Survey of Colletotrichum species diversity in Mid-Atlantic cucurbits

Delaware ranks 8th in watermelon production and over half of US watermelon production occurs along the east coast. Cucurbit crops can be impacted by anthracnose, caused by multiple species of the fungus Colletotrichum. Foliar lesions reduce the plant’s photosynthetic potential, while stem lesions disrupt the vascular system, and fruit infections result in sunken cankers that render fruit unmarketable. Colletotrichum is a complex genus and includes many species varying in pathogenicity and host range. C. orbiculare is the primary species associated with southern watermelon production and states to the south have experienced increased yield losses due to anthracnose in recent years. Species composition within Delaware cucurbit production has not been studied and in 2024 twelve fields were surveyed for anthracnose. Symptoms were present only on late season fruit where lesions were excised and brought to the lab for isolation. Isolates were obtained from watermelon (n=15), pumpkin (n=13), and acorn squash (n=10). Preliminary species identification was conducted based on morphology and amplification of HIS3, GAPDH, and ACT gene regions. Across all isolates, only one sample was identified as C. orbiculare. Within each host, 80-84% of isolates returned identifications of C. magnum, a less virulent species. This survey indicates that C. orbiculare represents a small portion of isolates and could help to explain why widespread yield losses have not yet been observed in DE.