Disease Management Now
No further applications of leaf spot fungicides will be needed for growers who plan to dig in the next two weeks, as long as leaf spot is still under good control. Leaf spot fungicides are effective for 14 days, which is also the PHI for most fungicides. If normal weather patterns return by the middle of the month, cooler nights and low humidity will greatly reduce further leaf spot activity. Any leaf spot currently not under good control can (and probably will) explode given the current weather. If growers are still 2 weeks or more from harvest, another spray could be justified once the weather clears. If they spray, they should use chlorothalonil (Bravo) alone or mixed with another product. Spraying an unmixed site-specific fungicide (group 3, 7, or 11) in high leaf spot situations greatly increases the risk of fungicide resistant leaf spot populations.
We have seen several cases this year of out of control late leaf spot that have been associated with use of unmixed tebuconazole, indicating lack of efficacy and presumably resistance to this fungicide. This illustrates the importance of mixing and/or alternating fungicides with different modes of action to prevent resistance development. In the specific case of tebuconazole, always mix with a fungicide from a different resistance group (7 or 11) or a nonspecific fungicide such as Bravo.
Due to the current weather, the risk of Sclerotinia blight is high, but no treatment is recommended if growers plan to harvest within the next two weeks. Typically we see little benefit from Sclerotinia sprays made this late in the season, and it is too late to apply Omega (PHI of 30 days) unless a very late harvest is anticipated. We will be running disease advisory programs in-house for a few more weeks and can provide updated advisories on request.
Article first appeared as North Carolina Peanut Note (PNNC-2015-141)