If there are two words out there that farmers never wish to hear used together, it’s PESTICIDE RESISTANCE. Unfortunately, growers had to face this issue head on with Cercospora Leaf Spot (CLS) in some portions of the upper Midwest during the 2016 growing season. The fungal pathogen Cercospora beticola, which causes CLS, is now confirmed resistant to a widely used fungicide. Click here to check out the entire current issue!
"Most of the [Cercospora resistance] problems were in the Southern Minn. and Minn-Dak growing regions," said North Dakota State University (NDSU)/University of Minnesota Extension sugarbeet specialist and NDSU Professor, Mohamed Khan. Growers in those regions of the upper Midwest saw severe pressure from the pathogen. As a result, some were forced to make up to six or seven fungicide applications this growing season, compared to the average of three to four. Environmental conditions helped fuel what was called an epidemic of CLS in those parts, which led to yield reductions of four to eight tons per acre. "Those areas usually start planting a few days earlier than the Red River Valley (RRV). As a result, their rows close earlier, their temperatures are higher, and they have more moisture in the form of dew and rainfall so most of the time they will have Cercospora leaf spot earlier and more severe than the RRV," continued Khan. The more CLS an area has can lead to more individual spores that contain the resistance code to a certain fungicide. That certain fungicide in the Southern Minn. and Minn-Dak regions is pyraclostrobin, which is the major ingredient of Headline and Priaxor fungicides and is a member of the QoI (Quinone outside Inhibitors) family. "You have this large population of the fungus and all you need are a few spores that are resistant to survive. Over time, the resistant population builds up. This past season, because it was so wet and warm from the end of June, July, August, into September, the population of the resistant fungi became very high. When you have a large resistant population and you use the chemistry that population is resistant to, you do not control the pathogen and this may result in field failures. In some areas in southern Minnesota, more than 90% of the fields had fungicide failure – leaves became brown followed by defoliation and re-growth which further reduces quality."
It is inevitable that those resistant spores will travel northward over time. Dr. Khan has a few tips for those growers in the northern portions of the Red River Valley, however. "The best thing is to use other fungicides that do not have any resistance issues. Furthermore, growers to the north can also protect themselves by using a couple of different techniques. They can still use the QoI fungicide in areas where the pathogen is sensitive, but they should never use it alone. Always mix it with a different chemistry to prolong the effectiveness and longevity of the QoIs. "If the population of the fungus is one billion spores, for example, maybe a quarter billion spores are resistant to the QoI fungicide. The QoI fungicide will still kill the sensitive population, while the second (and effective) chemistry with a different mode of action will control the QoI-resistant portion of the pathogen population." So you’re out in the field shortly after a CLS application, and you find that you have resistance. What is the first thing you do? "The best thing you can do is send in the samples to the NDSU Extension Service so we can tell you what the fungus is resistant to. CLS has a history of developing resistance to all chemistries. Hopefully it is not resistant to multiple chemistries and you can use one or two different modes of action for your next application. In some cases, we now recommend using a site specific and a broad spectrum fungicide. A broad spectrum is not quite as effective as a site specific, but it will help to wipe out most of those resistant strains. Normally we recommend 14 day intervals between applications, but when it is wet and hot we recommend shortening the interval to 10-12 days because the population can be so high." When those hot, wet conditions occur, there are temperature guidelines to follow to adjust your spraying interval. "You should consider shortening your interval when the daytime temperature is above 70-80° F with nights above 60° F with prolonged wet conditions in the form of rain or dew. When you have heavy rainfall, it can wash protectant fungicides off the leaf. When that is followed by warm temperatures, conditions may become favorable for an epidemic as occurred in 2016." Cercospora is not a disease you can procrastinate with. Staying on top of your management of the fungus is crucial, especially when adverse conditions arise.
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"A lot of growers use their own rigs to spray, but when it gets too wet and you cannot get into the field, your 14 day interval might get stretched out. In those situations, we recommend that the grower go as quickly as possible and get an aerial applicator to spray the fungicide. Once this fungus gets ahead of you, it’s very difficult to control." Another issue growers dealt with this growing season, aside from resistance, was the overall amount of pressure in which CLS was seen in some areas. This concern wasn’t necessarily related to resistance, but rather overwintering. "In 2015 we had a warm and wet September, which led to a high population of the pathogen later in the season and a high overwintering pathogen population. Now this year we had more Cercospora during the growing season than we did last year. With another warm and wet fall in 2016, next year does not look to be much better. Anytime you have conditions for a good sugarbeet crop in this region, those conditions are also very conducive for Cercospora Leaf Spot. If you have a high overwintering population of Cercospora, and then have a good growing season, you will have to fight for control of leaf spot." Farmers can be proactive when it comes to the overwintering of CLS. Incorporating your infected leaves into the soil after harvest may help decrease the pressure of Cercospora for the following growing season. "When you are defoliating the beets during harvest, those leaves will break off and stay on the ground. Infected leaves will be moved by wind to surrounding areas, including fields which did not have sugarbeets this year. If you incorporate the leaves into the soil in the fall, that will help break it down and reduce the problem. So you want to incorporate the infected residue as much as possible. If the residue remains on top of the soil then it will lead to higher disease incidence and possibly more damage next year." As of now Cercospora pressure is forecasted to be higher than both the 2015 and 2016 growing seasons. Giving yourself the best chance at controlling the disease comes down to a couple of key management decisions. "Because of the fact that next year we expect to see more leaf spot, number one, I would plan on looking for a more tolerant variety to CLS. Number two, make your first fungicide application at disease onset or when the first symptoms are observed in the factory district and keep spraying on time using mixtures of different chemistries," said Khan. "Growers are encouraged to complete the annual survey. Information provided for the different growing areas will help me in getting support from Minnesota and North Dakota Departments of Agriculture when requesting special exemptions for use of fungicides." – Mike Spieker