First Report of Stagonosporopsis caricae Causing Chayote Leaf Spot in Guizhou Province, China

Abstract

At present, chayote (Sechium edule (Jacq.) Swartz) have been widely planted in Guizhou Province, southwestern China, and the cultivation area in Huishui county ranks first among all the counties or cities in Guizhou Province. Chayote leaf spot was firstly observed in Huishui County (25.99°N, 106.64°E) from April to June in 2019. The disease incidence ranged from 52% to 58%, and the severity of leaf symptoms ranged from 34 to 41% across nine chayote plantations. Such levels disease development lead to considerable enocomic losses. Leaf lesions initially occurred at the leaf margins, and the lesions expanded gradually, becoming dark brown and irregularly shaped. To identify the leaf spot-associated pathogen, the samples were cut from lesion margins, sterilized with 75% ethanol followed by 0.5% sodium hypochlorite for 30 s, rinsed with sterile water three times, and transferred onto potato dextrose agar (PDA). They were then incubated at 25°C in darkness for 5 days. The hyphal tips from the margins of growing colonies were successively transferred to fresh PDA plates for obtaining isolates. All strains grew with a similar morphology on PDA, malt extract agar (MEA), and oatmeal agar (OA) plates, and the colonies presented smooth margins and abundant mycelia on all three media. The colonies were gray to light green on PDA and gray on MEA and OA at 5 days post-inoculation. At 11 days post-inoculation on 10% V8 medium at 25oC with a cycle of 14 h/ultraviolet light and 10 h/night, sexual morph was observed, ascomata pseudothecioid, subglobose, 121 × 142 μm, ostiolate, walls of brown textura angularis, and smooth. Asci were bitunicate, cylindrical to clavate, 7 × 90 μm, 8-spored, ascospores elliptical, straight to slightly curved, 5 × 17 μm, 1-septate, constricted at the septum, sub-hyaline, and smooth. Conidiomata were pycnidial, subglobose, 166 × 258 μm, ostiolate, wall of dark brown to black textura angularis, smooth. Conidia were short, cylindrical or slightly reniform, 6.18 ± 0.67 × 3.51 ± 0.33 μm (n = 50), 0-1 septate, hyaline, smooth. Chlamydospores were subhyaline to dark brown, verruculose or incidentally tuberculate, and solitary or in chains, and 14.16 ± 1.23 × 5.92 ± 0.49 μm (n = 50). The morphological characteristics of the strains were identical to those of Stagonosporopsis caricae (Aveskamp et al. 2010; Sivanesan 1990). The genes or DNA sequences of the partial 28S large subunit rDNA, the internal transcribed spacer, RNA polymerase II second largest subunit, and beta-tubulin were amplified (Liu et al. 1999; Rehner and Samuels 1994; Sung et al. 2007; Vilgalys and Hester, 1990; White et al. 1990; Woudenberg et al. 2009). The sequences were further deposited in GenBank (ITS: MZ619042-MZ619044, LSU: MZ620651-MZ620653, RPB2: MZ673652-MZ673654, and TUB: MZ673649-MZ673651). A phylogenetic analysis confirmed these strains to be identical to S. caricae reference strains CBS 248.90, CBS 282.76, and PD 06/03082531. Pathogenicity tests were performed on potted chayote and five-year-old chayote in the field. Mycelial plugs (6 mm diameter) were applied on wounded chayote leaves. Brown spots appeared on the wounded sites of chayote leaves after inoculation with mycelial plugs. No symptoms were observed on the leaves inoculated with PDA plugs lacking mycelia. The re-isolated pathogen from diseased plants was identical to the representative strains used for inoculation. To our knowledge, this is the first report of S. caricae causing leaf spot on chayote in China, and our findings will be useful for its management and further research.