V-C Peanut News Article Barbara Shew Peanut Notes No. 113 2018

— Written By Barbara Shew and last updated by
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Finishing up disease programs after a challenging summer

Not counting severe drought years, this summer has been among the most challenging I can remember for bringing a healthy peanut crop into the fall. We started out very wet and some peanuts got planted late. Hot dry weather in June and early July was followed by several weeks of extremely wet and humid weather. Leaf spot sprays were delayed or missed due to rain in July and early August. This means that growers will continue to face challenges in the fall even if we are fortunate enough to dodge tropical storms and hurricanes.

We used to rely on cooler, dryer weather in September to slow down leaf spot epidemics, making it possible to end spray programs around Labor Day. However, recent Septembers have tended to be warm and humid almost through the end of the month. Combine favorable leaf spot weather with greater disease loads due to missed or delayed sprays earlier in the summer and it’s likely that most growers will need to extend fungicide programs later than usual.

Many growers probably will need an extra leaf spot fungicide application in the middle of September to hold disease at bay until digging, particularly in late-planted peanut. A late maturing crop will be especially challenged with leaf spot epidemics because it will spend more time exposed to peak disease pressure. Leaf spot fungicide applications should continue until about three weeks before digging, or as long as weather remains favorable for disease.

Chlorothalonil (Bravo) is usually a good choice for late season leaf spot control since it also reduces the reduce the risk of fungicide resistance. This is even more important now, because we still are struggling to understand reports of poor performance of some fungicides.

Growers may be tempted to try rescue treatments in fields that are not in good shape, but so-called rescue treatments often are futile. For every spot you see, there probably is another spot incubating and waiting to be born. Fungicides will not harm an established incubating infection. While some fungicides can stop the very earliest stages of infection, other common fungicides mostly affect spore germination. This means that any fungicide applied as an attempted rescue treatment may prevent germinating spores from causing new infections, but will not stop infections that are already incubating.

Worse, exposing a large leaf spot population to a selective fungicide greatly increases selection for fungicide-resistant strains. If you feel you must attempt a rescue, mix the selective fungicide with a non-selective fungicide such as chlorothalonil. Measurable yield losses occur at about 25 – 30% defoliation and yield drops off sharply at 40 – 50% disease. Rather than making fruitless attempts at rescues, it is best to refrain from spraying and to consider digging early once disease approaches these levels. You can find much more information about digging decisions in Peanut Information and in Peanut Notes posted on the NC State University Peanut Portal.