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Title: First Report of Phytophthora pini Causing Foliage Blight and Shoot Dieback of Rhododendron pulchrum in China.
Author: Xu Y, Yang X, Li Y, Chen Z, Dai T.
Journal: Plant Dis; 2020 Dec 02; ():. PubMed ID: 33267642.
Abstract:
During a 2019-2020 survey for plant pathogenic oomycetes in Nanjing, China, severe foliage blight and dieback were observed on approximately 20 Rhododendron pulchrum plants at three public parks and gardens. Approximately 25% of leaves and shoots were affected. Symptoms included brown to black lesions on leaves and stems, dieback of shoot tips, and wilting. Diseased tissues were collected from a five-year-old shrub with typical disease symptoms at Xuanwuhu Park. They were cut into 10×10 mm2 squares, immersed in 70% ethanol for 30 sec, and placed onto fresh clarified V8 juice agar (cV8A) containing pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene. Phytophthora-like hypae were transferred to new cV8A plates daily. A total of five isolates were obtained after five days of incubation at 25°C. After approximately 20 days, all isolates were identical in morphological traits including semi-papillate sporangia and gametangia (homothallic). Thirty sporangia of a representative isolate Ppi were randomly selected and examined. They were mostly ovoid and sometimes obpyriform, averaging 41.0 ± 3.9 × 24.8 ± 3.2 µm. Antheridia of 30 randomly selected gametangia were paragynous, averaging 16.7 ± 0.7 × 12.4 ± 1.5 µm. Average diameters of oogonia and plerotic oospores were 29.2 ± 0.3 µm and 26.4 ± 1.6 µm, respectively. Chlamydospores were not observed. The above morphological traits suggested the causal agent belonging to the "P. citricola-complex". Isolate Ppi was subjected to sequencing of the rDNA internal transcribed spacer (ITS) region and the ras-related GTP-binding protein 1 (Ypt1) gene. ITS sequence of Ppi (GenBank ACN. MT672594) has 100% identity to that of P. pini (MG865565). It has a 3-nt difference from the ITS sequences of P. acerina (MG518642) and P. citricola (MG865475) and a 4-nt difference from that of P. plurivora (FJ665225). Ypt1 sequence of Ppi (MT680000) has 100% identity to that of P. pini (MK058416). Pathogenicity of Ppi on R. pulchrum was tested using both detached-leaf and whole-plant assays. In the former assay, each of six asymptomatic leaves was symmetrically wounded at both sides using a sterile inoculation needle. A 5×5 mm2 Ppi-colonized cV8A plug was placed on each wound of five leaves. Sterile agar plugs were used for a control leaf. All six leaves were placed on a wet filter paper in a closed container at 25°C. This assay was repeated twice. On the fifth day, all inoculated leaves had necrotic tissues around the wounds, while the control leaves remained asymptomatic. In the whole-plant assay, 20-inch-tall plants were used. Five attached leaves and the twig base of each plant were wounded. A control plant was inoculated in the same manner above, while sterile agar plugs were used. Each plant was covered with a plastic bag and maintained at 25°C. Wet cotton balls were placed in the bags to maintain humidity. After two days, the bag containing cotton balls was removed. This assay was repeated three times. After two weeks, all three inoculated plants in the three replicated trials had severe foliage blight and dieback, whereas control plants remained healthy. Phytophthora isolates recovered from artificially inoculated tissues were identical to isolate Ppi in morphological characters. Rhododendron diseases caused by P. pini were reported in the USA and Finland . This is the first report of P. pini causing foliage blight and dieback on R. pulchrum, an important nursery and landscape plant in China. Additional surveys are ongoing to determine the distribution of this pathogen in Nanjing. Management programs are under development to contain the spread of P. pini and treat diseased plants.

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