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

Recently, interest in cultivating blue honeysuckle (Lonicera caerulea L.) for horticulture and medicinal uses has grown (Sharma and Lee 2021). Between September 2022 and September 2023, a leaf spot disease (Fig. S1) was observed on approximately 20% of 'Lanjingling' blue honeysuckles grown in a 0.18 ha field in Qiqihar city (123.43°E, 47.92°N), Heilongjiang Province, China. Infected plants displayed black leaf spots that expanded to cover the entire leaf. Small, 3 to 4 mm segments of infected tissue 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) (Ma et al. 2023). To induce sporulation, nine purified cultures (Fig. S2) with similar culture characteristics were finally obtained from ten infected plants and they displayed a conidia morphology consistent with Neopestalotiopsis spp., no other fungi were isolated, and the isolation frequency was 90%. Conidiomata (Fig. S3) were brown to black and distributed in concentric rings with an average size of 261.98 (60.30-451.80) μm (n = 50). The conidia (Fig. S3) were fusoid and had four septa, straight to slightly curved, with an average size of 23.48 (13.50-30.30) × 5.42 (4.50-9.30) μm(n = 50), while basal and apical cells were hyaline and the three middle cells were brown with darker septa. PCR amplification was performed with ITS1/ITS4 (White et al. 1990), EFl-728F/EF1-986R (Carbone and Kohn 1999), and Btub2Fd/Btub4Rd (Glass and Donaldson 1995) primers from the genomic DNA of the LD-330. Sequences of ITS (PP033584), TEF (PP048757), and TUB (PP048758) revealed 99 to 100% (499/500, 255/255, and 481/486) shared identity with Neopestalotiopsis rosae sequences (NR145243, KM199524, and KM199430) (Rebollar-Alviter et al. 2020). Therefore, based on morphological characteristics and molecular phylogeny, LD-330 was identified as N. rosae. Six two-year-old healthy plants of the 'Lanjingling' cultivar were selected for a pathogenicity test (Yan et al. 2023). The leaves were surface disinfected with 75% alcohol and then wiped with sterilized water three times. Three plants were inoculated with 10 ml of LD-330 conidial suspension (1 × 106 spores/ml) or with sterile water as an experimental control, respectively. All plants were in closed plastic bag, incubated in a greenhouse at 28 ℃ and 75% relative humidity (RH) under a 12-h light/dark cycle, and each experiment was performed three times (Rebollar-Alviter et al. 2020). Typical leaf spot symptoms were observed on inoculated leaves after 14 days (Fig. S4), whereas no symptoms were detected on water-treated leaves. The same pathogen was reisolated from infected leaves, displayed the same morphological and molecular traits, and was again identified as N. rosae, confirming Koch's postulate. Neopestalotiopsis rosae was previously reported on pecan (Gao et al. 2022), causing black leaf spot disease in China. To our knowledge, this is the first report of a blue honeysuckle leaf spot caused by N. rosae in China and specifically in the Heilongjiang province which has the largest blue honeysuckle cultivation area in the country. Future research should be directed toward developing comprehensive management measures.