First Report of Phytopythium helicoides Causing Crown and Root Rot on Rhododendron pulchrum in China

Rapid detection of Phytophthora cinnamomi based on a new target gene Pcinn13739

Phytophthora cinnamomi causes crown and root wilting in more than 5,000 plant species and represents a significant threat to the health of natural ecosystems and horticultural crops. The early and accurate detection of P. cinnamomi is a fundamental step in disease prevention and appropriate management. In this study, based on public genomic sequence data and bioinformatic analysis of several Phytophthora, Phytopythium, and Pythium species, we have identified a new target gene, Pcinn13739; this allowed us to establish a recombinase polymerase amplification–lateral flow dipstick (RPA-LFD) assay for the detection of P. cinnamomi. Pcinn13739-RPA-LFD assay was highly specific to P. cinnamomi. Test results for 12 isolates of P. cinnamomi were positive, but negative for 50 isolates of 25 kinds of Phytophthora species, 13 isolates of 10 kinds of Phytopythium and Pythium species, 32 isolates of 26 kinds of fungi species, and 11 isolates of two kinds of Bursaphelenchus species. By detecting as little as 10 pg.µl−1 of genomic DNA from P. cinnamomi in a 50-µl reaction, the RPA-LFD assay was 100 times more sensitive than conventional PCR assays. By using RPA-LFD assay, P. cinnamomi was also detected on artificially inoculated fruit from Malus pumila, the leaves of Rhododendron pulchrum, the roots of sterile Lupinus polyphyllus, and the artificially inoculated soil. Results in this study indicated that this sensitive, specific, and rapid RPA-LFD assay has potentially significant applications to diagnosing P. cinnamomi, especially under time- and resource-limited conditions.

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

Photinia × fraseri Dress 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.