Factors influencing suppressiveness of soils to powdery scab of potato

Powdery scab, caused by Spongospora subterranea, is an important potato disease. Greenhouse experiments in 2017/18 and 2018/19 on (very susceptible) ‘Agria’ seed tubers assessed if field-collected soils had different powdery scab-suppressive capabilities and identified factors involved in disease suppression. 2017/18: 12 geographically diverse soils with either S. subterranea added at planting or not added; 2018/19: six single-type soils used, to determine if powdery scab suppression was ‘general’, or ‘specific’ (transferable; possibly involving microorganisms), and if suppression was associated with soil physical, chemical, or biological factors (bacteria/fungi). For both seasons, S. subterranea soil ammendment increased scab severity on harvested tubers in all soils but one. Powdery scab severity (percent tubers with > 5% surface area covered by scabs) ranged from 0 to 39%. Soil texture, pH, soil organic matter and nutrient contents were associated with powdery scab incidence for some soils but not others. Effects of previous crop rotations on powdery scab were variable: one soil with three recent previous potato crops in rotation was disease-suppressive. All 2018/19 soils displayed some microbe-mediated disease suppression, three being more suppressive than others. Two had possible ‘specific’ Spongospora suppression (less disease when added to the conducive soil). Thus Spongospora-suppressive soils are present in New Zealand, and abiotic and biotic soil factors influenced incidence/severity of powdery scab of potato.