First report of leaf spot disease caused by Pseudopithomyces chartarum on Lonicera caerulea L. in Heilongjiang Province, China.

Blue honeysuckle (Lonicera caerulea L.) cultivation has gradually expanded in China but continues to be limited by challenges such as leaf spot disease. Between September 2022 and September 2023, a leaf spot disease was observed on approximately 30% of 'Lanjingling' blue honeysuckles grown in a 2.66 ha field (a total of about 11,000 plants) in Jiamusi city (130.47°E, 46.16°N), Heilongjiang Province, China. Affected plants displayed brown necrotic lesions on their leaves that gradually expanded in area until the leaves fell off the plant entirely. Small, 3 to 4 mm segments of infected tissue from 50 randomly selected leaves were surface sterilized with 75% ethanol for 30 s and 5% sodium hypochlorite (NaOCl) for 3 min, rinsed three times with sterile distilled water, dried on paper towels, and plated in 9 cm Petri dishes containing potato dextrose agar (PDA) (Yan et al. 2022). Five pathogens (LD-232, LD-233, LD-234, LD-235, and LD-236) were isolated on PDA and displayed a conidia morphology consistent with Pseudopithomyces spp. (Perelló et al. 2017). The fungal colonies on PDA were villiform, white, and whorled and had sparse aerial mycelium on the surface with black conidiomata. The conidia were obpyriform and dark brown, had 0 to 3 transverse and 0 to 1 longitudinal septa, and measured 9.00 to 15.30 μm × 5.70 to 9.30 μm in size (n = 50). Genomic DNA was extracted from a representative isolate, LD-232, for molecular verification and PCR amplification was performed with ITS1/ITS4 (White et al. 1990), LROR/LR7 (Carbone and Kohn 1999), and RPB2-5F2/RPB2-7CR (Liu et al. 1999) primers. Sequences of LD-232 ITS (OR835654), LSU (OR835652), and RPB2 (OR859769) revealed 99.8% (530/531 nt), 98.8% (639/647 nt), and 99.8% (1015/1017 nt) shared identity with Pseudopithomyces chartarum sequences (OP269600, OP237014, and MK434892), respectively (Wu et al. 2023). Bayesian inference (BI) was used to construct the phylogenies using Mr. Bayes v. 3.2.7 to confirm the identity of the isolates (Ariyawansa et al. 2015). Phylogenetic trees cannot be constructed based on the genes' concatenated sequences because selective strains do not have complete rDNA-ITS, LSU, and RPB2 sequences. Therefore, based on the morphological characteristics and molecular phylogeny, LD-232 was identified as P. chartarum (Perelló et al. 2017; Wu et al. 2023). A pathogenicity test was performed with six healthy, two-year-old 'Lanjingling' blue honeysuckle plants. Three plants were inoculated by spraying the LD-232 conidial suspension (1 × 106 spores/ml) or clean water as an experimental control condition (Wu et al. 2023; Yan et al. 2023). All plants were cultured in a greenhouse at 28℃ under a 12-h light/dark cycle, and each experiment was replicated three times. Typical leaf spot symptoms were observed on inoculated leaves after 10 days. The same pathogens were reisolated from infected leaves, displayed the same morphological and molecular traits, and were again identified as P. chartarum, confirming Koch's postulate. P. chartarum previously caused leaf spot disease on Tetrapanax papyrifer in China (Wu et al. 2023). To our knowledge, this is the first report of blue honeysuckle leaf spot caused by P. chartarum in China. Identification of P. chartarum as a disease agent on blue honeysuckle will help guide future management of leaf diseases for this economically important small fruit tree.