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Sclerotinia Front Progress

Monday, March 26, 2018
filed under: Disease

A sunflower misting system at the NDSU Carrington station aids greatly in the inoculation phase for the Sclerotinia head rot management research conducted by Michael Wunsch and colleagues. (Photo: Blaine Schatz)
       Sunflower fields Since beginning his career at the North Dakota State University Carrington Research Extension Center in 2010, Michael Wunsch has conducted extensive research on improving the management of Sclerotinia head rot of sunflower. Now, eight years later, he’s happy to report some good news.
       “Our multi-year, multi-location research on Sclerotinia head rot management in sunflower has quantified the gains that can be obtained by planting oilseed hybrids with partial Sclerotinia head rot resistance,” Wunsch reports, “and has facilitated sharp improvements in our ability to identify hybrids with reduced susceptibility to head rot. Efforts are currently underway to develop fungicides as a tool for managing this disease.”
       Prior to Wunsch’s arrival in 2010, researchers had initiated an extensive effort to screen sunflower germplasm and commercial hybrids for susceptibility to head rot; but results were highly variable across screening locations and years. A hybrid that was highly susceptible in one screening nursery would often appear to be resistant in another nursery. The high variability in results had Wunsch scratching his head.
       “My efforts at the beginning centered around whether the variability in resistance screening results was due to inherent instability of resistance to head rot, or if it might be caused by the methods used to screen for resistance,” Wunsch recalls.
       Pathogen inoculations in the nurseries were conducted under the assumption that susceptibility to head rot in sunflower was similar throughout the R-5 growth stage (bloom) through the R-6 growth stage (bloom complete, disk and ray flowers wilted), and attempts were not made to standardize the growth stage at which sunflower plants were inoculated with the Sclerotinia pathogen.
       “In looking at the research, from 2008 to 2011 sunflower in the screening nurseries was generally inoculated without attention to the stage of bloom. During those years, results from multi-location nurseries evaluating the same hybrids were significantly correlated 32% of the time,” Wunsch explains. “In 2013, we decided to inoculate all heads at the same growth stage. Inoculations were conducted over multiple days such that all sunflower heads across all hybrids were inoculated twice, each time at a specific stage of bloom.
       “Since instituting this change, results from multi-location nurseries have been significantly correlated 100% of the time, with the best- and worst-performing hybrids at one screening location performing similarly at other screening locations.”
       Wunsch says this change in screening methodologies sharply improved the efficiency of screening for resistance. An effort that previously involved five screening locations funded by approximately $75,000 in grant dollars each year is now conducted at a single location, funded with a modest per-entry fee paid by private companies.
      “Sunflower increases in its susceptibility to head rot as the percent of disk flowers that are in bloom or have completed bloom increases, and susceptibility to head rot drops sharply as soon as bloom is complete and all disk and ray flowers are wilted. We didn’t know that before,” Wunsch says. “It is a lot of work to inoculate every single head at the same growth stage.  Sunflower plants aren’t completely uniform, and there is a sort of a bell curve in terms of when they bloom; it can be tempting to inoculate all sunflower that are in bloom at the same time, irrespective of the stage of bloom.
Michael Wunsch
       “We found that didn’t give us good results.  Hybrids that are inoculated in the last third of bloom, when sunflower is most susceptible to head rot, will generally develop more disease than hybrids that are inoculated in the first third of bloom, irrespective of the inherent relative susceptibility to head rot of the hybrids. 
       “Inoculating all heads at the same stage of bloom eliminated this bias, resulting in highly replicable results across screening nurseries and eliminating the need for multi-location nurseries. We cut back to just one location and focused on doing it well there. We saved a lot of money and got the same results we would have at multiple locations.”
       Starting in 2014, Wunsch and his team also began collecting seed yield and quality data from the nurseries in which commercial hybrids were screened for susceptibility to head rot.  Previously, only disease data were collected in the nurseries.  What they’ve learned so far is encouraging news for producers. 
       “Our research shows that what is out there in terms of commercial oilseed hybrids with partial resistance to head rot is actually pretty good,” Wunsch observes. “This is good news for oilseed producers. They can achieve satisfactory management of head rot in most situations by selecting a commercially available hybrid that has partial resistance; they can achieve 75% control of a disease just by selecting a hybrid that is partially resistant. That means that even under conditions that are highly susceptible for head rot, they can still get a good crop.
       “There is a way to manage this disease even in bad years,” Wunsch continues. “That’s one take-home. The other is that because we’ve improved our screening methodology, it is easier for companies to identify those materials rigorously and provide recommendations to growers on which hybrids to use when there is concern about this disease.” 
       Wunsch says it isn’t clear whether any commercial confection-type sunflower hybrids may exhibit sufficient partial resistance to head rot to make hybrid selection a viable management strategy for head rot.
       “We haven’t done as much testing with confection sunflower,” the NDSU researcher says.?“We have evaluated some experimental confection hybrids that exhibit sharply reduced susceptibility to head rot, but we haven’t done enough testing to know whether there are any commercial hybrids on the market that would work for confection growers concerned about head rot.”
       Wunsch and his team are currently engaged in a major effort to try to develop fungicides as a tool for managing Sclerotinia head rot. 
       “We’ve found ways to get good fungicide deposition to the front of sunflower heads, but there are challenges,” he notes.  “The fungicides we have tested have not exhibited satisfactory residual activity on sunflower, and we have only achieved satisfactory protection against head rot for two or three days after the fungicide is applied. That’s not acceptable. That’s the stumbling block we have run into.”
       In 2018, testing will be expanded to rigorously evaluate a new fungicide. “We did get permission to test a new experimental fungicide that will most likely make it to market, and we’re pretty excited about it.  It’s better than many other fungicides we’ve tested,” Wunsch reports.  “We will be evaluating that fungicide against the best product currently available.  We will test the fungicides utilizing multiple application methods and application timings, and we will be characterizing the residual activity of both fungicides.”
       Wunsch says that continued fungicide development research targeting head rot in sunflower is contingent upon the field trials being conducted in 2018. 
       “If we achieve satisfactory control of head rot with the new fungicide in this summer’s field trials, we’ll conduct follow-up research to confirm those results.  If not, we will focus our energies on other strategies for managing head rot.  We are interested in whether commercial confection hybrids may exhibit replicable differences in susceptibility in head rot, and whether hybrid selection might be able to help manage head rot in confection sunflower, just as it can in oilseed sunflower.”
Jody Kerzman                   
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