Infection risk potential of South American Spongospora subterranea f.sp. subterranea root gall and tuber lesion inoculum on potato (Solanum tuberosum ssp. tuberosum).

Spongospora subterranea f. sp. subterranea causes the potato diseases powdery scab on tubers and galls on roots, and occurs in most potato production areas worldwide. The pathogen was probably introduced to Europe from South America in the 16th century. Three different genotype clusters have been found worldwide: the genetically variable groups from South America (native), and, in contrast, the nearly clonal group outside South America (invasive). An inoculation experiment was carried out with the long-day potato host ‘Agria’ comparing three different native Spongospora resting spore inocula with an invasive one, to determine the infection risk potential on a widely grown potato subspecies. All inocula led to root infection. Invasive tuber lesion sporosori from ‘Agria’ produced the greatest amount of infection, whereas the tuber lesion inoculum from the Venezuelan S. tuberosum ssp. tuberosum host and the root gall inoculum from the Colombian S. phureja host caused the least infections. The inoculum genotypes corresponded to all of the three previously described groups. Most root galls showed the invasive group type, independent of the inoculum. These results suggest that the most successful invasive genotype is still present in native pathogen populations and emphasize the need for continued quarantine vigilance to prevent new virulent recombinants of the pathogen.