Low amounts of Spongospora subterranea sporosorus inoculum cause severe powdery scab, root galling and reduced water use in potato (Solanum tuberosum)

Abstract

This study measured effects of Spongospora subterranea soil inoculum levels on disease of potato tubers and roots, determining relationships between amount of sporosorus inoculum at planting, and both root function during plant growth and powdery scab on tubers at harvest. In a series of shadehouse experiments, different amounts of inoculum were applied to soil before planting tubers in large pots, and effects on plant growth and powdery scab incidence and severity were determined. In a glasshouse experiment different amounts of inoculum were applied to plants growing in sand/nutrient solution culture, and effects on water use, plant parameters and intensity of S. subterranea root gall formation were measured. In the shadehouse experiments, as few as 25 sporosori mL^-1 soil caused powdery scab on daughter tubers, and greater amounts of inoculum reduced plant productivity (tuber yields). In the glasshouse experiment, from one to ten sporosori g^-1 sand reduced water use by plants and shoot and root dry weights, and caused severe root galling. No consistently strong relationship was found between amount of inoculum at planting and disease incidence or severity at harvest. These results highlight the ability of S. subterranea to cause disease on host plants from low amounts of inoculum when conditions favour disease development, and emphasise the potential for the pathogen to reduce plant growth and tuber yields. They also show that inoculum detection methods should detect low soil inoculum amounts if they are to be useful for powdery scab prediction.