Influence of Herbicide Treatments on the Occurrence of Soil-borne Plant Diseases

Abstract

The effects of herbicides on the growth of soi-lborne plant pathogens and on the occurrence of damping-off of cucumber seedlings inoculated with the causal fungi were studied.
Among ten sorts of herbicides used, dinoseb, pentachlorophenol, linuron, and chloropropham inhibited the mycelial growth of Fusarium oxysporum, Rhizoctonia solani and Pythium aphanidermatum on culture medium, while 2, 4-D, MCPA, prometryne, and simazine seemed to promote the development of aerial hyphae of R. solani or P. aphanidermatum.
The effect of antifungal herbicides among above-described on the occurrence of pre- or post-emergence damping-off in cucumber was examined. Pythium+dinoseb, Pythium+linuron, and Fusarium+dinoseb combinations resulted in the increase in healthy seedlings compared with Pythium or Fusarium alone. This is probably due to the antifungal effect of herbicides. Rhizoctonia+dinoseb, Rhizoctonia+linuron, Pythium+pentachlorophenol, and Rhizoctonia+pentachlorophenol combinations, however, resulted in an appreciable reduction in cucumber stand due to damping-off compared with Rhizoctonia or Pythium alone. At present, it is not clear whether the herbicides acted on the pathogens or on the host.
It is possible that the herbicides affected the physiology of host plant, so that the susceptibility of emerging seedlings to the pathogens increased. The results may suggest the importance of secondary effects of herbicide.