Powdery mildew caused by Golovinomyces spadiceus on wild sunflower in Sinaloa, Mexico

Wild sunflower is a common weed that grows among agricultural crops in many parts of the world, where it is both highly invasive and competitive and can serve as a reservoir for plant pathogens. Recently, signs of powdery mildew (PM) were observed on wild sunflower plants in Sinaloa, Mexico. The aim of this study was to identify the causal agent based on morphology and molecular techniques, and to determine pathogenicity on sunflower plants. Phylogenetic analysis of the ITS region grouped the sequences within lineage III of the genus Golovinomyces, which comprises pathogenic species of the plant tribe Heliantheae of Asteraceae. However, within this lineage, the three species belonging to this group: G. circumfusus, G. ambrosiae and G. spadiceus, cannot be differentiated based on the ITS region due to their close phylogenetic relationship. Morphometric analyses, particularly conidium size and germ tube morphology, confirmed the identity of the fungus infecting wild sunflowers as G. spadiceus. Inoculation tests under greenhouse conditions confirmed the pathogenicity of G. spadiceus in wild sunflower and in the commercial sunflower hybrid SYN3950HO. Signs of powdery mildew displayed on sunflower were similar to those observed under field conditions and in two independent inoculation tests. Our results confirm that wild sunflower is a potential source of G. spadiceus inoculum for commercial sunflower during the autumn–winter growing season.