First Report of Target Spot Caused by Corynespora cassiicola on Senna obtusifolia L. (Sicklepod) in the United States
Authors: Patel Sejal , Bowen Kira L. , Koebernick Jenny 
Published: February 2025

Sicklepod (Senna obtusifolia L.) is a weed native to the American tropics which has become widespread in the southeastern United States, posing a significant challenge for row crop producers (McKinnon et al. 2012; Nice 1999). In August 2024, a foliar disease was observed on sicklepod throughout cotton (Gossypium hirsutum L.) plots in a nine-acre field at EV-Smith Plant Breeding Unit in Tallassee, Alabama (32.4967° N, 85.8905° W), U.S. The symptoms were round to irregular, 3 to 9 mm, brown to dark-brown lesions with alternating concentric rings, surrounded by a pale-yellow halo. The estimated disease incidence was 40%. Four symptomatic plants were sampled from different locations across the field for pathogen isolation. The leaves with lesions were surface sterilized by soaking in 1 % NaClO for 1 min, followed by 70 % ethanol for 1 min, and rinsed twice in sterile distilled water for 30 seconds. The lesions were excised and plated on V8 agar amended with 0.5 mL of lactic acid per 500 mL of medium. The plates were incubated at 22 °C under a 12-h light/dark cycle for ten days. Six fungal isolates with identical morphologies were obtained. During incubation, colonies developed gray aerial mycelium, turning dark gray to brown with age. The conidiophores were unbranched, erect or slightly curved, elongated, and brown, appearing singly or in clusters, with 3 to 14 pseudosepta. Conidia were obclavate to cylindrical to slightly curved, and multinucleate, containing 3 to 12 septa. Conidia were 50 to 193 µm in length and 5 to 14 µm in width. For molecular identification of pathogen, isolate SP1 was selected, and DNA was extracted from 100 mg of 10-day-old mycelium using the EZNA Fungal DNA Mini Kit (Omega Bio-tek, GA). Three regions, internal transcribed spacer (ITS; White et al. 1990), actin-encoding locus (act1; Carbine & Kohn, 1999), and hypervariable loci (ga4; Dixon et al. 2009) were used to confirm SP1 isolate. The sequences of ITS (PQ536090) and act1 (PQ876357) shared 100% identity, and 94.7% and 99.3% homology with ITS (ON316921) and act1 (MF320391), respectively. The ga4 sequence (PQ876356) shared 98% identity and 98.09% coverage with C. cassiicola (MH605239) in GenBank. For pathogenicity test, a conidial suspension of isolate SP1 (40,000 spores/mL) was sprayed onto sicklepod plants at the two-true-leaf stage in the greenhouse. Six plants were inoculated with the conidial suspension, while three additional plants were sprayed with sterile distilled water to serve as controls. Control and inoculated plants were transferred to a mist chamber constructed with PVC pipe (dimensions: 4 m × 4 m × 8 m) and enclosed with a transparent plastic sheet. The plants were arranged in a completely randomized design inside the chamber, and a mist system operated for 2 seconds every 10 minutes over three days to maintain humidity above 80%. Initial symptoms on the inoculated leaves were noted seven days post-inoculation, whereas no symptoms were observed on control plants. The experiment was repeated once, and the results were consistent. Based on morphology and sequence analyses, the fungal pathogen was reisolated from inoculated plants and identified as C. cassiicola, fulfilling Koch’s postulates. To our knowledge, this is the first global report of C. cassiicola infecting sicklepod. The detection of this pathogen on a new host highlights its notable adaptability. With an extensive host range of over 400 species (Dixon et al. 2009), the pathogen poses a significant risk to a wide range of plant communities and crops, illustrating the importance of closely monitoring C. cassiicola in host and weed control.