First Report of Leaf Spot on Panax notoginseng Caused by Caryophylloseptoria pseudolychnidis in Yunnan, China.

Panax notoginseng is a unique traditional medicinal plant in China, which has the effects of improving myocardial ischemia, protecting liver and preventing cardiovascular diseases (Jiang, 2020). In July 2021, gray-brown round spots were found on the leaves of P. notoginseng in the plantations of Lincang City (23º43´10˝N, 100º7´32˝E). By September, the symptoms were observed on more P. notoginseng plants, with incidence reaching 31%. Initial symptoms on leaves were small, brown spots that expanded, with black granular bulges on the lesions, often surrounded with yellow halo. As the disease progressed, multiple lesions merged, leaves became yellow, and abscission occurred. To isolate the causal pathogen, twelve symptomatic leaves were randomly obtained from twelve P. notoginseng plants. Small pieces of infected leaf tissues (about 5 mm2) were disinfected with 75% ethanol for 30 s, soaked in 2% sodium hypochlorite for 3 min, and then rinsed 3 times with sterile water and blotted dry. Sample tissues were plated on potato dextrose agar (PDA) plates incubated at 25℃ for 5 days with 12 h light/dark photoperiod. Hyphal-tips from the growing edge of colonies were transferred to fresh PDA to obtain pure cultures. Eight isolates were obtained with similar colony morphology, gray (top view) or black (back view) coloration, with a villous surface, and slow-growing on PDA. Conidia were hyaline, slender and obtuse to subobtuse at both ends, 10.3 to 52.62 (av. 25.2) µm × 1.4 to 4.0 (av. 2.4) µm (n=200) in size. Characteristics of the colonies and conidia were consistent with Caryophylloseptoria pseudolychnidis as described by Quaedvlieg et al. (2013) and Verkley et al. (2013). Genomic DNA of three representative isolates (LINC-4 to LINC-6) was extracted, and the rDNA-ITS region, ACT, and LSU gene regions were amplified and sequenced using the primer pairs ITS4/ITS5, 512F/783R, and LSU1Fd/LR5, respectively. Sequences have been deposited in GenBank (OK614104-OK614106 for ITS, OK614109-OK614111 for LSU, OK628350-OK628352 for ACT). BLAST search showed that all sequences were 98% to 100% homology with the corresponding sequences of C. pseudolychnidis. ITS sequences of the three isolates (LINC-4 to LINC-6) showed 99.21% identity (500/504 bp) to C. pseudolychnidis strain CBS 128630 (GenBank accession no. NR156266). LSU sequences of the three isolates showed 99.76% identity (823/825 bp) to C. pseudolychnidis strain CBS 128630 (MH876481). For ACT sequences, LINC-4 and LINC-5 showed 98.53% identity (201/204 bp) to C. pseudolychnidis strain 128614 (KF253599); LINC-6 showed 99.02% identity (202/204 bp) to C. pseudolychnidis strain 128614 (KF253599). Further, the neighbor-joining and maximum-likelihood method were used for multilocus phylogenetic analysis of the obtained sequences using MEGA-X (Kumar et al. 2018). The three isolates were clustered in the same clade with two C. pesudolychidis from database. Three isolates (LINC-4 to LINC-6) were tested for pathogenicity to confirm Koch's postulates. Annual potted P. notoginseng was inoculated with spore suspension (105 spores.mL-1). Each isolate was inoculated onto two leaves each of five P. notoginseng plants. The controls were similarly mock-inoculated with sterile water. To maintain high humidity (>90% RH), all plants were placed in transparent plastic boxes in a greenhouse at 25℃ with a 12 h light/dark photoperiod. Fifteen days post-inoculation, inoculated leaves showed similar symptoms to those observed in the field, and control plants remained healthy. The pathogen were reisolated from symptomatic leaf spots, and the colony characteristics were the same as those of the original isolates. Morphological characteristics, molecular data, and Koch's postulates tests confirmed C. pseudolychnidis as the cause of P. notoginseng leaf spot disease. To our knowledge, this is the first report of C. pseudolychnidis causing leaf spot on P. notoginseng in Yunnan, China. The spread of this disease might pose a serious threat to the production of P. notoginseng. The occurrence and spread of this pathogen should be further studied in order to formulate reasonable control measures.