Assessing the suppressive effects of biopesticides and phosphite on common scab development in potatoes

Abstract

ABSTRACT Potato common scab (CS) is caused by several saprophytic bacteria of the genus Streptomyces, including Streptomyces scabies. Various methods have been investigated to control CS, but their efficiency is usually low. Biopesticide formulations, including bacteria from the Bacillus spp., are increasingly used to control soil pathogens. Two Bacillus-based products, Double Nickel 55TM and Microflora PROTM, and the phosphite-based fungicide PhostrolTM, alone or in combination, were examined to determine their effects on CS development on tubers of the potato cultivars Red Pontiac, Prospect, Green Mountain, and Goldrush. CS disease incidence and severity were assessed using 4 parameters: percentage of tuber area covered by CS, necrotic areas, the number of deep pits and of shallow pits. The effects of treatments on tuber periderm morphology were assessed by determining the number of suberised cell layers and the thickness of suberised cell layers. Cultivar Goldrush performed much better than the other three cultivars, in all parameters scored. Among the six treatments, only Double Nickel 55TM + PhostrolTM reduced CS more consistently. Suberised cell size, shape, arrangement, number of layers, and phellem thickness, were found to vary largely across cultivars. A thicker phellem, but not the number of cells in the phellem, was found to be associated with improved resistance to CS. Treatments had limited effect on the number of layers of suberised cells and phellem thickness and these effects seemed to be cultivar specific. Overall, Double Nickel 55TM and Microflora PROTM were found to augment the number of suberised cell layers.