Application of Pyraclostrobin Fungicide at Miniature Floral Head Development Growth Stage Is Efficacious Against Phomopsis Stem Canker Fungi in Sunflower (Helianthus annuus)

Phomopsis stem canker, caused by multiple species of Diaporthe, is a yield-limiting disease of sunflower ( Helianthus annuus L.) in the U.S. Northern Great Plains. To develop fungicide recommendations, we performed meta-analyses on data collected from 56 foliar fungicide efficacy trials conducted in Minnesota, Nebraska, North Dakota, and South Dakota between 2009 and 2021. The specific objectives were to compare the fungicide application timings and active ingredients with a nontreated control (NTC) and determine their impact on the disease severity index (DSI) and yield. Effect sizes, Cohen's f or Hedges’ g, were calculated as the difference in DSI or yield between the fungicide treatment and NTC. We observed that fungicide application timings (DSI [ f = 0.43] and yield [ f = 0.26]) and active ingredients (DSI [ f = 0.51] and yield [ f = 0.39]) had a significant effect on DSI and yield ( P < 0.0001). In addition, application of pyraclostrobin (DSI [ g = −0.65] and yield [ g = 0.43]), pyraclostrobin + fluxapyroxad (DSI [ g = −0.55] and yield [ g = 0.36]), and mefentrifluconazole + pyraclostrobin + fluxapyroxad (DSI [ g = −2.23] and yield [ g = 0.85]) at the R1 (miniature floral head formation) growth stage outperformed NTC. This study suggests that farmers can rely on a single application of pyraclostrobin at R1 for Phomopsis stem canker management. Future studies on the economic benefits from fungicide use are important to help enhance farmer decisions.