First Report of Anthracnose Caused by <i>Colletotrichum spaethianum</i> on Ornamental Onion ( <i>Allium</i> × ‘Millenium’) in South Carolina, U.S.A.

In April 2024, symptoms of anthracnose were observed on ornamental onion plants (Allium × ‘Millenium’) in a commercial plant nursery in Edgefield County, South Carolina, USA. Primary symptoms were yellowing and necrosis of leaf tips, which died back and caused entire leaves to wither. The incidence was reported as 100% on 43,000 plants. Diseased plants were submitted to the Clemson University Plant and Pest Diagnostic Clinic. Under a stereomicroscope, acervuli with black setae of Colletotrichum species were observed on all diseased leaves. A leaf disc (approx. 5 × 5 mm2) was excised from the margin of necrotic tissue from 10 individual diseased plants, sterilized in 10% bleach for 1 min, rinsed in sterile deionized water, placed onto acidified potato dextrose agar [APDA; 0.085% (v/v) lactic acid], and incubated at 25°C. Colletotrichum colonies with identical colonial and conidial characteristics were isolated from eight out of the 10 discs. Four isolates (24-283-A to -D) from different plants were obtained by transferring individual hyphal tips to new APDA plates. After six days, they had almost identical colonial characteristics including circular, flat, filiform, matte, and opaque colonies with a pale green (front) or dark pink (back) center and a pale orange to white edge. Thereafter, isolates 24-283-A and 24-283-C were selected as representatives in the following characterizations. Conidia of 24-283-A and 24-283-C (n = 30 per isolate) measured 19.8 ± 0.5 µm and 18.4 ± 0.7 µm in length and 4.9 ± 0.1 µm and 5.7 ± 0.2 µm in width, respectively. They were unicellular and oblong, some slightly curved and elongated, while others were compact. Setae (n = 30) on APDA were 135.4 ± 34.5 µm (24-283-A) and 187.1 ± 50.8 µm (24-283-C) in length. Sequences of three genetic markers, internal transcribed spacer region, glyceraldehyde 3-phosphate dehydrogenase, and actin from both isolates shared 99.6 to 100% similarity with those of C. spaethianum ex-epitype strain CBS 167.49 (Damm et al. 2009). In a phylogenetic tree, both isolates were grouped with the ex-epitype in a distinct cluster. Their identity as C. spaethianum was also confirmed by the U.S. Department of Agriculture’s National Identification Services. The pathogenicity of each isolate was tested on five pots, each containing five ‘Millenium’ plants (five- to six-leaf stage). Each pot was sprayed with 10 ml of 1×106 conidia/ml suspension. Five pots of non-inoculated control plants were sprayed with 10 ml of deionized water per pot. Plants were incubated in a closed moist chamber for 8 days at 25°C. Two days after inoculation (DAI), wilting was observed on inoculated leaves. Sixteen DAI, leaves inoculated with either isolate had yellowing at the tips. Colletotrichum acervuli with black setae were observed on only inoculated leaves. Fungal isolates with morphological characteristics identical to those of the representative isolates were recovered from inoculated leaves using the method described above, but not from non-inoculated control plants. The pathogenicity test was repeated once. Colletotrichum spaethianum is known to cause anthracnose on Allium species, namely A. fistulosum in Brazil (Santana et al. 2016) and A. ledebourianum in India (Salunkhe et al. 2018). In the USA, it has been detected on Iris sp. (California Department of Food and Agriculture, 2015), Glycine max (Boufleur et al. 2021), and Beta vulgaris (Bhuiyan Ziaur et al. 2025). This is the first record of C. spaethianum on any plant species in South Carolina. Its exact distribution in the USA warrants further investigation.