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.