Sunflower is at the growth stage (usually at or after blooming) when most foliar diseases start to develop. Areas of the state that have received frequent rains may begin to see a few diseases including Phomopsis stem canker and sunflower rust. These diseases are promoted by wet and humid weather and high humidity within the crop canopy.
Phomopsis Stem Canker
Phomopsis stem canker is characterized by a tan to light brown lesion on the stem. The disease starts from the leaf (Figure 1) and spreads to the stem through the petiole (Figure 2). The stem lesion develops after flowering and can girdle the entire stem making the plant prone to lodging. The causal pathogen of Phomopsis stem canker can reside on crop residues. Therefore, cultural practices like crop rotation are essential to avoid serious infestations. A four-year rotation is recommended between sunflower crops to help reduce Phomopsis and other diseases. In conventional till systems, tillage may help reduce Phomopsis stem canker incidence. Heavy nitrogen fertilizer applications and high plant populations can also increase the incidence of this disease. No in-season management options have been found effective, but further research is underway to determine the efficacy and best timing of a fungicide to manage this disease.
Fig. 1. Phomopsis stem canker symptoms on leaf. Credit: F. Mathew
Fig. 2. Phomopsis stem canker symptoms on stem. Credit: F. Mathew
Sunflower rust starts to develop on lower leaves and can reach yield-reducing levels in a relatively short time. Sunflower rust forms small pustules that are dark red on both the upper and the underside of the leaf. These can coalesce leading to leaf yellowing and premature death (Figure 3). Confection type varieties are more susceptible to rust than oilseed types.
Sunflower rust pathogen comes from volunteer sunflower and wild sunflower. Spores can also be blown by wind from distant fields or from within a field, therefore crop rotation does not help in managing sunflower rust. Sunflower rust can be best managed through the use of resistant hybrids and application of a well-timed fungicide. The threshold for applying a fungicide for sunflower rust is 1% average diseased leaf area on the upper four, fully expanded leaves at or before bloom (R5). Fungicides effective against sunflower rust can be found in 2017 South Dakota Pest Management Guide: Sunflower & Oilseeds/Alfalfa & Range.
Figure 3. Sunflower leaf yellowing due to severe sunflower rust.