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First Report of Downy Mildew Caused by Plasmopara angustiterminalis on Ligularia fischeri in China

Chen, Y., Wu, C. Y., Bai, X., Bai, Q. R., Zhao, T. C.
Plant disease 2018 v.102 no.1 pp. 254
Ambrosia artemisiifolia, DNA, DNA primers, Ligularia fischeri, Plasmopara angustiterminalis, Xanthium strumarium, container-grown plants, cuisine, cytochrome-c oxidase, death, downy mildew, financial economics, greenhouses, harvest date, herbaria, inoculum, leaves, oospores, ornamental plants, pathogenicity, pathogens, polymerase chain reaction, protein subunits, ribosomal proteins, runoff, sporangia, sporulation, spraying, stomata, vegetable crops, Austria, Bulgaria, China, Czech Republic, Poland, Ukraine
Ligularia fischeri (Ledeb.) Turcz is an edible, medicinal, and ornamental plant in the family Asteraceae. It is popularly used in Korean cuisine. Since 2010, commercial cultivation of L. fischeri as a vegetable crop has increased rapidly in Antu County (43°04′11.27″N, 128°43′25.08″E), Jilin Province, China. In 2016, L. fischeri cultivation exceeded 50 ha in Jilin. In June through September of 2015 and 2016, severe downy mildew symptoms were observed on the foliage of L. fischeri. The incidence increased from 60 to 80% in the late harvest stage, which caused unmarketable leaves and serious economic losses. Representative specimens (HMJAU-75202) were deposited in the Jilin Agricultural University herbarium. Symptoms first appeared as large angular chlorotic spots bordered by veins on the upper leaf surface. Lesions expanded rapidly, causing early death of leaves. The underside of infected leaves showed downy-looking whitish sporulation of the pathogen. Sporangiophores, emerging through stomata, measured up to 620 μm long and 8 to 12 μm wide, and were hyaline, aseptate, straight, with 3 to 6 monopodial ramifications occurring mainly at right angles, the final branch ending in 3 or 4 ultimate branchlets. Sporangia were globose to ovoid, hyaline, smooth, 16 to 30 μm long and 12 to 16 μm wide (n = 100). Oospores were not observed. Morphological characteristics were consistent with the original description of Plasmopara angustiterminalis Novot (Novotelnova 1962). Total genomic DNA was extracted from symptomatic plant tissue using Chelex-100 (Walsh et al. 1991). The large ribosomal subunit DNA was amplified by PCR using primers NL1 and NL4 (Khodaparast et al. 2005). The resulting amplicon of 764 bp (KY432530) had 99% similarity to other isolates of P. angustiterminalis (AY178535 and EU743806). The cytochrome oxidase subunit 2 was also amplified using primers P-COX2 F/R (Hudspeth et al. 2000) and the resulting amplicon of 602 bp (KY091622) had 100% similarity with P. angustiterminalis (HM628738, EU743812). In pathogenicity tests, 10 healthy potted L. fisheri plants were inoculated with a sporangial suspension (1 × 10⁵ sporangia/ml washed from infected leaves) by spraying until runoff. Ten additional potted plants were inoculated with sterile water as the control. Plants were incubated in a moist chamber at 21 ± 2°C for 48 h and then maintained in a greenhouse at 22 to 25°C until symptom development. All inoculated plants developed symptoms of downy mildew within 8 days. Symptoms were similar to those of naturally infected plants. The downy mildew on the inoculated plants exhibited identical morphological characteristics to the original inoculum. No symptoms or signs were observed on control plants. P. angustiterminalis was reported to infect Ambrosia artemisiifolia in Ukraine, Xanthium strumarium in Austria, Bulgaria, China, the Czech Republic, and Poland, and X. strumarium subsp. italicum in Bulgaria (Farr and Rossman 2017). To our knowledge, this is the first report of downy mildew caused by P. angustiterminalis on L. fischeri in China.