Development and validation of a novel rapid in vitro assay for determining resistance of potato cultivars to root attachment by Spongospora subterranea zoospores

Spongospora subterranea f. sp. subterranea is a major pathogen of potatoes leading to losses in tuber quality and yield. Disease can be expressed as root infection, root galling and tuber lesions, the latter known as powdery scab. Attachment of zoospores to potato root hairs is the first step before infection of roots and disease development. Root hair infection results in root dysfunction leading to impaired plant productivity and yield. Varieties vary in their susceptibility to root and tuber disease; however, varietal screening is both time and resource intensive. Furthermore, traditional screens assess root galling or tuber disease and not root infection. In this study, we determined optimal conditions for zoospore release and attachment of zoospores to plant roots and used this information to develop an in vitro bioassay to assess resistance to zoospore root attachment among 153 potato lines and cultivars. Optimal zoospore release occurred at 20°C in Hoagland's solution in a rapid and synchronized manner over the first 2 days, followed by a steep decline. The extent of zoospore root attachment varied with cultivar (Iwa > Agria > Russet Burbank > Gladiator), region of the root maturation zone (lower > middle > upper) and temperature (greatest zoospore root attachment occurring at 15°C). Further comparisons suggested efficiency of zoospore root attachment was also generally associated with known variety resistance to powdery scab, zoosporangial infection and root galling, with a few notable exceptions. The bioassay proved to be a rapid and robust method for screening cultivar resistance to zoospore root attachment.