By Mike Spieker | Photos by Mohamed Khan. Editor’s Note: In the November/December 2016 issue, we recapped the severity of Cercospora during the 2016 growing season throughout North Dakota and Minnesota. In 2017, the disease was much less significant in some areas. Several factors played into that result, such as management and/or weather. In this article, we cover Cercospora’s effects on the 2017 growing season from northern North Dakota to southern Minnesota.
In 2016, conditions were very favorable for crop growth; it was warm and wet. In 122 years of record-keeping, 2016 was the region’s warmest and wettest year to date, says North Dakota State University / University of Minnesota Extension sugarbeet specialist and NDSU professor, Mohamed Khan. "When it’s warm and it’s wet, that’s good for sugarbeets," Khan said. "But it also promotes the growth of Cercospora. In 2016, American Crystal [growers] averaged around three applications, while Minn-Dak growers were right around four to five. Growers in the Southern Minn. area were upwards of six applications." In 2016, the region saw significant resistance to the QoI (strobilurin) fungicides, such as Headline and Priaxor. The resistance to that chemistry resulted in a cascade of disease control. However, Khan’s 2016 trials revealed that if a broad-spectrum fungicide, such as Tin, is mixed with the triazoles, that provided the best control. "We used that information from the 2016 trails, worked with all the co-ops, and for 2017 they decided to start the fungicide applications early – as soon as they saw the first symptoms after row closure – and to always mix two different fungicides," Khan said. While the number of applications remained about the same for all three North Dakota and Minnesota co-ops this year, the amount of control increased significantly. "The growers spent a lot of money on control this year. Last year, it wasn’t as expensive but the control was not as good," Khan said. "In 2017, they followed our recommendations and I would say they had good to excellent control in most areas." Despite the better control in 2017, Cercospora was the upper Midwest’s number one concern for growers. "It was unanimous for all three co-ops," Khan said. Most of the fungicides on the market are protectants, meaning they need to be on the leaf surface before the disease arrives. Khan and his team are working on ways to provide better coverage to ensure the fungicide covers and protects as much surface area on the leaf as possible. "One of the cheapest and most available resources to use is water. We encourage growers not to cut their water volume. We recommend a rate of about 20 gallons per acre," Khan continued. "We are also doing work to find if adjuvants can help to give better deposition and to help prevent wash-off of fungicides after a heavy rainfall." After reviewing his 2016 data, Khan found something alarming. He noticed a very small percentage – around five percent – of Cercospora fungi are resistant to all modes of action growers are currently using. "We have to be very careful to always be using mixtures and to always include a broad-spectrum fungicide," he said. One major red flag regarding Cercospora is the fact that there are no new chemistries being created at this time. As a result, growers are forced to use old chemistries and dated techniques against a very old and seasoned enemy. Southern Minnesota Beet Sugar Cooperative Over the past few growing seasons, southern Minnesota has had the heaviest Cercospora pressures in the region. The same held true, albeit to a lesser extent, in 2017. Southern Minnesota Beet Sugar Cooperative (SMBSC) estimates that Cercospora cost growers between $250-300 per acre on average in 2016. In 2017, an aggressive recommended regimen was laid out for growers in attempt to regain control of this adaptive disease. Overall, everybody’s hard work paid off. "Fields that received six applications with a tank mix were able to stay green until harvest," said Mark Bloomquist, research director at SMBSC. "In the 2017 growing season, about 20 percent of our fields in the cooperative turned brown by October 1st. We believe a lot of those fields were fields that either had application timing issues or were not tank mixed on every application. That’s still a huge victory, compared to 2016 where about 90 percent of our fields turned brown. We made tremendous progress this year." Bloomquist echoed that SMBSC growers averaged six fungicide applications this season, which almost mirrors the six to seven they averaged in 2016. However, it was the ingredients of the applications that made the difference in 2017. Very few of the applications in 2016 included a tank mix partner. Growers rotated between the different families, but there was just a single fungicide at each application. "The big difference for us this past year were the tank mixes. That helped us get over the major disaster we had in 2016," Bloomquist said. Even though just 20 percent of the SMBSC fields turned brown this fall, Bloomquist stresses that is still more than enough innoculum for Cercospora to continue to be an issue for the co-op in 2018. Resistance to the strobilurins remains a concern for growers. In 2016, that chemistry basically failed in the SMBSC region. Strobilurin tests were conducted again this past growing season in research trials and Bloomquist says the results were no better than the untreated check. "We did not recommend any strobilurins in 2017," said Bloomquist. "I do not believe we will recommend any of that fungicide family in 2018 either."