By Rosalie I. Tennison
No question that early blight is a yield robber, but, with careful management, most growers can prevent losses. An independent potato researcher has some useful advice on how to manage early blight even more effectively, but it requires planning.
Dr. Jeff Miller of Miller Research LLC in Idaho says of the different ways growers can manage early blight infections, the best is to keep fertility at optimum levels right from the beginning. “Fertility management can reduce early blight problems effectively,” he says. “But, it has to be done properly.”
“Too high levels of fertility can cause problems in crop yield,” Miller explains. “So, manage fertility for optimum production and then use fungicides for control of early blight.”
However, the greatest concern about this strategy is overuse of a limited number of fungicides, which could lead to resistance development. Miller says there are some fungicides that are showing early blight resistance in the midwestern United States and that resistance could show up in Canadian fields in the near future. “We are recommending that growers use a tank mix for early blight management to help reduce the development of resistance,” says Miller. “Use two chemistries at the high end of the label rate to ensure resistance doesn’t develop.” He adds that if growers start early to control early blight, they can sometimes get late blight control as well.
Currently, the most effective fungicides for early blight are in Group 7. Of these, boscalid (sold as Cantus in Canada) has proven most effective, but resistance to this product is developing, perhaps due to injudicious use. However, while losing its effectiveness on early blight due to resistance issues, boscalid remains effective on white mould. Therefore, a tank mix with boscalid along with another product effective on early blight from another chemical group should control both.
When a new chemistry is introduced, such as fluopyram + pyrimethanil (sold as Luna Tranquility), which is effective on both early blight and white mould, growers need to use it frugally to minimize resistance development. Despite being a Group 7 chemistry, there has been no recorded resistance to Luna Tranquility, which means it could be a good tank mix choice with another chemistry, Miller says. He suggests tank mixing with a protectant fungicide, such as one based on chlorothalonil or mancozeb. “When we have an effective product we need to use it carefully to ensure its continuance,” Miller comments.
Miller says Group 3 chemistries have good activity on early blight, but not on white mould. Again, tank mixing a Group 3 and a Group 7 should provide the control needed and ensure the longevity and effectiveness of the chemistries. Group 11 strobilurin chemistry products, while registered for early blight control, do not get a vote of confidence from Miller because of fungicide resistance in the early blight pathogen. He says he would not recommend using them because the risk is too great.
Miller recommends the “protectant approach” for optimum disease management, particularly for early blight control. “I would always begin my early blight management with good fertility,” he concludes. “Then, my next step would be to choose an effective fungicide and apply it early in my planned program at row closure and then 10 to 14 days later. After that, I would switch to other products used as protectants. My first products would be strong on early blight.” He likens this approach to using sunscreen on your skin. It’s not effective if you put it on after you have been out in the sun for several minutes, but it is highly effective if it is put on before sun exposure occurs.
“If you wait until you see early blight before you begin control, it is already too late,” Miller says. “You have to plan your early blight management when the crop is in the ground beginning with fertility and then continue with the protectant approach before early blight becomes a yield reducing problem.”
While most growers understand the importance of rotating chemistries to prevent resistance development, tank mixing two different chemistries and then switching to a mixture of two other chemistries may be a new approach. It could be one that proves most effective as growers begin to plan the management for their latest potato crop.