Global Spread and Genetic Differentiation of Powdery Mildew Infecting Strawberries and Raspberries

Powdery mildew poses a substantial challenge to strawberry (Fragaria ×ananassa) and raspberry (Rubus idaeus) growers. Both strawberries and raspberries are globally significant crops; annual strawberry production exceeds 9 million tons and raspberry production exceeds 880,000 tons. Until recently, the powdery mildews infecting these economically important hosts were thought to be Podosphaera aphanis. However, in our initial ITS analyses, we found multiple different host specific powdery mildews, outside of the P. aphanis clade, infecting strawberries, raspberries, and other Rubus species. We proceeded to sequence multiple loci of both freshly collected and herbarium specimens, ranging up to 100 years old, collected from throughout the world. Our analyses revealed the worldwide spread of these powdery mildews. Most notably, powdery mildews infecting strawberries and raspberries in Europe and China formed separate genetic groups to those within the United States. Future research and breeding programs should consider the different genetic groups of powdery mildew, their localities, and their virulence, when developing cultivars for the different regions. As such this research is vital for enhancing breeding programs and disease management, which has significant implications given the popularity of strawberries and raspberries.