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Bee Vector / Head Rot Research Hits Roadblock

Wednesday, February 1, 2023
filed under: Disease

bagging sf heads in field
Bagging Sunflower - G. Henson, M. Wunsch / Credit: Suanne Kallis

      The quest to find an effective and practical strategy for managing Sclerotinia head rot in sunflower has been a major focus of plant pathologist Michael Wunsch’s efforts since he began work at the NDSU Carrington Research Extension Center in 2010.  It has been a quest marked by some early successes and many struggles.
        “Early on, we had significant success figuring out how to rigorously identify the hybrids and breeding lines that carry partial resistance,” Wunsch says.  “While no sunflower is fully resistant, hybrids and breeding lines differ in susceptibility to Sclerotinia head rot.  Efforts to identify less-susceptible hybrids and breeding lines were stymied by highly variable results across screening nurseries. 
        “The lines that were most susceptible in one nursery would sometimes be the top performers in another nursery,” Wunsch notes.  He and his colleagues conducted replicated studies in which sunflower was inoculated with the Sclerotinia pathogen at different growth stages, and they learned that susceptibility to head rot increases as bloom progresses and sharply declines once bloom ends. 
        “Once we inoculated every sunflower head in our resistance screening nurseries at the same growth stage, the top-performing hybrids were the same across every nursery,” Wunsch reports. 
        Wunsch and his colleagues identified multiple commercial hybrids, mostly oilseed types, with sharply reduced susceptibility to head rot.   However, when conditions were highly favorable for head rot, even the best hybrids developed commercially unacceptable levels of head rot.  “In a really bad year, head rot incidence was 70 to 90% in the most susceptible hybrids and 15 to 40% in the least susceptible hybrids,” Wunsch explains.  “The partially resistant hybrids were much better but still developed too much disease.”
        Looking for another management tool that could be combined with partial resistance, Wunsch conducted extensive field studies evaluating foliar fungicides.  “I was very hopeful we could find a way to make foliar fungicides work against head rot,” Wunsch says.  “We tested everything — different fungicides, adjuvants, application timings, application methods — but nothing worked.”  The best he could do was to get slight reductions in disease under low levels of disease pressure.
        “The problem with foliar fungicides is the difficulty achieving satisfactory fungicide deposition to the front of sunflower heads,” Wunsch explains.  “You’ve got to get fungicide onto all of the flower buds and blossoms, which is very difficult given the outward- to downward-facing orientation of heads.”
        Seeking a different solution, Wunsch turned his attention to the use of biological control.  Venkata Chapara, plant pathologist at the NDSU Langdon Research Extension Center, had obtained promising preliminary results from using bees to carry a biological control agent to sunflower heads.
        “Using bees solves the application problem that we had with fungicides,” Wunsch observes.  “The bees visit the sunflower daily, carrying the biological control agent to each new ring of disk flowers as the flowers open.”  A special dispenser is affixed to the exit of bee hives.  This dispenser releases a granulated formulation of the biological control agent several times daily, and the bees must crawl through the biological control agent as they exit the hive.   As bees forage for nectar, they carry the biological control agent with them.
        “We got great results,” Wunsch notes.  In 10 studies conducted across three locations and six years, he and Chapara observed an average 48% reduction in Sclerotinia head rot.  Critically, confection sunflowers exposed to the biological control averaged 53% lower contamination of the grain with sclerotia at harvest.  Sclerotia are black, hard fungal resting structures of the Sclerotinia pathogen produced in diseased tissues, and confection sunflower exceeding 4% contamination (by weight) is rejected by processors.
        Two of the bee-vectoring studies were conducted as non-replicated trials consisting of spatially isolated blocks of sunflowers.  Sunflower was managed identically in each block, and one block was exposed to bees and biological control while another was not.  In the remaining eight studies, the non-treated control was established by placing perforated bags over every sunflower head to exclude bees from the heads.  To avoid moisture retention that would favor disease development, perforated bags that allowed for significant airflow around sunflower heads were selected.
        To confirm that the reductions in disease in sunflower exposed to bee-vectored biological control were caused by the biological control and not by the absence of bags over the sunflower heads, Wunsch conducted a companion study evaluating the impact of the bags in 2021.  Sunflower was managed identically as in the bee-vectoring studies — except that no bees or biological control were introduced.  No major impact of the bags was found, but testing was conducted on a relatively small number of heads per plot.  Results were highly variable across replicates of the study, and no conclusions could be drawn.
        “We expanded our testing on the impact of the bags in 2022, and what we found was very disappointing,” Wunsch reports.  “In each of two large, rigorous studies evaluating the impact of the bags on disease in the absence of biological control, head rot incidence and contamination of the grain with sclerotia were sharply higher in sunflower that was bagged.”
Inoculating Sunflower / Credit: Suanne Kallis
        The results suggest that much of the reduction in head rot observed in the biological control studies may have actually been a reduction in disease associated with not having those heads bagged.  Follow-up research is planned for 2023, but for now, Wunsch’s hope in this management strategy has diminished.
          “Unfortunately, it looks like we may have to look for new strategies for head rot management,” Wunsch states.  “I had great hope for pairing bee-vectored biological control with partially resistant hybrids for management of head rot.  However, unless follow-up studies planned for 2023 give a different result, it looks like this biological control strategy might not provide the level of disease control that we need.” — Jody Kerzman & Michael Wunsch
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