Effects of delaying fungicide treatment on the incidence of gangrene in stored potato tubers

Potato titbers infested with Phoma exigua var. foveata were uniformly wounded and sprayed or dipped in fungicide suspensions either immediately or after periods of up to 21 days' storage at 5, 10, 15 or 20 °C. Tubers were then stored at 5 °C and gangrene assessed after 12 wk. Incidence of gangrene on untreated tubers was progressively decreased by increasing the length of storage at 15 or 20° (curing) but was not affected by 3 days' storage at any temperature. Fungicide treatment immediately after wounding gave best control of the disease; treatment after 3 days' delay was less effective and after 14 or 21 days was usually ineffective. Gangrene was decreased by fungicides more on tubers stored for 3 or 7 days at 5 °C than at higher temperatures. Control of gangrene by curing or fungicides diminished when the amount of inoculum on tubers was increased. Increasing the amount of fungicide applied improved control and fungicides were more effective in decreasing gangrene on cut and crush wounds than on cut wounds. At the arbitrary concentrations used in these experiments imazalil gave better disease control than thiabendazole, prochloraz, carbendazim plus quinolin 8-ol or triadimefon.