Friday, July 13, 2012

Time for Peanut Disease Control

Written by: Barbara Shew, Peanut Extension Specialist, NC State University
Leaf spots
Now is the time for peanut growers to begin foliar disease control programs. The first fungicide spray should go on when peanuts reach R3, or when about half the plants in a particular planting have at least one pod starting to develop. Most fields in North Carolina will be at or near R3 by now. In most cases, a group M (multisite) fungicide such as Bravo (chlorothalonil) or Tilt Bravo (groups 3 and M) should be used for the first spray in a 5-spray calendar leaf spot control program. You may be able to reduce the number of leaf spot sprays applied in a season by using the North Carolina leaf spot advisory after the first spray.
Example of a plant at R3 (very early pod stage) (Photo: B. Shew)
Southern stem rot
Southern stem rot usually starts at the crown of the plant or on stems touching the ground near the crown. Infected stems are the color of a brown paper bag. The white fungus growth is thick, stringy, or fan-shaped. Later, fungus structures about the size and color of mustard seed will develop.
Signs and symptoms of southern stem rot (Photo: B. Shew)

Southern stem rot is very active in hot, wet weather. Most peanut fields benefit from applications of fungicides that control stem rot during the hottest part of the season – usually from mid-July to late August. In North Carolina, we recommend starting stem rot control at the second leaf spot spray, or approximately two weeks from now. Fields at high risk for stem rot problems – those planted to a highly susceptible cultivar like Gregory, irrigated fields, fields or cultivars where heavy vine growth can be expected, and fields that have a history of stem rot and/or vegetable production – may benefit from using a soil fungicide for the first spray. Foliar fungicides effective against stem rot include Abound (group 11) and Provost (group 3). Artisan (groups 3 and 7), Convoy (group 7) and tebuconazole (group 3) are also effective but must be mixed with a foliar fungicide such as chlorothalonil or Headline (group 11). Fontelis (group 7), which recently was labeled on peanut, also is active against leaf spots and stem rot. Growers need to alternate fungicides from different groups or mix with a group M fungicide during the season to optimize leaf spot and stem rot control, and to avoid potential resistance problems. Remember that group 11 fungicides can only be applied twice in a 5-spray program. See the Agricultural Chemicals Manual for rates and other details. 

The new cultivar Bailey has moderate leaf spot resistance and good resistance to southern stem rot. The first spray can be delayed two weeks (R3+2) on Bailey. The new cultivar Sugg also has resistance to these diseases. Fungicide programs have not been tested as extensively on Sugg as on Bailey, but I believe that sprays can also be delayed on Sugg. Growers who chose to delay sprays on Sugg should be sure to scout carefully until we have more experience with this cultivar.

Begin scouting for Sclerotinia blight!
This week’s cool rainy weather has been highly favorable for Sclerotinia blight, so it’s time to begin scouting. Outbreaks of Sclerotinia blight can develop even in early July if weather is favorable and rows are within 6 inches of touching. It is important to check early for Sclerotinia blight because we have shown that the fungicide application made at the time of the first outbreak is the one most critical for control during the season. 

To scout for Sclerotinia blight, check several 100-foot sections of row. Early Sclerotinia infections often are found on limbs away from the crown. The fluffy white fungus is easiest to see early in the morning or after a rain. You must look inside the canopy to spot these infections. A 3 or 4 foot section of half-inch dowel or PVC pipe is a good tool for pushing vines aside for scouting. Infected stems develop a bleached appearance as the disease progresses. In fields with a history of Sclerotinia blight problems, apply a fungicide if you see active infections on an average of about 1% of the plants in a row. Continue to scout and monitor Sclerotinia advisories as the season continues. 
Early infect of Sclerotinia blight. Note fluffy fungus and bleached stems (Photo: Damon Smith)
Leaf spot and Sclerotinia advisories are available daily during the summer. Email Barbara Shew or your county agent if you would like to receive advisory emails. Advisories are also available on-line here