Rust in sunflower is caused by the fungus Puccinia helianthi, which overwinters on plant stubble and produces five spore‐stages throughout the year. Early in the season, orange rust pustules appear on the upper surfaces of cotyledons of seedlings and volunteer plants, with later infections moving to the underside of the leaf (Figure 1). The most yield-damaging stage occurs in late July to early August, when symptoms of infection show up as dusty, dark brown pustules on leaf surfaces, petioles and flower bracts (Figure 2). As the disease develops, black teliospores form, overwintering on the crop residue.
Sunflower rust becomes a more severe issue in later-planted crops, or crops with weaker genetic resistance. Warm, moist weather favours rapid multiplication of rust spores, and windblown spores can travel quickly from field to field. Early stages of sunflower rust in 2018 have been observed in the Cypress River area the week of July 3rd.
High plant populations and dense, leafy canopies allow humid conditions to remain in the crop throughout the day, compounding injury from rust spores.
Scout sunflower fields regularly to monitor the development and stage of rust infection. Watch for dense clusters of brown, powdery pustules scattered over all plant surfaces. Orange-brown ‘dust’ on clothing after being in a sunflower field is a key indicator that rust is present, and more careful scouting is needed. Withered lower leaves are an indication that the surface is heavily infected.
Controlling rust after infection is primarily done using triazole-based and strobilurin-based fungicides. Recommended action in rust-infected crops is to use a fungicide from the triazole group after the first onset of symptoms, at the 2-3% pustule coverage on the upper four leaves at flowering (R5). Strobilurin-based fungicides act more as a ‘protectant’, applied earlier before widespread infection occurs.
See more information on sunflower rust at https://www.gov.mb.ca/agriculture/crops/guides-and-publications/#gfcp
See Guide to Field Crop Protection for more information on fungicides registered at https://www.gov.mb.ca/agriculture/crops/guides-and-publications/#gfcp
National Sunflower Association of Canada information on Sunflower Diseases: http://www.canadasunflower.com/wp-content/uploads/2012/11/Disease.pdf