Control of clubroot caused by Plasmodiophora brassicae with nonionic surfactants

Liquid formulations of the nonionic surfactants Agral, Citowett Plus, AquaGro 2000-L, and the granular formulation AquaGro 2000-G were tested in greenhouse and field trials for their control of clubroot [Plasmodiophora brassicae]. After 6 weeks in greenhouse tests, disease severity of nontreated Chinese cabbage plants increased with increasing inoculum concentration (0, 10(3), 10(5), and 10(7) spores/g soil), but disease severity was markedly reduced in proportion to surfactant concentrations of 0.1%, 0.2%, and 0.5%. Foliar weights of nontreated plants decreased sharply with increasing inoculum levels, but the weights of treated plants decreased much less. AquaGro 2000-G was tested separately, but it appeared to be more effective and less phytotoxic than the liquid formulations. A similar result occurred in a 1995 field trial of broccoli. In this trial the liquid formulations most effectively controlled disease, with the least amount of phytotoxicity, when applied as a preplant soak to the root ball and to the transplant hole at a concentration of 0.2%. The preplant soak alone was ineffective. In a 1996 field trial, the liquid formulations applied at 0.5% to the transplant hole or as a split application to the hole (at 0.2% followed by a surface drench at 0.2% 10 days later) resulted in the greatest disease reduction and greatest yield. Numbers of marketable heads increased from 4.7% in the check to 86.7% overall in the treated plots. However, in a trial in which the soil was free of P. brassicae these treatments were phytotoxic, resulting in lower head weight and quality than a single application at 0.2% or a split application at 0.1%. This indicates that for lower inoculum levels the lower application rate is more appropriate. Of the liquid formulations, AquaGro 2000-L controlled disease most effectively with the least amount of phytotoxicity.