Identification of Strawberry Wilt Caused by Plectosphaerella cucumerina in China.

Strawberry (Fragaria × ananassa Duch.) is a widely cultivated economic crop in China. In April 2022, an unusual wilt disease was observed on strawberry plants (6 months' old) in Chenzui town, Wuqing district, Tianjin, China (117°1'E, 39°17'N). The incidence across the greenhouses (≈0.34 ha) was approximately 50 to 75%. The first wilt symptoms were observed on the outer leaves, then the whole seedlings wilted and died. The rhizome of the diseased seedlings changed color and became necrotic and rotted. Symptomatic roots were surface disinfected with 75% ethanol for 30 s, washed with sterile distilled water for three times, and then cut into 3 mm2 pieces (four pieces per seedling) and placed on petri dish with potato dextrose agar (PDA) containing 50 mg/L of streptomycin sulfate and incubated at 26℃ in the dark. After 6 days' incubation, hyphal tips of the growing colonies were transferred onto PDA. Eighty-four isolates belong to five fungal species were obtained from 20 diseased root samples based on their morphological characteristics. To confirm their pathogenicity, ten two-month-old healthy seedlings of strawberry (cv Red Face) planted in sterilized nutrient soil, were inoculated by pouring 50 mL of conidial suspension (107 conidia/mL) (Cai et al. 2021). Another ten seedlings poured with sterile distilled water were used as controls. Each treatment was repeated three times in a greenhouse at 25 to 28℃ and 75% relative humidity under a 12-h photoperiod. After 15 days，only seedlings inoculated with Plectosphaerella (an original percentage of 35.71%) exhibited similar symptoms to those of diseased seedlings originally observed in the field. Seedlings had no symptoms in the control and other fungi inoculation treatments. To fulfill the Koch's postulates, Plectosphaerella isolates were reisolated from each inoculated, symptomatic seedling with a percentage of 100%, but were not recovered from any of the control seedlings. The experiments were repeated twice with similar results. The results indicated that the genus Plectosphaerella was the pathogen causing strawberry wilt. Colonies of the genus Plectosphaerella isolates on PDA were white to cream, and then gradually became salmon pink, with few aerial hyphae and slimy surfaces. Colonies produced numerous hyphal coils with conidiophores. Conidia were 4.56 to 10.07 μm × 1.11 to 4.54 μm (avg. 7.10 × 2.56 μm, n=100), septate or aseptate, ellipsoidal, hyaline and smooth. Such morphological characteristics were identical to those of Plectosphaerella spp. (Palm et al. 1995). For species identification, the ITS region and D1/D2 domain of the 28S rRNA gene of representative isolates (CM2, CM3, CM4, CM5 and CM6) were amplified and sequenced with the primer pair ITS1/ITS4 and NL1/NL4, respectively (White et al. 1990; O'Donnell and Gray 1993). By BLASTn analysis, the obtained sequences of ITS amplicon (ON629742, ON629743, ON629744, ON629745, and ON629746) and D1/D2 domain amplicon (OQ519896, OQ519897, OQ519898, OQ519899, and OQ519900) showed 99.14% to 99.81% identity to sequences of P. cucumerina (MW320463.1 and HQ239025.1) in the NCBI database. A multilocus phylogenetic tree performed by the UPGMA analysis showed that the representative isolates were assigned to the group of P. cucumerina. To our knowledge, this is the first report of P. cucumerina causing strawberry wilt worldwide. This disease may induce serious economic losses in strawberry production, thus effective management strategies should be taken.