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Phomopsis Continues to Challenge the Dakotas

Saturday, February 1, 2025
filed under: Disease

Fungicide Strategies Show Promise
 
By Febina Mathew(1), Samuel Markell(2), Robert Harveson(3), Megan McCaghey(4), Peter Kovacs(5) and Karthika Mohan(6)
 
phomopsis
These images compare (left) a field trial plot treated with a fungicide containing pyraclostrobin (Headline) at R1, followed by a single application of fluopyram + tebuconazole (Luna Experience, Bayer) at R6, and (right) a nontreated control (NTC) plot. The trial, conducted in Grandin, North Dakota in 2024, was planted with a commercial oil-type sunflower hybrid susceptible to Phomopsis stem canker. The NTC plots exhibited plants with visible Phomopsis lesions on the stems, while the fungicide-treated plots showed little to no
stem discoloration (Mathew lab).

        Phomopsis stem canker continues to be a significant concern for sunflower farmers in North Dakota and South Dakota.  A recent survey by North Dakota State University (NDSU), which sampled 50 fields across the Dakotas, found that the average disease prevalence had reached 70%.  This marks a substantial increase from the 30% prevalence recorded in 2023 and 35% in 2021, based on previous surveys by the National Sunflower Association. 
        One of the primary factors contributing to the increased prevalence of Phomopsis stem canker in 2024 has been the combination of warmer temperatures (averaging 25°C) and normal to above-normal precipitation levels.  Common symptoms of the disease observed in affected fields included stem lesions, wilting, and pith degradation (Figures 1-4).
 
phomopsis stages
Stem lesions of varying size and color, wilting, and pith degradation of sunflower plants, as observed in affected fields in North Dakota.
Photo Credit: Mathews Lab, NDSU

        Both Phomopsis helianthi and Phomopsis gulyae have been identified as the causal pathogens of Phomopsis stem canker, with P. helianthi being the most prevalent in North Dakota and P. gulyae in South Dakota, according to the 2024 survey.
        The continued spread of Phomopsis stem canker in the Dakotas, which poses the risk of up to 40% yield loss, highlights the need to incorporate fungicide applications into integrated pest management (IPM) practices.
 
        In 2024, we assessed the effectiveness of new fungicide chemistries and application timings for managing Phomopsis stem canker under field conditions in Minnesota, North Dakota, Nebraska and South Dakota.  The trials used a Phomopsis-susceptible hybrid (Nuseed Genetics), seeded at a rate of 18,000 to 22,000 seeds per acre in Crookston, Minn., Grandin, N.D., Scottsbluff, Neb.,and Brookings, S.D.  Foliar fungicides containing QoI (FRAC 11), triazole (FRAC 3) and SDHI (FRAC 7) were applied at the V8 (eight true leaves), R1 (miniature floral head) and R6 (flowering complete, ray flowers wilting) growth stages, with two sequential applications (V8 + R1 and R1 + R6). The field trial was conducted using a randomized complete block design, with 10 treatments and four replications per treatment.

2024 Field Trial Highlights
        One application each of pyraclostrobin (Headline) at the R1 growth stage, followed by a single application of fluopyram + tebuconazole (Luna Experience, Bayer) at R6, resulted in a yield increase of up to 37% compared to the no-fungicide control across locations (Figure 5). These results are consistent with previous years. In 2022, yield increases ranged from 8% to 30%, and in 2023, increases were between 3% and 10%, following the same fungicide treatment protocol. These data support the effectiveness of applying fungicides at specific growth stages to enhance sunflower yield and protect against Phomopsis stem canker.
 
fungicide field trial components

— Acknowledgment —
        This research was funded in part by the National Sunflower Association and the USDA-ARFI-CARE Program [Grant no. 2024-69008-42655]. We thank the South Dakota Oilseed Council, Mr. Brayden Espeland, Mr. Rick Weber, Mr. Justin Ogan, and Mr. Sam Ireland, for their assistance in surveying Phomopsis stem canker in South Dakota in 2024.  We extend our thanks to the companies that supported our trials by providing chemicals, land for planting, assistance with planting and harvesting, and seeds for the fungicide trials. Our gratitude also goes to the personnel from Dr. Mathew’s lab (Dr. Denis Colombo, Taofeek Mukaila, Dilorom Rasuleva), Dr. Markell’s lab (Bryan Hansen, Jessica Halvorson, Zach Ittel), Dr. Harveson’s lab (Clay Carlson, Alison Rickey, Tyler Patrick), Dr. McCaghey’s lab (Peter Aspholm), UMN NWROC’s Michael Leiseth, and Dr. Kovacs’s lab (Nicolas Passone) for their invaluable assistance.  Additionally, we acknowledge the support of the Sunflower Working Group, funded by USDA-NIFA-CPPM through the North Central IPM Center (2018-70006-28883), for Extension activities.
 
(1)Associate Professor, North Dakota State University
(2)Professor and Extension Plant Pathologist, North Dakota State University
(3)Professor and Extension Plant Pathologist, University of Nebraska, Panhandle Research and Extension Center
(4)Assistant Professor, Univ. of Minnesota
(5)Associate Professor, Precision Agriculture Cropping Systems, South Dakota State University
(6)Graduate Research Assistant (Mathew lab), North Dakota State University
 
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