First Report of Leaf Blight of Hedychium coronarium Caused by Alternaria alternata in China.

Abstract

Hedychium coronarium is an economically significant crop that is widely cultivated for its ornamental, aromatic, and medicinal value (Abbas et al. 2021). From 2020 to 2023, a leaf blight was observed in about 85% of H. coronarium growing in a production field (approximately 500 m2) at Southwest University, Chongqing, China (29° 150'-29° 41' N, 105° 17'-105° 44' E). Symptoms included dark brown necrotic tissue with a clear yellow border. When the disease became severe, affected leaves became dry and abscised. Symptomatic pieces (2 to 5 mm2) between necrotic and healthy tissues were collected from 20 leaf samples, then were immersed in 70% ethanol for 10 s, 0.1% mercury bichloride for 3 min, rinsed in sterile water three times, and placed onto potato dextrose agar (PDA). Four Alternaria isolates were obtained by transferring hyphal tips to new plates. All isolates had identical morphological traits. Cultures on PDA were initially white mycelium on the rim with a light brown center. At around the fourth day, the colony margin changed into light gray and the central part turned sooty black. Conidiophores were branched. Conidia were dark brown, ovoid or ellipsoid in shape, 3.4 to 13.2 μm × 4.1 to 23.5 μm (n = 50) with zero to four transverse and longitudinal septa. For molecular identification, DNA was extracted using the PlantGen DNA Kit CW0553A (Cwbio, Taizhou, China) for PCR amplification of internal transcribed spacer (ITS) region, and 28S large subunit ribosomal RNA (LSU), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), Alternaria major allergen (Alt a 1) and actin (ACT) genes (Choi et al. 2022; Xie et al. 2022; Zhang et al. 2021). BLASTn searches showed that ITS, LSU, GAPDH, Alt a 1 and ACT of four isolates had 100% homology with the corresponding sequences of A. alternata strains MZ578214, KP940477, MK903028, MN655781 and MF564199, respectively. Representative sequences of one strain (AH1) with accession numbers of OK639009, OK639186, OK664976, OK664977 and OK664978 for ITS, LSU, GAPDH, Alt a 1 and ACT regions were deposited in GenBank. The maximum-likelihood tree generated by MEGA 5.10 demonstrated that the pathogenic isolate AH1 obtained from H. coronarium leaf was grouped in the same clade with A. alternata strain CBS121348, which was supported by 100% bootstrap values. To fulfill Koch's postulates, conidia were collected from a 7-day-old culture, suspended in sterile distilled water, and adjusted to 1 × 106 conidia/mL. Leaves on 6-month-old H. coronarium were surface disinfected with 1% sodium hypochlorite solution for 1 min, rinsed twice in water, and then inoculated with AH1 using a sprayer, while leaves treated with sterile water served as negative controls.The experiment was conducted four times, and each repeat contained 10 plants. Pathogenicity testswereperformedinthe greenhouseat25°C with a 12 h photoperiod. Partial yellow lesions were observed 3 days of post-inoculation. As the disease progressed, the tawny color gradually spread across the leaf and the tip became dark brown within 7 days. The necrosis expanded and some small leaves were completely affected within 2 weeks. The pathogen was re-isolated from the lesions and re-identified through morphological traits and sequence analysis. A. alternata have been reported to cause leaf diseases in a variety of cereal crops, vegetables, and fruits across China (Sun et al. 2021; Zheng et al. 2015), which cause significant crop loss. To our knowledge, this is the first report of A. alternata causing leaf blight of H. coronarium in the world. More surveys are needed to explore the epidemiology and management strategies for disease caused by A. alternata in Southwest China.