Effect of registered potato seed piece fungicides on tuber-borne Phytophthora infestans

Curative applications of thiophanate-methyl + mancozeb to blighted seed pieces of three potato (Solanum tuberosum) cultivars significantly reduced the amount of surface area colonized by Phytophthora infestans compared with treatment with thiophanate-methyl or no fungicide under laboratory conditions. Percent blighted tuber surface area over six tests averaged 3.5, 11.4, and 21.2% for the three treatments, respectively. Seed pieces inoculated with P. infestans US-8 or US-11 incubated, and then treated with the same fungicides and planted at Oregon and Washington field sites, respectively, had higher emergence across locations for thiophanate-methyl + mancozeb than for thiophanate-methyl (30 versus 12.5%). Emergence of healthy thiophanate-methyl + mancozeb-treated seed pieces averaged 93%. When protective applications of these fungicides, mancozeb, or fludioxinil were made to healthy potato seed pieces prior to inoculation with P. infestans, plant emergence in the greenhouse was significantly increased with thiophanate-methyl + mancozeb compared with fludioxinil (92 versus 36%) in Oregon and with thiophanate-methyl or fludioxinil (90 versus 20 and 24%) in Washington. Inoculation of healthy seed pieces reduced plant stand in all greenhouse tests; however, treatment with thiophanate-methyl + mancozeb or mancozeb alone prior to inoculation resulted in sprout emergence similar to that of the noninoculated control. In a late blight management program, treatment of seed pieces with a registered fungicide that has activity against P. infestans contributes to an increase in plant emergence and improved crop uniformity. However, for maximum benefit, the fungicide must be in place before coming in contact with the pathogen.