Susceptibility of potato (Solanum tuberosum) cultivars to powdery scab (caused by Spongospora subterranea f. sp subterranea), and relationships between tuber and root infection

Ninety-nine potato ( Solanum tuberosum) cultivars and 13 breeding lines were assessed in field trials for susceptibility to powdery scab ( caused by Spongospora subterranea f. sp. subterranea). The trials were carried out over 11 growing seasons ( each cultivar/line tested in at least two growing seasons) in soil inoculated with S. subterranea and regularly irrigated during crop growth. The cultivars and lines were categorised as 'very resistant' (21%), 'moderately resistant' ( 28%), 'moderately susceptible' (33%) or 'very susceptible' (19%). Scaled average severity scores for the cultivars and lines followed a continuum between very resistant and very susceptible, suggesting that resistance to powdery scab is quantitative. In a glasshouse experiment where small plants were inoculated with S. subterranea sporosori, 15 cultivars selected to cover the spectrum of field-assessed susceptibility were assessed for intensity of infection by S. subterranea zoosporangium in roots and for numbers of root galls on root systems. All of the cultivars developed zoosporangia and root galls (i.e. no cultivar was immune from infection), and root infection was usually related to tuber infection. Field-resistant cultivars ( tuber infection) generally had low numbers of root zoosporangia and root galls in the glasshouse experiment, and cultivars that were very susceptible to tuber infection in the field had high levels of root infection in the glasshouse experiment. An exception was the early maturing cv. Swift, which had very low levels of tuber infection in the field, but had high numbers of root zoosporangia and root galls in the glasshouse. These results demonstrate the relative reaction of a large number of potato cultivars to powdery scab and reaffirm the potential for using plant resistance as a powdery scab management strategy. Development of S. subterranea in host roots, even in cultivars with resistance to tuber infection, is likely to be important in the epidemiology of powdery scab.