Effects of Soil-Applied Late Blight Foliar Fungicides on Infection of Potato Tubers by Phytophthora infestans

Potato tuber infection was assessed under greenhouse and outdoor conditions when late blight foliar fungicides were applied to soil 24 h prior to soil infestation with a suspension of zoospores and sporangia of Phytophthora infestans. Spore viability of P. infestans in soil treated with various fungicides was determined using buried healthy whole tubers and by assaying infested soil applied to freshly cut tuber disks. Protection of tubers and tuber disks from infection was more effective when soil was treated with mancozeb, metiram, and cyazofamid than with other fungicides. Whole tuber infections were significantly less in soils treated with mancozeb, metiram, fluazinam, and fenamidone than when treated with distilled water. Infection of buried tubers and tuber disks was prevented for 3 to 5 days following a single soil application of mancozeb or metiram under outdoor conditions. The tuber disk method was more sensitive in determining the efficacy of a fungicide in inhibiting infection and spore viability than using whole buried tubers. However, both methods of determining viability may determine different modes of action of some fungicides that inhibit infection since whole tubers were not infected when protected by some fungicides but tuber disks were infected.