The role of soil, seed-potato tuber and haulm in the transmission of Phoma foveata from infected seed to daughter tubers

Abstract

Infestation of ridge soil by Phoma foveata was examined over 4 years by collecting soil samples at various distances from seed tubers which were either naturally infected or inoculated with a distinctive strain. Samples were taken on four occasions during each growing season. Infestation was greatest immediately around the tuber and was largely confined to a distance of 15 cm until haulm destruction, after which P. foveata was detected more widely in the soil. The effects of various seed-tuber and haulm treatments on the incidence of daughter-tuber contamination by P. foveata were assessed over 5 years. Removing the infected seed tuber at emergence significantly (P < 0.05) reduced the contamination at haulm destruction of daughter tubers harvested by hand in only 1 out of 4years. In all years, tuber contamination was much less(P < 0.05) when stems were pulled and removed than when they were desiccated by applying diquat dibromide. The incidence of tuber contamination did not increase between haulm destruction and harvest when stems were pulled and removed. In another experiment, tuber contamination increased linearly with the delay in pulling haulms after applying diquat dibromide. Cutting and removal of stems tended to reduce gangrene contamination, relative to the desiccation of stems by applying diquat dibromide, in all years where the seed tuber had been removed, but in only 2 out of 5 years where the tuber was present.