First Report of Needle Blight on <i>Pinus sylvestris</i> var. <i>mongolica</i> caused by <i>Thyronectria pinicola</i> in Inner Mongolia, China

Pinus sylvestris var. mongolica is one of the main afforestation tree species in North China, with significant ecological and economic value (Zhang et al. 2025). In June 2025, we observed irregularly distributed red bands on the needles of P. sylvestris var. mongolica in Toudaoqiao Forest Farm (48.275750°N, 120.000670°E), Inner Mongolia, China, which seriously restricted the local ecological construction. Initially, red bands appear on the needles, followed by gradual chlorosis, eventually leading to extensive wilting. Almost 55% of the trees in the planting area were symptomatic. Randomly collected 30 symptomatic needles from different plants. These samples were cut into 3 mm pieces, surface-sterilized in 75% alcohol for 30 s. After rinsing thrice in sterile distilled water and drying, the pieces were transferred onto potato dextrose agar (PDA) and incubated at 25 ℃ for 5 days. A total of 16 morphologically similar isolates were obtained. Initially, the colonies of the fungi appeared white, after 10 days of cultivation, the mycelium gradually turned orange from the center of the colony, white conidia were evenly distributed outward. Under the microscope, the spores appeared transparent, oval to oblong, without septa. These morphological features align with the typical features of Thyronectria pinicola (Carnegie et al. 2015; Zapata. 2017). The genomic DNA of two representative isolates (NEFU-ZQY-1 and NEFU-ZQY-2) were extracted using the Fungal DNA Extraction Kit (D3390-01, OMEGA, Norcross, GA). The internal transcribed spacer (ITS) , large subunit (LSU) regions of nuclear ribosomal DNA and β-tubulin (TUB2) gene were amplified with ITS1/ITS4, NL1/NL4 (Carnegie et al. 2015) and Bt2a/Bt2b (Wang et al. 2023), respectively BLAST search results showed a homology of over 99% with T. pinicola, with a maximum homology of 100% (KY514388.1, 501/503; KP751377.1, 521/521; JF832863.1, 486/486). The sequences of ITS, LSU and tub2 were deposited in GenBank (NEFU-ZQY-1: PX589408, PX589416 and PX609846; NEFU-ZQY-2: PX589411, PX589417 and PX609847). Moreover, a phylogenetic tree based on the three genes revealed that all two isolates were closest to T. pinicola. Based on these results, the two isolates were identified as T. pinicola. To confirm pathogenicity, healthy trees of 3-year-old P. sylvestris var. mongolica were surface sterilized, healthy needles were punctured with sterile needles, and the spore suspensions (10 7 conidia/ml) of NEFU-ZQY-1 was sprayed onto the damaged needles (n=5). A control group was also established, where sterile water was sprayed instead of the conidial suspension (n=5). The seedlings were then placed in a greenhouse at 25°C. After thirty days, the inoculated plants showed symptoms similar to those of the original infected plant, whereas the control showed no symptoms. The fungus reisolated was morphologically identical to the original isolate, satisfying Koch's hypothesis. There have been reports indicating that a canker disease caused by T. pinicola on Pinus bungeana (He. et al. 2016). To our knowledge, this is the first report of needle blight on P. sylvestris var. mongolica caused by T. pinicola in Inner Mongolia, China. The voucher specimens were deposited in the Heilongjiang Province Key Laboratory of Forest Protection. Honghua'erji Scots Pine Forest is the largest sandy land Scots pine forest in Asia. Those studies could help develop effective measures to prevent and control this disease.