Diaporthe gulyae colonizes seeds of sunflower (Helianthus annuus) systematically through leaf, petiole and stem infection

Diaporthe gulyae, first identified as causing stem lesions and mid-stem lodging on sunflower in Australia, is a highly virulent pathogen now also reported on sunflower in Argentina, Canada, China and North America. This report details the first observations of sunflower leaf infection by D. gulyae in the field and confirms a leaf-to-stem infection pathway by glasshouse experiments. Further, capitula tissues and seed were also infected by D. gulyae following stem infection, demonstrating that hyphae associated with stem lesions can colonize upwards into the capitulum under favourable conditions. Leaf and stem lesion symptoms caused by D. gulyae are very similar to those caused by D. helianthi, the cause of Phomopsis stem canker (PSC) of sunflower in China, Europe, North and South America and Russia. Leaf colonization followed by stem infection is also the recognized infection pathway of D. helianthi and the almost identical symptoms and signs caused by D. gulyae and D. helianthi on sunflower highlight the difficulties of differentiating these species in the field. Additionally, seedlings that emerged from infected seeds with pericarp halves attached were shown to be a viable source of D. gulyae inoculum, which may contribute to further spread both locally and internationally. The name Diaporthe Stem Canker (DSC) is attributed here to the disease caused by D. gulyae to enable distinction from PSC caused by D. helianthi.