DNA barcode identification and virulence testing of Phytophthora spp. and Pythium spp. isolated from soil of strawberry fields

Oomycete species belonging to Phytophthora and Pythium genera can cause root, crown or fruit rots of strawberry. In this study, we have isolated and identified by sequencing twelve novel oomycete isolates from strawberry plantations showing disease symptoms or weak growth. Virulences of four new Phytophthora and two Pythium isolates were tested using detached leaflets of four strawberry cultivars ('Bounty', 'Honeoye', 'Jonsok', 'Polka') and also with runners and roots of 'Polka'. Two of the Phytophthora isolates (GE5 and SO18) showed very low virulence, whereas Phytophthora cactorum isolate Pc407, used as a positive control, was the most aggressive towards leaflets and runners. It caused significant necrotic lesions in all cultivars but 'Bounty', which is known to be quite resistant against crown rot. The most sensitive cultivar was 'Polka'. P. megasperma isolates (SO6 and GE9) and two Pythium isolates (LG1 and SO28) represented intermediate virulence in the runners of 'Polka', but in the leaflets lesions were quite small compared to Pc407. In contrast, LG1, SO28 and SO6 rapidly induced lesions in the detached roots, suggesting that they may be more virulent towards roots than leaf tissues. The ability of these isolates to cause root rot and stunting symptoms on strawberry should be further studied using whole plants.