Chlorothalonil Peanut Notes No. 130 2023

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There is a lot of data showing the effectiveness of a spray program with chlorothalonil applied bi-weekly throughout the season. There are datasets from 40 years ago that demonstrate this. When a generic tebuconazole is applied during the middle sprays (2, 3, and 4 in a 5-spray program), leaf spot and stem rot control is generally covered. While this approach can be inexpensive and effective, there are several reasons why you should use caution in terms of using a full-season chlorothalonil program. First, chlorothalonil is a protectant only and you have to stay on a strict 2-week schedule. If weather does not allow this, it is likely you will get behind on leaf spot disease. Newer fungicides offer greater flexibility in this regard compared with chlorothalonil. Second, of all the fungicides we spray, chlorothalonil is the one that is more likely to flare spider mites under drier conditions. A fungus keeps mites in check when conditions are favorable for pathogen development. When conditions get drier, the pathogen is less active and this gives mites some “breathing room.”  When we apply chlorothalonil under drier conditions, we knock out the “good” fungus and this really opens the door for mites to be active. Other fungicides can do this, but the general consensus is that chlorothalonil is the fungicide that is most prone to do so.

With all this said, chlorothalonil is important for the first and last sprays. Perhaps one more application in the number 2, 3, or 4 slot can be effective if well timed when applied with tebuconazole. Logistically, there are risks to a more intensive chlorothalonil program and we have to always keep mites in mind.