Phomopsis Stem Canker of Sunflower: Is Fungicide Resistance a Problem?

Dr. Febina Mathew, associate professor at North Dakota State University, presented “Phomopsis Stem Canker of Sunflower – Is Fungicide Resistance Becoming an Emerging Problem?” during the August “Pests and Progress” webinar. Dr. Mathew summarized 15 years of research on Phomopsis stem canker, its impacts on sunflower crop yields, and options for management. 

The United States is in the top 10 for sunflower production in the world; Minnesota, North Dakota, and South Dakota produce 80% of the United State’s sunflower crops. Sunflowers are an important crop for these states, and Phomopsis stem canker reduces yields and is the most limiting factor for sunflower production in Minnesota. 

Symptoms of Phomopsis stem canker include discolored and broken stalks. Sunflowers also die early, at least two weeks before harvest. These symptoms also result in smaller seed heads, fewer seeds and a about 10% reduction in oil content. More information about Phomopsis stem canker is available on the National Sunflower Association website.

Sclerotinia is a disease that also infects sunflowers and has similar symptoms. Phomopsis stem canker is often mistaken for  Sclerotinia, and differences are subtle. It is even possible for a plant to be infected with both.

Surveys from recent years show that Phomopsis is becoming more common, although increased outbreaks are expected during warm, humid conditions. Unfortunately, once a Phomopsis outbreak occurs, the area tends to remain infected. This is because other crops and weeds can host Phomopsis and support its life cycle, and the disease is capable of overwintering in crop residues.  Wheat and corn are not known to support the Phomopsis life cycle, but for crop rotation to be effective, areas near the field would need to be unsuitable for Phomopsis as well.

As Phomopsis continues to spread, options for management are needed. Ideally, a sunflower variety resistant to Phomopsis would be used for production, but no resistant varieties have been confirmed. This leaves fungicide application as the primary control option.

Many field trials have been conducted in the past 15 years looking for an effective fungicide and spray timing for Phomopsis stem canker. A Quinone outside inhibitor (Qoi) called Pyraclostrobin consistently protects yields when applied directly to the flower head at the R1 growth stage, when floral head starts to form. 

While the effectiveness of Pyraclostrobin is good news, diseases are known to become resistant to Qoi fungicides. Use of Pyraclostrobin should be limited, and its effectiveness is not expected to last long term. 

Other fungicides, mixtures and active ingredients have been tested and were described in the webinar.  Rotating or combining active ingredients with different FRAC (Fungicide Resistance Action Committee) groups can help prolong the effectiveness of pyraclostrobin. Preliminary findings indicate that applying pyraclostrobin (QoI) fungicide during the floral head formation growth stage, followed by fluopyram (Succinate Dehydrogenase Inhibitor) + tebuconazole (triazole/DeMethylation Inhibitors) fungicide during the post-flowering growth stage, can potentially increase yields by 10% or more compared to the non-treated control.

It is also helpful to review these results because some of the tested fungicides had no impact on Phomopsis.

Efforts will continue to test fungicide efficacy and ideally to find or develop a sunflower variety that is resistant to Phomopsis.