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

Curative applications of thiophanate-methyl + mancozeb to blighted seed pie ces of three potato (Solanum tuberosum) cultivars significantly reduced the amount of surface area colonized by Phytophthora infestans compared with t reatment with thiophanate-methyl or no fungicide under laboratory condition s. Percent blighted tuber surface area over six tests averaged 35, 11.4, an d 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 fung icides and planted at Oregon and Washington field sites, respectively, had higher emergence across locations for thiophanate-methyl + mancozeb than fo r thiophanate methyl (30 versus 12.5%). Emergence of healthy thiophanate me thyl + mancozeb-treated seed pieces averaged 93%. When protective applicati ons of these fungicides, mancozeb, or fludioxinil were made to healthy pota to seed pieces prior to inoculation with P: infestans, plant emergence in t he greenhouse was significantly increased with thiophanate-methyl + mancoze b 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, t reatment with thiophanate-methyl + mancozeb or mancozeb alone prior to inoc ulation resulted in sprout emergence similar to that of the noninoculated c ontrol. In a late blight management program, treatment of seed pieces with a registered fungicide that has activity against P, infestans contributes t o an increase in plant emergence and improved crop uniformity. However, for maximum benefit, the fungicide must be in place before coming in contact w ith the pathogen.