Fumigant dosages below maximum label rate control some soilborne pathogens

The activity of commercial soil fumigants on some key soilborne pathogens was assessed in sandy loam soil under controlled conditions. Seven soil fumigants that are registered in California or are being or have been considered for registration were used in this study: dimethyl disulfide (DMDS) mixed with chloropicrin (Pic) (79% DMDS and 21% Pic), Tri-Con (50% methyl bromide and 50% Pic), Midas Gold (33% methyl iodide [MI] and 67% Pic), Midas Bronze (50% MI and 50% Pic), Midas (MI, active ingredient [a.i.] 97.8%), Pic (a.i. 99% trichloronitromethane) and Pic-Clor 60 (57% Pic and 37% 1,3-dichloropropene [1–3,D]). Dose-response models were calculated for pathogen mortality after 24 hours of exposure to fumigants. Overall, the tested fumigants achieved good efficacy with dosages below the maximum label rate against the tested pathogens. In this study, Pythium ultimum and citrus nematode were sensitive to all the fumigants and Verticillium dahliae was resistant. For most fumigants, California regulations restrict application rates to less than the maximum (federal) label rate, meaning that it is possible that the fumigants may not control major plant pathogens. This research provides information on the effectiveness of these alternatives at these lower application rates. The results from this study will help growers optimize application rates for registered fumigants (such as Pic and 1,3-D) and will help accelerate the adoption of new fumigants (such as DMDS) if they are registered in California.