First Report of Leaf Anthracnose Caused by Colletotrichum siamense on Michelia alba in Yunnan, China

Michelia × alba (M. alba), an evergreen magnolia, is widely planted as courtyard and street ornamental tree across subtropical South China. It grows in open ground in southern provinces and pots in the Yangtze River basin (Liang and Nooteboom 1993). In February 2026, severe anthracnose was observed on M. alba in the Qujing community courtyard, Qujing City (103.783028 E, 25.472859 N), Yunnan Province, China. Disease symptoms were observed on 60 - 80% of leaves in planting plots (~ 0.015 - 0.020 ha) with 15 - 20 mature M. alba trees. The disease forms small water-soaked leaf spots expanding into irregular brown lesions with clear yellow halos. Lesions range 1.2 × 1.0–4.5 × 3.8 cm, with sunken necrotic centers and concentric rings. Under humid conditions, lesions coalesce to trigger widespread leaf blight and early leaf drop, with dark fungal fruiting bodies occasionally present on dead tissue. To isolate the fungus, 9 symptomatic leaves were randomly collected from three trees and rinsed with sterile water. Small, infected tissue segments (~ 5 mm²) were surface sterilized with 75% ethanol for 30 s, then 3% hypochlorous acid for 3 min, rinsed thrice with sterile water, and inoculated on potato dextrose agar (PDA). Plates were incubated at 28°C with a photoperiod of 12 h. 9 strains with similar morphology characterizations were isolated, and two representative isolates (BLYB1 and BLYB2) were purified. Purified colonies initially had sparse edges on PDA, becoming dense after 3 days, covering the dish by 12 days, with round, flat edges. Microscopically, the conidia were observed to be hyaline, cylindrical, and obtuse at the apex, measuring (12.30 - 16.90) × (5.56 - 8.36) μm, avg. (14.85 ± 2.30) × (6.96 ± 1.40) μm (n = 50). These morphological characteristics resembled Colletotrichum siamense. To confirm identification, genomic DNA from mycelium of these two isolates was extracted, and the sequences of internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), chitin synthase (CHS1), and calmodulin (CAL) were amplified (Zhang et al. 2020), with GenBank accessions: ITS (PZ537092, PZ537093), GAPDH (PZ584306, PZ584307), ACT (PZ584308, PZ584309), CHS1 (PZ584310,PZ584311), CAL (PZ584312, PZ584313). Pathogenicity of isolates BLYB1 and BLYB2 was assayed on healthy one-year-old leaves with needle wounding, using 5 mm mycelial plugs in three biological replicates. Inoculated leaves were cultured at 25 - 30℃, 85 - 95% relative humidity and a 12 h photoperiod. Typical leaf spots formed on all inoculated leaves at 5 days post-inoculation, whereas control leaves treated with blank PDA plugs or sterile water showed no lesions. The target fungus re-isolated from diseased tissues was identified as C. siamense, which fulfilled Koch’s postulates. The fungus was re-isolated and identified as C. siamense, fullfilling Koch's postulates. C. siamense infects at least 60 plant species across the world (Ji et al. 2019). To our knowledge, this is the first report of C. siamense causing leaf spot on M. alba in Yunnan, China via multilocus phylogenetic analysis. This finding aids targeted disease management to protect M. alba plantations, as C. siamense anthracnose reduces the ornamental value of M. alba, and supports the need for research on host-pathogen interactions and resistance breeding.