Spongospora subterranea genetic diversity
Powdery scab is one of the most serious diseases of potatoes wherever intensive potato cropping is carried out. It is caused by the pathogen Spongospora subterranea - which may be seed-borne or soil-borne. Consequently planting disease-free and uncontaminated seed-tubers in infested soil; or infected or contaminated tubers in uninfested soil can both lead to the development of disease symptoms. Transporting diseased potatoes or contaminated soil has probably been the main route of spread of S. subterranea. Given this, the extent of the genetic diversity of S. subterranea in different locations is difficult to predict. However, knowing about its diversity could be useful, for example when plant breeders are looking for sources of resistance to pathogens it is useful to know how genetically variable the pathogens are, as it may impact on the durability of resistance.
AHDB Potatoes was one of several international sponsors who provided support for a PhD student (Rebecca Gau) to carry out research on the genetic diversity of S. subterranea. Results from her work are summarised below.
Global genetic characterisation of the powdery scab pathogen
This study was the first to characterise the powdery scab pathogen on a global scale. Collections of the pathogen were obtained from 19 countries, from all continents. Close to 700 samples (from tuber lesions or root galls) were analysed using molecular techniques. Collections from South America were variable (six “strains” were differentiated), while collections from almost all other areas of the world were genetically identical to each other. This indicates that the pathogen probably originated in the highlands of northern South America (where greatest diversity occurs), the same area where potatoes were first developed as food crops.
Computer analysis procedures were used to test different scenarios of pathogen evolution and spread around the world. The pathogen was probably transported out of South America during the 1500s, when the conquistadors introduced potatoes to Europe. Only a very few original introductions from South America to Europe were likely, which accounts for the very low diversity in all countries other than in South America. Further spread probably occurred when European colonialists took potatoes from Europe to other areas. More recently, the export of seed potatoes from Europe has probably caused further dissemination of the pathogen.
Implications for potato breeding
The low diversity of the pathogen in all areas except South America has important implications. Potato breeders working to develop powdery scab resistant cultivars who are working in areas outside South America can proceed with the knowledge that the pathogen they are selecting against is genetically very uniform. This suggests that cultivars selected for resistance to powdery scab in one area are likely to be resistant in most other areas of the world. South America is likely to be a good area to find potato varieties that are resistant to powdery scab. Plant types from that area are likely to have been exposed to different strains of the pathogen and may possess usefully broad resistant characteristics.
Another important outcome is that spread of different strains from South America to other countries should be prevented. This emphasises that plant quarantine regulations should be robust particularly to prevent import of potentially infested soil or plant material from South America.
Dr Rebecca Gau worked in Switzerland at the Eidgenössische Technische Hochschule (ETH) in Zurich, and in New Zealand at the NZ Institute for Plant & Food Research at Lincoln. Her project was supervised by Dr Ueli Merz, Dr Patrick Brunner and Prof Richard Falloon, and was funded by the ETH; Horticulture Australia Ltd; AHDB Potatoes; the New Zealand Ministry for Science and Innovation; Potatoes New Zealand (Horticulture New Zealand); and the Swiss Ministry for Agriculture.
Details of Dr Gau’s research have been published by PLoS ONE and are available online