Effects of Phytophthora species on seed germination and seedling survival and performance in five sweet chestnut varieties.

The oomycete genus Phytophthora includes numerous pathogens, some causing severe tree diseases. In forestry, their impact is usually assessed on adult trees, while their effects on seed germination and seedling development-crucial phases in forest regeneration-remain poorly investigated. Here, we tested the effects of the two main causal agents of ink disease in sweet chestnut, P. cinnamomi and P. x cambivora, along with three other Phytophthora species commonly found in European sweet chestnut forests (P. plurivora, P. cactorum, P. cryptogea), on seed germination and seedling development of five Swiss sweet chestnut varieties, which we genetically characterized using microsatellite markers. A total of 180 seeds of each variety were sown in soil substrates artificially infected with different Phytophthora species and incubated for 11 weeks in a climate chamber. Seed germination, seedling survival and seedling performance (height, root length, above and below ground dry biomass) were then assessed. While P. cinnamomi had a significant negative effect on seed germination in all sweet chestnut varieties, no effect was observed for P. cactorum. The remaining Phytophthora species exhibited variety-dependent effects. After successful germination, chestnut seeds of all varieties showed a high probability of surviving and developing into seedlings, except when exposed to P. x cambivora and especially P. cinnamomi, which killed all seedlings. Phytophthora cryptogea and P. plurivora significantly impacted seedling performance in specific varieties. Our study shows that soilborne Phytophthora species can strongly reduce seed germination and, in the case of P. cinnamomi and P. x cambivora, seedling survival in sweet chestnut. In infested stands, this may hinder the recruitment of future tree generations and threaten the long-term persistence of sweet chestnut populations.