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Fungicides Work Against Phomopsis Stem Canker

Wednesday, March 30, 2022
filed under: Disease

By Febina Mathew(1), Samuel Markell(2), Nabin Dangal(3), Ruchika Kashyap(3) and Robert Harveson(4)
        Phomopsis stem canker has increased in prevalence (i.e., number of fields with disease) in the U.S. since the 2010 disease epidemic.  According to the National Sunflower Association’s 2021 production survey, the disease incidence (i.e., number of diseased plants) approached 100% in the sunflower producing areas of the Dakotas.
        Symptoms of Phomopsis stem canker include brown-colored, irregular spots on leaves (Figure 1) and a light-to-dark brown-colored “canker” on the stem (Figure 2), which are observed when the plants enter the reproductive growth stages.  As the disease continues to develop, the fungus moves into the stems of the sunflower plants and degrades tissues, causing them to become hollow and to lodge.
phomopsis stem canker
Figure 1: Phomopsis stem canker lesion seen on the lower leaf of a sunflower plant. (Photo: Febina Mathew)
phomopsis stem canker
Figure 2: Phomopsis stem canker on a sunflower stem. (Photo: Sam Markell)

        These symptoms can be confused with those of Phoma black stem.  However, unlike Phomopsis stem canker, lesions of Phoma black stem are black-colored, relatively smaller, appear glossy and shield shaped.  Lodging is rare with Phoma black stem since the lesions are superficial and the pathogen does not penetrate deeply into stems. 
        Since the 2010 disease epidemic, the National Sunflower Association has focused on developing solutions for farmers to manage Phomopsis stem canker.
        Foliar fungicides trials were conducted by North Dakota State University, South Dakota State University,and the University of Nebraska-Lincoln (Scottsbluff) between 2009, 2015 and 2020 for a total of 49 location-years.  These trials were conducted in rain-fed (Minnesota and the two Dakotas) or irrigated (Nebraska) sites with a history of Phomopsis stem canker, using both oil-type and nonoil-type hybrids.
        Foliar fungicides labeled for sunflower with three distinct chemical families — including the DMI/triazoles (FRAC 3), strobilurins/QoI (FRAC 11) and SDHI (FRAC 7) — were evaluated for their efficacy in Phomopsis stem canker management.  These products were applied at labeled rates with a non-ionic surfactant using a CO2-powered backpack sprayer or a high-boy sprayer.
        In our trials, the disease pressure from Phomopsis stem canker was quantified using a disease severity index (DSI) based on the severity of the lesions on stem (Figure 2).  The DSI takes into consideration both disease incidence (the number of diseased plants) and disease severity (how severe the symptoms were in each of the diseased plants).  Yield was adjusted to 10% moisture.
Highlights from Foliar Fungicide Research Trials
r1 growth stage
Figure 3: The R1 growth stage of the sunflower when the minature floral head is formed on the plant. (Photo: Don Lilleboe)
      1. We evaluated three fungicide application timings, including the V8 (eight true leaves), R1 (miniature floral head; Figure 3) and R5 (beginning of flowering) growth stages, and two to three sequential applications (i.e., V8 + R1, R1 + R5, V8 + R5, V8 + R1 + R5).  We observed that a single application of fungicide at R1 growth stage was associated with an increase in yield when compared to nontreated control.
sf sprayed
Figure 4: Sunflower plots sprayed with pyraclostrobin (Headline®) showed greater reduction in severity of Phomopsis stem canker when compared to no-fungicide control. (Photo.· Febina Mathew) 19-BC

     2. We evaluated fungicide products belonging to DMI, QoI and SDHI groups. Among the products, fungicides containing pyraclostrobin (QoI; Figure 4) and mefentrifluconazole + pyraclostrobin + fluxapyroxad (DMI+QoI+SDHI) was associated with a disease severity reduction of >30% and a yield gain of ≥5% when compared to nontreated control.
      3. We observed that the severity of Phomopsis stem canker could possibly affect the yield and profitability costs obtained from a foliar fungicide application.

table 1 comparison yield results
    In Table 1, for select trials, to determine the sunflower yield response from a single application of pyraclostrobin fungicide, we subtracted the mean yield of the nontreated control (i.e., no-fungicide control) in a trial from the mean yield of the pyraclostrobin fungicide treatment in that same trial. 
        We then calculated the profitability associated with application of pyraclostrobin fungicide using the 2020 and 2021 grain prices for oil-type ($20.4/cwt in 2020 and $31.9/cwt in 2021) and nonoil-type ($26.3/cwt in 2020 and $30.4/cwt in 2021).  The price for pyraclostrobin (Headline®) was obtained from three South Dakota retailers ($128/gal), which when converted to a per-acre basis using the label-recommended rate of 6 fl oz/A, the fungicide cost was $6/A.
        In Minnesota (15-MN-5), where disease pressure was high (DSI>70%) and a susceptible oil-type hybrid was used, the difference in mean yield between the pyraclostrobin treatment and nontreated control was 842.3 lb/A.  In contrast, in Minnesota (16-MN-3), where there was no disease pressure and a susceptible oil-type hybrid was used, the mean yield difference was 342.5 lb/A.  The net return of including a pyraclostrobin fungicide application was 60% greater in a high disease pressure environment when compared to a no disease pressure environment (Table 1).
        In South Dakota (19-SD-2), where disease pressure was high (DSI>90%) and a susceptible nonoil-type hybrid was used, the difference in mean yield between the pyraclostrobin treatment and non-treated control was 543.8 lb/A. In contrast, in South Dakota (17-SD-1), where there was low disease pressure (DSI<10%) and a susceptible nonoil-type hybrid was used, the mean yield difference was 155.1 lb/A. The net return was 75% greater in a high disease pressure environment when compared to a low disease pressure environment (Table 1).
        Overall, our study was able to establish that foliar fungicides containing pyraclostrobin, when applied once at R1 growth stage, are effective against Phomopsis stem canker.  It is important to note that R1 growth stage is the time when disease begins and symptoms are observed on the leaves of the plants (Figure 1).
        Thus, based on our observations,
        •  Foliar fungicides are likely to pay off when applied at R1 growth stage (Figure 3) if the farm has a disease history of Phomopsis stem canker.
        •  If fungicides are considered at R1 growth stage without knowing about the disease history, farmers must consider the economics associated with chemical application, which includes grain price, machinery and labor costs for ground or aerial application (per acre) and the cost of fungicide product (per acre).
        The fungicide trials were funded, in parts, by the National Sunflower Association, the USDA-CARE Program [grant no. 2016-08651], and the South Dakota Oilseeds Council.  We thank the companies for providing us chemicals, seeds and farms for the trials.  We also thank the personnel from Mathew’s lab (Nathan Braun, Brian Kontz, Renan Guidini, Karthika Mohan), Markell’s lab (Bryan Hansen, Jessica Halvorson, Michelle Gilley) and Harveson’s lab (Clay Carlson, Alison Rickey, Tyler Patrick) for their assistance.  We also acknowledge the support of the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883) for extension updates.
(1) Associate professor and field crops pathologist, South Dakota State University;
(2) Professor and extension plant pathologist, North Dakota State University;
(3) Graduate research assistant, South Dakota State University
(4) Professor and extension plant pathologist, University of Nebraska-Scottsbluff.
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