First Report of <i>Colletotrichum fioriniae</i> Causing Leaf Anthracnose on <i>Spiraea japonica</i> in China

Spiraea japonica is widely used for landscaping in China, particularly in urban gardens and parks. During the summer of 2024, leaf blight symptoms were observed on S. japonica in Tongbai County, Henan Province, China (32°22′21″ N, 113°24′10″ E). Field surveys across different areas recorded a disease incidence of 51% (n = 100 plants). Initial symptoms appeared as small, dark brown to nearly black, circular to irregular leaf spots that expanded with tan centers and darker margins; lesions frequently coalesced, producing large, blighted areas and occasional premature defoliation. Diseased leaves (n = 15) were collected and surface sterilized. From each leaf, two pieces were excised from the lesion margin and placed on potato dextrose agar (PDA). Twenty-seven Colletotrichum isolates were recovered (positive isolation rate 90%); three representative isolates from distinct sites (X-21, X-22, X-23) were selected for detailed analyses. On PDA after 5 days of incubation, colonies were fast-growing with aerial mycelium; colonies exhibited a salmon-pink surface with a vinaceous-red reverse. Conidia were hyaline, aseptate, smooth-walled, and cylindrical to fusiform, measuring 9.1–16.6 × 1.6–5.2 μm (n = 100). Appressoria were medium to dark brown, ovoid with entire margins. These features are consistent with members of the Colletotrichum acutatum species complex (Talhinhas et al. 2023). For molecular identification, five loci—the internal transcribed spacer (ITS), dehydrogenase (gapdh), chitin synthase (chs-1), actin (act), and calmodulin (cal)—were amplified for each isolate. All newly generated sequences have been deposited in GenBank under the following accession numbers: PV342493-PV342495 (ITS), PV798316-PV798318 (gapdh), PV575167- PV575169 (chs-1), PV417281, PV417282, PV468789 (act), PV755113, PV660595, PV755114 (cal). A maximum-likelihood phylogeny based on the concatenated dataset placed X-21, X-22, and X-23 within the Colletotrichum fioriniae clade, clearly separated from other congeners. BLAST results showed ≥99–100% identity to reference C. fioriniae strains for all loci. Together, morphology and multilocus phylogeny support the identification of these isolates as C. fioriniae (Zhang et al. 2023). Pathogenicity was assessed by applying 50 μL droplets of a conidial suspension (10⁶ conidia mL⁻¹) onto the adaxial surface of unwounded leaves on five potted S. japonica plants; leaves on five additional plants received 50 μL of sterile water as mock-inoculated controls. Plants were maintained at 28 °C and ~90% relative humidity, lesion consistent with symptoms observed in the field development was evaluated 3 days post-inoculation, whereas control plants remained symptomless. The pathogen was consistently re-isolated from symptomatic tissues and identified as Colletotrichum spp. based on morphology and ITS sequences, thereby fulfilling Koch’s postulates. C. fioriniae causes anthracnose in various hosts in China, including walnut, persimmon, Camellia oleifera, Rhus chinensis (Zhu et al. 2015; Fan et al. 2024; Liu et al. 2025; Xu et al. 2023). Because S. japonica is extensively cultivated for its decorative foliage and profuse flowering, the emergence of anthracnose poses a tangible threat to aesthetic function in public landscapes. Accurate identification of C. fioriniae is thus critical to guide fungicide selection, and support early intervention in landscape maintenance programs.