Arkansas Peanuts: Hot Conditions Favor Southern Blight

Currently, the 2018 Arkansas peanut crop is looking good to excellent. The majority of the peanut fields range in maturity from full pod to beginning seed (Fig. 1). Blooming has slowed due to the hot weather and has stopped in areas of the field where plants are stressed. Based on the stage of growth and weather forecast, irrigation will be an important factor throughout July.

In addition to water, diseases can be a limiting factor and now that vines are meeting across the furrow crowding out weeds, growers’ attention has turned to disease control.

Although hot, humid weather conditions are not favorable for many fungal diseases, those with the name “southern” tend to be more active. Southern blight caused by Sclerotium rolfsii is one of those diseases that is more active in hot weather. This pathogen has a wide host range that includes soybean, thus peanut following soybean can have more issues with southern blight.

To the trained eye “flagging” is the first symptom in peanut (Fig. 2). Most have applied their first preventative fungicide and when the summer is cooler as in 2017, the impact from southern blight is lower; however, the current hot weather pattern is more favorable for southern blight. So, overlapping fungicides at least at 21 day intervals in a field with a history of the disease will be more important this year compared to last year. See MP 154 for fungicide recommendations for southern blight on peanut.

Currently, there have been no reports of early or late leaf spot in the state. However, there are lots of leaf spot doppelgangers (Fig. 3), so before treating based on seeing spots, make sure those leaf spots are caused by the early or late leaf spot pathogen. Late leaf spot is the more common of the two in the state and typically shows up in September. See earlier articles on this website for information about early and late leaf spot.

Another foliar disease of peanut popping up in peanut fields is caused by Lithosphaerulina crassiasca. This fungus causes two diseases of peanut, pepper spot and leaf scorch. These are minor diseases and do not warrant a fungicide, but the leaf scorch symptoms look impressive in the field. It may develop along the leaf edge or more commonly in the center of the leaf resulting in the “v-shaped” lesion with yellow margin (Fig. 4).

Figure 1. Peanut plants collected in Arkansas with pods at various stages of maturity.

Figure 2. Wilted leaves (“flagging”, blue arrow) caused by southern blight.

Figure 3. Spots on peanut leaves that look like early leaf spot, but are not caused by the early leaf spot pathogen. These can be found in all peanut fields in Arkansas.

Figure 4. Leaf scorch of peanut caused by fungus Leptosphaerulina cassiasca, a minor foliar disease that does not warrant a fungicide.

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