Development of Techlofthalam and Studies on Its Mode of Action

Abstract

Studies were begun to determine the effectiveness of N-aryl-3, 4, 5, 6-tetrachlorophthalamic acids against bacterial leaf blight of rice, caused by Xanthomonas campestris pv. oryzae. The structure-activity relationship of many derivatives indicated that the presence of both the carboxyl radical and four chlorine atoms at phthalic acid moiety was structurally essential for effectiveness against the disease, and optimization of activity was obtained in a derivative (techlofthalam) having 2, 3-dichlorophenyl as the aryl group. Techlofthalam showed no activity in vitro against the causal bacterium, when estimated by a turbidimetric method. Soil application with techlofthalam was found to be considerably effective at 4kg per hectare both in the greenhouse and the field. With foliar application, however, extremely different efficacies were shown with two inoculation methods; excellent control against lesion development was realized by spray inoculation, but only poor control was obtained by needle prick inoculation. Intensive field evaluation proved that techlofthalam spray at 60 to 120g per hectare showed a stable and excellent effect. In order to know the mode of action of techlofthalam, the change in population and aggressiveness of the bacterium in leaves sprayed with a 100ppm solution were examined. There was a definite trend of change in bacterial populations at all spray times, 1 day before and 3 days after inoculation, and after lesion formation; the bacterial population started to decrease a few days after spraying, and then was stable at much lower levels. The bacterium isolated from a sprayed leaf had lower aggressiveness than one from an unsprayed leaf. In consideration of generation time (approximately 10 hours in the exponential growth phase) of the bacterium in rice leaf, these results suggested techlofthalam's direct inhibitory effect on the multiplication of bacterium on the leaf. Retardation of the increase in viable bacterial population was found by a static culture using Suwa's synthetic medium containing techlofthalam at 0.1ppm; this was practically the same concentration as that gaschromatographically determined in guttation droplets on rice leaves a few days after spraying. Some other results also gave support to the conclusion that techlofthalam displays bacteriostatic action in rice leaf. Techlofthalam has been practically used in Korea since 1979.