Population genetics of the stem nematode Ditylenchus dipsaci – unravaling the race complex

The stem nematode Ditylenchus dipsaci is an obligate plant parasite. Although problematic in the Netherlands, it is a cosmopolitan species mostly prevalent and damaging in temperate regions across the world. Stem nematodes can cause severe losses in bulbous and tuberous crops/ornamentals, but also affect cereals, legumes and green-manure crops. In literature over 30 distinct ’host races’ have been described that infect 500 different plant species. However, the inconsistency of these host races has limited their practical value and lead many authors to question their validity. Therefore gaining a genetic and biological insight into these races would be valuable for disease management. To this end we are in the process of collecting diverse D. dipsaci populations, which are genetically characterised using whole-genome sequencing, a first for the species. So far we have constructed a draft reference genome, collected over 43 distinct populations, and sequenced 31 of these. Our results indicate the existence of defined subclusters within D. dipsaci, pointing towards the existence a smaller number of races. However, primary isolation host and geography are confounded in the current dataset, making it unclear which of the two is driving genetic diversity.