LAUSR

Abstract Powdery scab, caused by Spongospora subterranea, causes significant economic damage in potato production worldwide due to reduced quality and marketability of seed and ware tubers. Three field experiments were conducted in naturally infested soil during winter 2008, 2012 and 2013 to evaluate the efficacy of soil fumigation on powdery scab on tubers, root galling and the pathogen populations in soil. Soil fumigation with metam sodium (MS) or chloropicrin (PIC) was highly effective in controlling the disease under field conditions. Powdery scab incidence and severity on tubers were significantly reduced by MS applied at 219–222 L a.i. ha−1 by 66–98%. In application of PIC at 99–396 kg a.i. ha−1 followed with immediate plastic cover, disease incidence on tubers was significantly reduced by 56–90%, whereas, uncovered PIC treatments had a lower efficacy. Root galling was also affected by the fumigants and was significantly lower in MS and PIC treatments as compared to the control. In-furrow fluazinam treatments were less effective than the fumigants, with only 18–20% reduction in disease incidence on tubers. Pathogen levels in the soil, determined shortly after fumigation, were significantly lower than in the control. However, at the end of the growing season, they were increased to high levels with no significant differences among all treatments.