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.