Abstract
Black dot, caused by Colletotrichum coccodes, is an increasingly important potato blemish disease that reduces market quality, particularly in light-skinned fresh-pack cultivars. Multi-year field and storage studies at the University of Idaho Aberdeen Research and Extension Center quantified infection timing, effects of season length, and efficacy and timing of recently registered fungicides. Inoculation trials in 2012 and 2013 showed that tubers were highly susceptible before skin set, while disease progress was minimal after skin set, supporting that symptoms observed at harvest and in storage originate primarily from field infections. Season-length experiments in 2011 and 2012 with Agata and Rosara showed that extending the crop beyond about 100 days increased black dot incidence and severity without improving total or marketable yield, indicating that calendar-day targets should be interpreted in relation to cultivar maturity and skin set. Fungicide trials in 2021 and 2022 showed that programs containing pyraclostrobin, trifloxystrobin, or azoxystrobin reduced C. coccodes DNA in stems and tuber skin when initiated early, whereas programs first applied at canopy closure, including Luna Tranquility + Echo 720, provided little benefit. Across trials, applications beginning at the 15 to 20 cm plant-height stage provided the most consistent suppression. Collectively, these results indicate that effective management depends on protecting plants early in the season and aligning harvest timing with cultivar-specific maturity and skin set to limit blemish development under conditions typical of Idaho and similar potato-producing regions.