First Report of Anthracnose Caused by <i>Colletotrichum fioriniae</i> on Flowering Dogwood ( <i>Cornus florida</i> ) in the Southeastern United States

The flowering dogwood (Cornus florida L.) is an important understory tree native to south and east pine and deciduous forest ecosystems of the United States and Canada (Jenkins & White et al. 2002). This tree species has important cultural values associated with its use by Native Americans in natural remedies from bark and root extracts, dyes and arrow shafts (Elbert et al. 1980; King et al. 2003), and a popular ornamental landscape tree planted in the southern and eastern United States. Studies have reported localized decline of dogwood populations due to the introduction of the fungus Discula destructiva Redlin, the causal agent of dogwood anthracnose in the eastern United States (Oswalt et al. 2012; Daughtrey et al. 1996). During May 2024, dogwood trees showing branch dieback and irregular necrotic spots (> 30% disease severity) on foliage were found on the Kisatchie National Forest, Louisiana (31.16912° N, -92.65568° W). Symptomatic leaf tissue sections (4 mm2) from collected samples were surface disinfected with 70% ethanol and 10% sodium hypochlorite and rinsed with sterile-distilled water. Tissue sections were transferred to potato dextrose agar (PDA) and incubated at 25°C for four days. A species of Colletotrichum was consistently isolated and identified based on conidia morphology. DNA analysis was performed for species identification through PCR amplifications of two loci, the rDNA Internal Transcribed Region (ITS) using primers ITS1/ITS4 (White et al. 1990) and partial sequence of the gene encoding actin (ACT) using primers ACT-512F/ACT-783R (Carbone and Kohn 1999). Both ITS and ACT sequences of the Colletotrichum species isolate DW13 (GenBank Accession nos. PV854696.1 and PX441942.1) showed 100% and 99.5% homology to the Colletotrichum fioriniae isolates (GenBank accession nos. MN684842.1 and PV541637.1) respectively. The pathogenicity of C. fioriniae was tested on healthy, one-year-old dogwood trees planted in three-gallon pots kept in an incubation room at 25°C. The saplings were inoculated by placing two mycelial disks (4 mm2) of a C. fioriniae isolate on one non-wounded and one scalpel-tip wounded disinfected leaves. Four individual saplings were inoculated as replicates. Pure agar disks were used as control treatment using the same methods. Initial symptoms observed five days post-inoculation showed a necrotic annular ring around the point of inoculation which is typical of Colletotrichum infection. Symptoms were observed in all four replicates and caused premature fall of inoculated leaves at 21 days post-inoculation. To complete Koch postulates, the pathogen was re-isolated from symptomatic leaves, and the newly established cultures were identified as C. fioriniae known worldwide for causing anthracnosis in a wide range of hosts. This is the first report of C. fioriniae associated with flowering dogwood decline in Louisiana and the southeastern United States with potential forest health implications.