Barak, E., Department of Environmental Biology, University of Guelph, Guelph, ON, NIG2W1, Canada
Edgington, L.V., Department of Environmental Biology, University of Guelph, Guelph, ON, NIG2W1, Canada
Ripley, B.D., Department of Environmental Biology, University of Guelph, Guelph, ON, NIG2W1, Canada, Agricultural Laboratory Services Provincial Pesticide Residue Testing Laboratory, Ontario Ministry of Agriculture and Food, University of Guelph, Guelph, ON, NIG2WI, Canada

The systemic fungicide metalaxyl is highly effective against fungal pathogens belonging to the Peronosporales. Foliar applications of metalaxyl resulted in residue levels in potato tubers (2.5-12 jug.kg"1) sufficient to protect tubers against Phytophthora infestans over more than 7 months of cold storage. Moreover, tubers containing metalaxyl residue were resistant to decays caused by Fusarium sambucinum, F. culmorum and Alternariasolani. This resistance was lost gradually after 4 months of storage, when the metalaxyl level had decreased to less than 8 jug.kg'1. Metalaxyl applied to the surface of potato tuber medullary tissue prior to inoculation caused the same phenomenon of broad spectrum resistance. The presence of metalaxyl in the tuber tissue stimulated the activity of the oxidative enzyme polyphenol oxidase. It seems that metalaxyl, which is directly effective against P. infestans even at very low levels, but which has no direct fungitoxicity against Fusarium or Alternaria spp., may stimulate the defence reaction of the host tubers against fungal parasites through enhancement of the lignin synthesis pathway. © Taylor & Francis Group, LLC.