First Report of Crown and Root Rot Caused by Fusarium oxysporum on Photinia × fraseri Dress in China.

ABSTRACT

Photinia × fraseri Dress was introduced to China in 1998 and has been widely used in gardens as an ornamental plant. From April 2021 to 2022, a diseasecausing blight and root discoloration in approximately 80% of P. × fraseri at several landscape sites in Xuanwu Lake Park. Symptomatic root tissues were immersed in 75% ethanol for 30 s followed by 1% NaClO for 90 s, rinsed with sterile water 3 times, and placed on Potato Dextrose Agar (PDA). After 3 days of dark incubation at 25 °C, white Fusarium-like colonies grew out from the symptomatic root tissue pieces. Three representative isolates (SG1, SG6, and SG23) were obtained and deposited in China's Forestry Culture Collection Center. The hyphae grew radially and the aerial hyphae were velvety, white or pinkish-white . After 20 days, macroconidia, microconidia, and chlamydospores were produced in the colonies on PDA. Macroconidia are sickle-shaped, slightly curved, 23-50.6 µm × 4-6 µm in size. Microconidia were numerous, oval or kidney-shaped, 6.7-12.6 µm × 3.5-5.7 µm in size, with germinating from one or both ends (Fig S1E, F). Chlamydospores were spherical, smooth, and round, in chains or solitary in hyphae (Fig S1G). All three isolates had identical morphological features. Phylogenetic analysis of concatenated CAMD, RPB2 and TEF1 sequences showed that the three isolates clustered in the same clade as F. oxysporum. Two-year old P. × fraseri potted seedlings (30-cm tall, n=12) were placed at the greenhouse (temperature; 26°C, daylight; 14 hours) for the pathogenicity tests . Roots of P. × fraseri were dipped in a 10 mL of the conidial suspension (106 conidia/mL) of each isolate for 2 hours, and the control plants were inoculated with sterile water. Results showed that after 21 days post-inoculation, all inoculated seedlings (n=9) showed crown and root rot . In contrast, none of the control seedlings (n=3) were affected. Re-isolation of three fungal isolates (infected root) showed that their morphology and gene markers sequence were identical to the original isolates thus fulfilled Koch's postulates. Globally, this is the first report of F. oxysporum causing crown blight and root rot of P. × fraseri, which is also a potential threat to the two parent hosts (P. serratifolia and P. glabra). Additional surveys are being conducted for mapping the distribution of F. oxysporum in the Nanjing Province of China.