators and it was found to increase PR1 gene expression (Gianinazzi and Kassanis, 1974). Since this first study, other synthetic chemicals have been shown to trigger the plant immune system over the years (Langcake and Wickins, 1975;Watanabe et al, 1977). Probenazole (PBZ), discovered in 1977, increased the defense response against Magnaporthe oryzae by activating enzymes related to plant immunity (Watanabe et al, 1977).…”

A laboratory method of screening fungicides for systemic activity against Bretnia lactucae

“…Recent studies have shown that a systemic compound active in vivo may not necessarily be active at the same concentration in vitro (Langcake & Wickins, 1975). For this reason an in vitro test alone is not always satisfactory as many potentially useful chemicals may be overlooked.…”