Oklahoma Peanuts: Little Disease Pressure, but Conditions Good for Development

Peanut acreage is up this year and the crop is looking good with few disease problems thus far except for early leaf spot (Figure 1). Recent rains and relatively cool temperatures in southwestern OK peanut production areas have resulted in periods of favorable weather for early leaf spot development.

Leaf spot causes leaflet defoliation, which can drastically reduce yield when defoliation levels exceed 50%. In plots at the Caddo Research Station, the Peanut Leaf Spot Advisor has already recommended 2 to 3 sprays for leaf spot depending on planting date. With more rain in the forecast, favorable weather for leaf spot is expected to increase (Figure 2).

Leaf spot is already showing up in our plots that are in continuous peanuts. Leaf spot is also showing up in commercial fields with short or no rotations, but not yet in well rotated fields. There are a lot of acres planted to spanish varieties which are particularly susceptible compared to runners.

Growers are encouraged to stay ahead of leaf spot by making timely preventative fungicide applications.

Photo: Oklahoma State University Figure 1: early leaf spot

Figure 2: Leaf Spot Advisor. Click Image to Enlarge

We have not yet seen Sclerotinia or southern blights (Figures 3 and 4), but look for those to develop shortly with recent rains and cool weather. Soilborne diseases such as pod rot, southern blight, and Sclerotinia blight generally begin to appear in August and treatment programs for those diseases should be based on field history and early symptom detection.

Spanish varieties typically do not respond to fungicide treatment for Sclerotinia blight. The new runner variety “Lariat” is also highly resistant. Other runner and virginia varieties should be treated for Sclerotinia blight shortly after symptoms first appear.

Due to the widespread use of generic tebuconazole for leaf spot control in Oklahoma, growers have generally been getting good southern blight control in the process of spraying for leaf spot.

Photo: Oklahoma State University Figure 3: Sclerotinia

Photo: Oklahoma State University Figure 4: Southern Blight

Growers should also watch for web blotch (Figure 5) on spanish varieties. The disease is favored by cool rainy weather and is not effectively controlled by tebuconazole. If web blotch is observed in spanish peanuts, growers should immediately revert to a 14-day schedule with chlorothalonil (e.g. Bravo).

Include fungicides containing pyraclostrobin fungicide (e.g. Headline or Priaxor) into the spray program for enhanced control. Once web blotch gets started, it is difficult to control. The disease is rarely seen on runner types.

Photo: Oklahoma State University Figure 5: Web Blotch