Spongospora subterranea f. sp. subterranea (Sss) causes two diseases on potato (Solanum tuberosum), lesions on tubers and galls on roots, which are economically important worldwide. Knowledge of global genetic diversity and population structure of pathogens is essential for disease management including resistance breeding. A combination of microsatellite and DNA sequence data was used to investigate the structure and invasion history of Sss. South American populations (four countries, 132 samples) were consistently more diverse than those from all other regions (15 countries, 566 samples), in agreement with the hypothesis that Sss originated in South America where potato was domesticated. A substantial genetic differenciation was found between root and tuber-derived samples from South America. Estimates of past and recent gene flow suggested that Sss was probably introduced from South America into Europe. Subsequently, Europe is likely to have been the recent source of migrants of the pathogen, acting as a “bridgehead” for further global dissemination. Quarantine measures must continue to be focussed on maintaining low global genetic diversity and avoiding exchange of genetic material between the native and introduced regions. Nevertheless, the current low global genetic diversity of Sss allows potato breeders to select for resistance, which is likely to be durable.

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.Resumen Spongospora subterranea f. sp. Subterranean causa las enfermedades de la papa de la roña polvorienta en tubérculos y agallas en la raíz, y se presenta en la mayor parte de las áreas productoras de papa en todo el mundo. Posiblemente el patógeno se introdujo a Europa de América del Sur, en el siglo XVI. En el mudo se han encontrado tres diferentes grupos de genotipos: los grupos genéticamente variables de Sudamérica (nativos) y, en contraste, el grupo casi clonal fuera de América del Sur (invasor). Se efectuó un experimento de inoculación con la papa hospedera de día largo "Agria" comparando inóculo de esporas en reposo de tres diferentes Spongospora nativas con una invasora, para determinar el riesgo potencial de infección en una subespecie ampliamente cultivada de papa. Todos los inocula condujeron a infección de la raíz. Los sporosori invasores de lesión de tubérculo de "Agria" produjeron la mayor cantidad de infección, mientras que el inóculo de la lesión del tubérculo de la hospedera venezolana S. tuberosum ssp. tuberosum, y el inóculo de las agallas de la raíz de la hospedera colombiana S. phureja causó las menores infecciones. El inóculo de los genotipos correspondió a los tres grupos descritos previamente. La mayoría de las agallas de la raíz mostraron el tipo del grupo invasor, independientemente del inóculo. Estos resultados sugieren que el genotipo invasor de mayor éxito aún está presente en poblaciones patogénicas nativas y se enfatiza la necesidad de vigilancia cuarentenaria continua para prevenir nuevas recombinaciones virulentas del patógeno.

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