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Update: Phomopsis Fungicide Research

Thursday, March 30, 2023
filed under: Disease

By Karthika Mohan/1/, Samuel Markell/2/, Robert Harveson/3/, Megan McCaghey/4/ and Febina Mathew/5/

Phomopsis stem canker is a priority disease for many U.S. sunflower growers. Yield losses may exceed 40% when the disease is present.
        Over the years, multiple fungi belonging to the Diaporthe / Phomopsis family have been implicated to cause this disease. Among the fungi, Diaporthe helianthi and D. gulyae are the predominant species in Minnesota, Nebraska, North Dakota and South Dakota.
        Symptoms of Phomopsis stem canker, which include brown-colored discoloration on the lower leaves (Figure 1), are first observed when the miniature floral head has initiated on the sunflower plant. As the sunflower begins to bloom, the fungus moves from the leaves into the stems, thus causing the plant to lodge (Figure 2).

Management options are currently limited to the use of foliar fungicides, commercial hybrids with partial disease resistance, weed management and crop rotation to non-hosts (e.g., wheat).
        Between 2009 and 2020, field trials were conducted in the two Dakotas, Minnesota and Nebraska for a total of 87 location-years. The trials were planted to either susceptible or partially resistant sunflowers of confection/oil seed type under natural Phomopsis stem canker disease pressure. We observed that foliar fungicides containing pyraclostrobin (FRAC 11) are effective against Phomopsis when applied at miniature floral head (R1) growth stage (Figure 3).

r1 growth stage — Figure 3. Showing the R1 growth stage of the sunflower when the miniature floral head is formed on the plant. (Photo: Don Lilleboe)

        In 2022 we evaluated the effectiveness of new fungicide chemistries and application timing to manage Phomopsis stem canker under field conditions in Dakota, Minnesota and Nebraska. The trials were planted to a Phomopsis-susceptible hybrid at a seeding rate of 18,000 to 22,000 seeds per acre at 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 V8 (eight true leaves), R1 (miniature floral head) or R6 (flowering is complete and the ray flowers are wilting) growth stages, and two sequential applications (i.e., V8 + R1 and R1 + R6).

        The specific treatments used for the trials included:
        (1) Headline / FRAC 11 (R1) @ 6 fl oz/Ac
        (2) Luna experience / FRAC 3 + 7   (V8) @ 9 fl oz/Ac
        (3) Luna experience (V8) @ 9 fl oz/Ac + Headline (R1) @ 6 fl oz/Ac
        (4) Luna experience (R6) @ 9 fl oz/Ac +Headline (R1) @ 6 fl oz/Ac
        (5) Luna experience (R1) @ 9 fl oz/Ac
        (6) Luna experience (R6) @ 9 fl oz/Ac
        (7) Folicur / FRAC 3 (V8) @ 4 fl oz/Ac
        (8) Folicur (R6) @ 4 fl oz/Ac
        (9) Luna experience (R1) @ 9 fl oz/A + Headline (R6) @ 6 fl oz/Ac
        (10) no fungicide treatment control (NTC)

        All of the fungicide treatments were sprayed with two adjuvants [NIS (0.25% V/V, Induce) and Crop oil (0.08% V/V, Interlock)] with a backpack or hi-boy sprayer [Water volume of 15 gal/A]. The severity of Phomopsis stem canker was evaluated after R6, and yield at harvest was adjusted to 10% moisture.
        One application of Headline at R1 followed by a single application of Luna experience at R6 showed 8 to 30% greater yield when compared to no-fungicide control.
        The study will be repeated in the four states in 2023.

Acknowledgements
        The fungicide trials were funded, in part, by the National Sunflower Association, the USDA-CARE Program [grant no. 2016-08651] and the South Dakota Oilseeds Council. We thank the companies for supporting our trials by providing chemicals and seeds.
We also thank the personnel from the Mathew lab (Brian Kontz, Dr. Ruchika Kashyap), the Markell lab (Bryan Hansen, Jessica Halvorson), the Harveson lab (Clay Carlson, Alison Rickey, Tyler Patrick), and Minnesota experimental advisors (John Swanson, Minnesota Sunflower Council, and Mike Leiseth, NWROC) for their assistance.
        We additionally acknowledge the support of the USDA NIFA CPPM through the North Central IPM Center (2018-70006-28883) for extension updates.

/1/ Graduate research assistant, North Dakota State University
/2/ Professor & Extension plant pathologist, North Dakota State University
/3/ Professor & Extension plant pathologist, University of Nebraska-Scottsbluff
/4/ Assistant professor and plant pathologist, University of Minnesota
/5/ Associate professor and broadleaf/oilseeds crop pathologist, North Dakota State University