First Report of Apiospora arundinis Causing Leaf Spot on Polygonatum cyrtonema Hua in China

Abstract

Polygonatum cyrtonema Hua, which is a perennial herb of Liliaceae, can be used as clinical Chinese medicine for treating diabetes and asthma. It is widely cultivated in China, with 700 ha planted in Tonggu County, Jiangxi province (Chen et al. 2022). In June 2022, leaf spot symptoms were observed on P. cyrtonema in Tonggu county (28°71′42″N, 114°56′19″E), and the disease incidence was estimated to be above 35%. In the early stages of infection, small brown spots appear on the edge or tip of the leaves. As the lesion matures, the spots gradually expand to form wedge-shaped or elliptic to irregular lesions with brown edges and yellow halos. To identify the pathogen species, leaf pieces (5 × 5 mm) from the lesion borders were surface sterilized in 75% ethanol for 30 s, followed by 2% NaClO for 2 min, then rinsed with sterile distilled water for 3 times and dried with sterile filter paper. The tissues were placed on potato dextrose agar (PDA) and incubated at 25°C. Pure cultures were obtained by monosporic isolation, and the representative isolates, HJYB2-1, HJYB2-2 were used for morphological studies and phylogenetic analyses. Colonies on PDA of the two isolates were white with fluffy aerial mycelia. The hyphae were smooth, hyaline, and branched. The conidiophores were hyaline or pale brown and produced conidiogenous cells. The conidiogenous cells were pale brown, smooth, and ampulliform, 5.8-11.7 × 3.0-4.9 μm (n=50). The conidia were brown, smooth, and ellipsoidal to spherical, 4.7-7.3 × 2.3-4.5 μm (n=50). Morphological features were similar to Apiospora arundinis species complex (Crous et al. 2013, Pintos et al. 2021). For molecular identification, the ITS, TUB2 and TEF 1-α genes were amplified with the primer pairs ITS1/ITS4, T1/Bt-2b and EF1-728F/EF-2 (White et al. 1990), respectively. The generated sequences were deposited in GenBank with accession numbers OR229623, OR229624 (ITS), OR260104, OR260105 (TUB2), and OR290789, OR290790 (TEF1-α). Maximum likelihood and Bayesian posterior probability analyses using IQtree v. 1.6.8 and Mr. Bayes v. 3.2.6 with the concatenated sequences placed HJYB2-1 and HJYB2-2 in the clade of A. arundinis. The two isolates were identified as A. arundinis based on morphological and multilocus phylogenetic analyses. For pathogenicity testing, healthy leaves of six P. cyrtonema plants (five leaves each, n=30) in the field were pin-pricked with a sterile needle and inoculated with 100 µL spore suspension on the wound (1×106 conidia per mL). Healthy leaves of another three P. cyrtonema plants (five leaves each, n=15) in the field were pin-pricked with a sterile needle, inoculated with sterile distilled water and served as the control. All the inoculated leaves were covered with plastic bags to keep a high-humidity environment with a temperature of about 28℃. The test was repeated three times. More than 90% of inoculated leaves showed similar symptoms to those observed in the field, whereas control leaves were asymptomatic for 7 days. Apiospora arundinis was reisolated from the leaf lesions on the inoculated plants. No pathogenic fungus was isolated from the control leaves. A. arundinis has been reported causing disease on Camellia sinensis (Thangaraj et al. 2019), Prunus persica (Ji et al. 2020), Saccharum officinarum (Liao et al. 2022) but has not previously been reported causing disease on P. cyrtonema. To our knowledge, this is the first report that A. arundinis can cause leaf spot on P. cyrtonema in China. Our result should help with future monitoring and control of this disease.