The effect of soil‐ and tuber‐borne inoculum on the incidence of potato gangrene

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Abstract
SUMMARY: Under optimum growing conditions neither tuber‐ nor soil‐borne Phoma exigua var. foveata inoculum appreciably affected stand or yield of the subsequent potato crop. Seed tubers with gangrene rots caused high levels of stem and tuber symptoms when planted in var. foveata contaminated or uncontaminated land; contaminated seed tubers with no rots also produced progeny with a high gangrene potential.Sufficient soil‐borne inoculum was carried over in land that produced a gangrene affected crop in the previous year to override the effect of tuber disinfection. Effective gangrene control was achieved by a combination of tuber disinfection shortly after harvest over successive years with a 1 in 5 yr potato crop rotation.Gangrene rots usually developed through injuries to the tuber periderm, rots in other tubers being associated with pustules of powdery scab (Spon‐gospora subterranea).