V-C News Barbara Shew Peanut Notes No. 154 2019
A Different Kind of Season (So Far)
NC State University
After a couple of growing seasons with mild wet weather, 2019 has been hot and dry through early August. There are hints that we are headed for some relief in August, but effects of the previous hot weather will continue to affect decisions this fall.
Delayed planting and poor pod development earlier in the summer could set us for a long harvest season and late maturity. As of early August, leaf spot has been fairly quiet, but just a little rain and some milder temperatures could lead to rapid increases in disease. It is critical to maintain good disease control and vine health through harvest. This could mean that leaf spot sprays will be needed well into September for those fields that need more time to mature.
Chlorothalonil (Bravo) is the best choice for fall leaf spot control since it helps to reduce selection for fungicide resistance. Now that we have confirmed that some leaf spot populations in North Carolina are resistant to group 11 fungicides, the need for resistance management is more critical than ever. Site-specific fungicides from any resistance group applied after September 1 should be mixed with chlorothalonil to guard against resistance development and to protect against any resistant populations that may already be present. This usually is true even in fields where Sclerotinia blight is a concern.
Peanut crops that remain in the field through October or into November are at risk for Sclerotinia blight as long as cool weather and heavy dews prevail. However, Sclerotinia sprays are most effective when applied when disease first becomes apparent. Assuming that disease has not started already, the time to be extra vigilant for Sclerotinia blight is in late August and early September. Sprays applied after this time often do not result in yield increases. Lektivar 40SC (fluazinam; same a.i. as in Omega 500F) is now available in North Carolina. The amount of a.i. per gallon and the application rate of 16 to 24 oz/A are the same as for Omega. At the time this article was written, we have not had the opportunity to test it against Sclerotinia blight on peanut, but hope to have some results by the end of the season.
Minor diseases tend to pop up when peanuts are grown late into the fall. Botrytis blight and other vine declines are most commonly associated with injuries such as previous disease and tractor or frost damage. Maintaining plant health by controlling leaf spot and avoiding excess vine growth and injury may help to prevent these problems.
As always, be on the lookout for problem areas in the field as the season ends. Check patches of yellowed, stunted, or dead plants for possible disease problems. Areas affected by nematodes may show stunting or yellowing. Gently pull up several plants and check the roots and pods for galls caused by root-knot nematodes, small pod lesions caused by lesion nematodes, or severe root pruning caused by sting and various other nematodes. Areas of wilted, yellowed, or dying plants may have CBR. Brittle, blackened roots, pods rotted or missing, and brick-red fungus on the roots, pods or lower stems indicate that CBR may be present. Spotted wilt can cause late-season yellowing that can be confused with CBR. Look for pod stunting and discoloration and for foliar symptoms such as spotted patterns, purple discoloration on the underside of the leaf, and twisted petioles. Note the location and severity of these problems so that this information can be used when planning rotations and future plans for disease management.