Development of a New Fungicide, Pencycuron

Abstract

Pencycuron (1-(4-chlorobenzyl)-1-cyclopentyl-3-phenylurea) is a new fungicide specifically active against Rhizoctonia solani causing a number of important plant diseases such as sheath blight of rice, black scurf of potato and seedlings damping-off of various crops. In the course of screening trials of benzylurea derivatives originally intended to search for herbicidal candidates, some compounds turned out to be fungicidal to rice sheath blight, and this fact led us to study further on the structure-activity relationships. In the general formula of Xn_??_-CH2(R)NC(Y)N(R1)R2, a para substitution of the benzyl moiety was indispensable to a potent activity, and halogens especially the chlorine substitution provided the highest activity. The alkyl moiety should be bulky with 3 to 6 carbons in total and branched at the first carbon having not more than two carbon chains. sec-Butyl and cyclopentyl group were most active. R1 should be a non-substituted phenyl group. Any substituents either electron-withdrawing or electron-donating on benzene ring acted only against the activity. R2 should be hydrogen and any other substituents could not contribute to an elevation of the activity. Although urea (Y=O) was more active than thiourea (Y=S) in general, the same tendency in the activity relations was shown with both types. The activity of pencycuron was extremely selective even among the group of R. solani. Most of the important groups as crop pathogen were highly sensitive but some others were tolerant. Pencycuron is essentially a contact fungicide and non-systemic. The majority of the applied substance in plant was localized at the treated zone and was mostly retained as the parent form. The GLC profiles of metabolites derived from both isolates of R. solani sensitive and non-sensitive to pencycuron were not much different and no metabolite exceeding the amount of pencycuron could be found. The characteristic of pencycuron in disease control should be due to an excellent prevention against infection with a remarkable long-lasting efficacy. The highly selective activity to R. solani with very low potential to non-target organisms may serve for a high degree of safety to users, less hazard to the enviroment and favourable compatibility with many kinds of crop.