First Report of Neopestalotiopsis clavispora Causing Leaf Spot on Phoebe bournei in China

Abstract

Phoebe bournei (Hemsl.) Yang is a typical evergreen broadleaf species widely distributed in subtropical China for its ornamental and economic value (Zhang et al. 2021). The wood of P. bournei is considered a good material for architectural decoration and furniture (Li et al. 2018). In June 2020, leaf spot symptoms were observed in Dexing (28°41′22.056″N, 115°51′52.524″E), Jiangxi province, China. Initial disease symptoms were small brown spots on the leaves. Then, the spots enlarged and coalesced into regular or irregular dark brown necrotic lesions with dark margins. Disease incidence in the field in Dexing was estimated 25%. Leaf pieces (5 × 5 mm) from the lesion borders were surface-sterilized in 70% ethanol for 30 s, followed by 2% NaOCl for 1 min, and then rinsed three times with sterile water. Tissues were placed on potato dextrose agar (PDA) and incubated at 25°C under a 14/10 h light/dark cycle for 4 days. Pure cultures were obtained by monosporic isolation, and the representative isolates, JX-N2, JX-N7, and JX-N11 were used for morphological studies and phylogenetic analyses. The colonies of three isolates grown on PDA were white, cottony, and flocculent, contained undulate edges with dense aerial mycelium on the surface. Conidia were 5-celled, clavate to fusiform, smooth, 18.7-24.6 × 5.9-8.8 μm (n = 100). The 3 median cells were dark brown to olivaceous, central cell was darker than other 2 cells, and the basal and apical cells were hyaline. All conidia developed one basal appendage (3.4-8.3 μm long; n = 100), and 2-3 apical appendages (17-30 μm long; n = 100), filiform. Morphological features were similar to Neopestalotiopsis sp. (Maharachchikumbura et al. 2014). The internal transcribed spacer (ITS) regions, β-tubulin 2 (TUB2) and translation elongation factor 1-alpha (TEF1-α) were amplified from genomic DNA for the three isolates using primers ITS1/ITS4, T1/Bt-2b, EF1-728F/EF-2 (Maharachchikumbura et al. 2014), respectively. All sequences were deposited into GenBank (ITS, OQ355048 - OQ355050; TUB2, OQ357665 - OQ357667; TEF1-α, OQ362987 - OQ362989). A maximum likelihood and Bayesian posterior probability-based phylogenetic analyses using IQtree v. 1.6.8 and Mr. Bayes v. 3.2.6 with the concatenated sequences (ITS, TUB2, TEF1-α) placed JX-N2, JX-N7, and JX-N11 in the clade of N. clavispora. Based on the multi-locus phylogeny and morphology, the representative isolates were identified as N. clavispora. The pathogenicity of three isolates were tested on six 9-year-old P. bournei plants, which were grown in the field. Three leaves per plant were wounded with a sterile needle (Φ=0.5 mm) and inoculated with 20 μL conidial suspension per leaf (106 conidia/mL). Another six control plants were inoculated with sterile water. Each leaf was covered with plastic bags to keep a humidity environment for 2 days. All the inoculated leaves showed similar symptoms to those observed in the field, whereas control leaves were asymptomatic for 9 days. N. clavispora was reisolated from the lesions, whereas no fungus was isolated from control leaves. N. clavispora can cause leaf diseases in a variety of hosts, including Machilus thunbergii (Wang et al. 2019), Fragaria × ananassa (Shi et al. 2022), Taxus media (Li et al. 2022). However, this is the first report of N. clavispora infecting P. bournei in China. This work provided crucial information for epidemiologic studies and appropriate control strategies for this newly emerging disease.