First Report of Leaf Spot on Lasia spinosa Caused by Fusarium fujikuroi in China

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Lasia spinosa (L.) Thwaites (Araceae) is an ornamental medicinal plant grown in tropical and subtropical countries. The leaf extract of L. spinosa has significant anthelmintic efficacy (Yadav and Temjenmongla 2012). During July 2018 and 2019, L. spinosa plants at the Guangxi Botanical Garden of Medicinal Plants of Guangxi province, China, showed leaves with brown spots or irregular white necrotic lesions that were scattered with some shot holes. Sixty-eight plants from a total of 200 plants showed such symptoms. The overall incidence was about 34%. To isolate the pathogen, 15 infected leaves were excised near the margin of necrotic lesions, surface sterilized with 75% ethanol for 1 min, and treated with 0.5% sodium hypochlorite for 10 min followed with three rinses with sterile water. The leaf sections were then plated on potato dextrose agar (PDA) medium and incubated at 28°C. Further, to obtain pure cultures, hyphal tips from recently germinated spores were excised and transferred onto fresh PDA and incubated at 28°C. Ten fungal isolates with similar morphological characteristics were obtained. Colonies on PDA developed white fluffy aerial mycelia and produced abundant conidia 2 weeks later. Microconidia were abundant, oval, clavate, solitary, and hyaline, with an average size of 7.7 × 3.1 μm (n = 46). Macroconidia were mostly three to five septate, slender, almost straight, and sickle shaped, with an average size of 26.7 × 3.6 μm (n = 30). No chlamydospores were observed. These morphological characteristics were consistent with those of Fusarium fujikuroi (Leslie and Summerell 2006). For further identification, five DNA regions of isolate CYA, corresponding to the internal transcribed spacer (ITS), β-tubulin (TUB2), calmodulin (CL), translation elongation factor 1-alpha (TEF1), and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) were PCR amplified and sequenced (Jiang et al. 2018; Weir et al. 2012). BLASTN analyses of ITS and CL sequences showed 100% similarity with corresponding sequences of F. fujikuroi accession numbers MK630103.1 and MF984412.1; the TUB2 and TEF1 sequences shared 99.71% identity (MH398245.1 and MH341224.1), and the GAPDH sequence shared 98.65% identity (CP023092.1). Sequences were submitted to GenBank under the accession numbers MN704565 (ITS), MN490089 (TUB2), MN490091 (CL), MN490092 (TEF1), and MN490090 (GAPDH). According to those morphological and molecular characteristics, the pathogen was identified as F. fujikuroi. Further, to verify the pathogenicity, spore suspensions (10⁵ conidia/ml) were prepared from 10-day-old PDA cultures and inoculated onto leaves of nine 2-month-old L. spinosa plants using the wound/drop inoculation method (10 μl per wound). Three more plants were inoculated with sterile water as controls. All plants were grown in a growth chamber with a temperature of 28°C and 90% humidity. After incubating for 7 days, leaf spot symptoms were observed on the inoculated leaves, and no symptoms were observed on the control plants. F. fujikuroi was reisolated from leaf lesions on the inoculated leaves, thus confirming Koch’s postulates. The pathogenic experiment was subsequently repeated under the same conditions. To our knowledge, this is the first report of F. fujikuroi causing leaf spot of L. spinosa. Severe leaf spot disease on L. spinosa has affected its medicinal and ornamental values, which suggests breeders should take this disease into consideration in L. spinosa breeding programs.