Phomopsis Stem Canker - Infection begins at the leaf margins. Yield losses result from smaller heads, lighter seed, and lodging. Phomopsis is identified by large tan to light brown lesions which typically surround the leaf petiole. The lesion may reach 6 inches in length, is light in color, and has an indefinite and sunken border. As the crop matures, the entire stalk may develop a silvery appearance and become very brittle.
Management is achieved through crop rotations and tillage to bury crop residue.
Later, rust spores are spread by wind from the volunteer or wild sunflower plants to the current year's sunflower crop. Warm temperatures and rain or dew favor fast multiplication of the rust. High nitrogen rates and high seeding rates increase leaf area which increases humidity in the crop canopy favoring rust development. Rust infections are recognized by cinnamon colored spots on the leaves, stems, petioles, bracts, and the back of the head. The spots turn black with the arrival of cool temperatures in late summer.