Development of a Greenhouse Assay to Evaluate Potato Germplasm for Susceptibility to Powdery Scab

Potato resistance to powdery scab, caused by the protist Spongospora subterranea f.sp. subterranea, has become extremely important in recent years due to the increased damage caused by this disease. Since field conditions cannot be controlled, they are sometimes sub-optimal for scab susceptibility detection. Thus, a greenhouse assay has been developed that consistently provides optimal conditions so that the susceptibility potential of a clone in the field can be efficiently evaluated. Four potato cultivars were evaluated which varied in levels of powdery scab susceptibility. Two soil types and three inoculum levels were also evaluated. Other factors, such as soil temperature and moisture, were examined to make certain that a range ideal for powdery scab development as reported in current literature was obtained. These greenhouse results were compared with three years of field data collected from trials conducted in the San Luis Valley, Colorado, USA. Each of the cultivars was evaluated for root galling, tuber lesion incidence, and severity. Greenhouse results most consistently matched field results when potatoes were planted in a soil with 50% sand, an inoculum level of one sporeball per gram, and high irrigation coupled with appropriate soil temperatures (ranging from 10–18°C) during the tuber initiation to bulking phase. This soil mix was utilized to test 14 cultivars in the greenhouse with varying levels of resistance to powdery scab. When greenhouse results were correlated with field results a pattern emerged which demonstrated that the cultivars were accurately ranked based on susceptibility to scab (e.g., cultivars with the most resistance to scab in the field also showed the most resistance in the greenhouse). A conversion table using the greenhouse score was developed to estimate the potential susceptibility of a clone under optimum field conditions. Results demonstrate that a greenhouse assay can be successfully used for evaluating advanced potato germplasm for resistance to powdery scab.