First Report of Crown and Root Rot Caused by Phytopythium helicoides on Photinia × fraseriDress in China.

Abstract

Photinia × fraseriDress is a hybrid species of Rosaceae and Photinia genus which is widely cultivated in China.During 2020 and 2021, approximately 80% of plants growing in Xuanwu district of Nanjing, China, exhibited disease symptoms including blight, necrosis, and dieback of crowns and roots. Symptomatic root tissues collected from 2-year-old plants were rinsed with water, cut into 2-mm tissues which were surface-sterilized in 70% ethanol for 60 s, and plated onto 10% V8 PARP agar and incubated in the dark at 26°C for 3 days. Hyphae emerged from 70% of the samples. Two representative isolates (PF-he2, PF-he3) were obtained and deposited. Ten agar plugs (2×2 mm2) of each isolate were transferred into 10 mL of 10% V8 juice to produce mycelial mats. To stimulate sporangial production, 3-5 drops of soil extract solution (soil collected from healthy fields, immersed in sterile water, and filtered) were added to each plate. Sporangia were terminal, ovoid to globose or papillate. The zoospores were 7.1-9.3 µm in diameter. Each oogonium contained a single, smooth, spherical and aplerotic oospore with a diameter of 24.5-32.6 μm. These morphological properties resemble those of Phytopythium helicoides (CBS286.31 from S. F. Ashby). For molecular identification, the large subunit (LSU) rDNA, cytochrome c oxidase I (COXI) gene, and COXII gene were amplified using the primer pairs NL1/NL4 , FM55/FM52R , and FM66/FM58 . The LSU, COXI, and COXII sequences of isolate PF-he2 were 100% (763/763 nt), 98.07% (1066/1087 nt), 99.64% (561/563 nt) identical to isolate CBS 286.31 (AY598665.2), GDGJ6 (KT750956.1), and TC3 (MN952224.1), respectively. Based on the morphological and molecular analysis, the two isolates shared 100% homology were identified as Phytopythium helicoides. The pathogenicity of two isolates were tested on potted 2-yr-old (40-cm tall) P. × fraseri. Six plants were dug up to expose root balls which were wounded before inoculations with a sterile needle, and then inoculated with zoospore suspension (106 zoospore/mL). Controls were treated with ddH2O. Three seedlings/isolate were used for each treatment including controls. All plants were repotted using the original sterilized potting mix and pots. After inoculation, the plants were covered with plastic bags, and sterilized H2O was sprayed into the bags twice per day to maintain humidity and kept in a greenhouse at the day/night temperatures at 25/16 °C. All the inoculated plants showed lesions similar to those observed in the field after 23 days , whereas controls were asymptomatic. The isolates were reisolated from the lesions and sequenced as P. helicoides which has found causing root rot on Nelumbo nucifera, Rhododendron pulchrum, Zea mays in China, and also on Fragaria × ananassa in America, Peach Rootstock in California. Globally, this is the first report of P. helicoides causing crown blight and root rot of P. × fraseri. Management programs are under development to contain the spread of P. helicoides and treat diseased plants.