By Sam Markell
Extension Plant Pathologist, Broad-leaf Crops
While surveying for downy mildew last week, Michelle Gilley (Graduate Student and Research Specialist) began noticing an early spore stage of the sunflower rust pathogen (aecia) on sunflowers. Aecia look like a collection of orange cups and are found on the underside of the leaf (Figure 1). While certainly not found in every field, aecia were found in multiple locations in the state.
Once aecia are observed, the typical rust pustules with cinnamon-brown spores are often found a week or two later (Figure 2). Favorable environmental conditions include relatively warm temperatures and frequent dews; conditions that are occurring in much of the state.
Sunflowers are still young, but it is important to scout for rust in the upcoming weeks. When rust reaches approximately 1% severity on the upper four leaves at or before bloom (R5) yield loss can occur. Fortunately, rust can be managed with well-timed fungicide applications. Triazoles (Tebuzol, etc...) and Strobilurins (Headline, Quadris) or fungicides with a strobilurin component (Priaxor) are efficacious.
I will include more information about fungicides and timing when the crop is closer to bloom.