Treat fungi with the same chemical and eventually they'll develop resistance to it. That's the reasoning behind recommendations to mix or alternate fungicides in dealing with leafspot and other diseases in peanuts, says Barbara Shew, North Carolina State University Extension plant pathologist.
When she's giving talks to peanut growers, all she has to do is mention one word: Benlate. Peanut growers began using it in 1970 in conjunction with emerging high-yielding varieties. By 1973, widespread loss of control was observed with Benlate. In only two years, leafspot fungi developed resistance due to over-use.
Generally, Extension recommends putting a chlorothalonil product at the beginning and the end of the spray schedule. “The reason for that is you want to kill any resistant strains that may over-winter,” Shew says. “You start and finish with chlorothalonil to eliminate as many resistant strains as possible.”
Chlorothalonil, “our old faithful,” came on the market in the 1970s and is still around because of its broad-based activity, Shew says. Bravo is the most well known brand of the material.
Starting with chlorothalonil at the beginning, peanut producers can use other products to round out their leafspot spray schedule.
About 10 years ago, the crop protection industry began a move to gentler materials with specific modes of action. Tebuconazole or Folicur, made by Bayer CropScience, was the one of the first of the DMIs to come on the market with a specific mode of action and less effect on the environment, Shew says. Propiconazole or Tilt is also a DMI. Propiconazole is sold with chlorothalonil (Tilt/Bravo) to increase activity and for resistance management. Tebuconazole inhibits the production of sterols, which are essential for fungal growth and development.
The strobilurin family of chemistry, which Syngenta's Abound (azoxystrobin) is a member, inhibits respiration of the Cerospora fungi that causes leafspot. The most recent strobilurin on the market is Headline, which is a tyrazlostrobin.
Soon, it will be easier for peanut growers to alternate chemicals with different modes of action. Headline, which is made by BASF, is the first fungicide using a group system to identify fungicides, says Shew. Headline is a Group 11 fungicide. “Over a period of time, more and more fungicides will have these labels that clearly identify what chemistry family the fungicide belongs to.”
Crop protection companies have also introduced pre-mixed or tank-mixed versions of different products. For example Stratego, made by Bayer, is a mixture of two active ingredients. “These mixtures give a broader spectrum of activity and reduce the chances of the fungi developing resistance,” Shew says.
“We'll always have to be vigilant about resistance developing,” Shew says.
Plant pathologists constantly monitor the development of resistance. “Currently, I have a person working with me establishing a baseline sensitivity on Headline,” Shew says. “So far, we haven't found any indication of resistance in the isolates we have tested.”