Monday, September 12, 2011

Late season disease control

Planning fall fungicide applications will be a bit tricky this year. 

In a normal year, the 5th calendar spray would usually be applied in the first week of September. Since fungicides generally provide about 2 weeks of protection, a spray in early September provides protection until the third or fourth week of the month. By that time, most growers are within a week or so of digging. In addition, night temperatures usually have dropped into the 50's with low daytime humidity, which drastically slows leaf spot epidemics. For this reason, no additional sprays are necessary in most years. 

This year many growers applied a fungicide just before Hurricane Irene.  It has now been about two weeks since that application was made. Unless a field is nearly ready for digging, an additional fungicide application may be necessary to carry protection though the end of the season. 

An additional foliar fungicide spray could also be necessary if digging will be delayed due to earlier drought stress – or if we have another tropical storm. 

Growers who want to continue a calendar schedule should apply a spray if it has been more than two weeks since the last fungicide application. Be especially cautious with NC-V11 or VA 98R since these cultivars are susceptible to web blotch. Perry and Gregory, which are susceptible to late leaf spot, also could be vulnerable. 

As already mentioned, leaf spot epidemics do slow down in typical late September weather. Many days will not favor leaf spot development, and sprays will not be necessary. However, a warm humid spell anytime before digging can get disease going again. The best way to determine whether a late-season spray is needed is to watch leaf spot advisories, even if a calendar spray schedule was used throughout the summer.

Either way, use a broad-spectrum fungicide such as Bravo (chlorothalonil) for the final spray.  Some growers may be interested in mixing Bravo and Headline for the last spray. While not as risky as using Headline alone, there still is a fungicide resistance risk with this approach because Headline persists longer than Bravo. This means that the Headline could be unprotected for a period of time after the Bravo is no longer effective. Here are some guidelines for resistance management in late season: 

  • The best way to prevent fungicide resistance is to use Bravo (or generic equivalent) as the last spray
  • Do not use any group 3 (tebuconazole, Provost) or group 11 (Headline, Abound, Evito) fungicide as the last spray unless it is mixed with Bravo (or generic)
  • Do not use a Bravo + Headline mixture as the last spray if digging will be delayed past 2 weeks. A follow-up application of Bravo may be necessary for longer intervals.
  • Do not make more than two applications of any group 11 fungicide per season.


Sclerotinia blight has been very active in many locations due to cool weather following Irene’s heavy rains.  Follow disease advisories, but remember that the PHI (time to threshing) for Omega is 30 days. I’ve gotten several questions about Quash as a possible alternative to Omega due to cost. I have done only limited testing with Quash for Sclerotinia control. In my trials, it provided suppression of Sclerotinia but did not give control equal to Omega (1 to 1.5 pt/a). Quash at 4 oz/a could be an alternative for fields with only a light amount of disease. If you can find it, Endura at 8-10 oz/a also controls Sclerotinia. PHI for Quash and Endura is 14 days. 

BE SURE to scout very carefully to determine the extent of the problem before making Sclerotinia control decisions. Even severe epidemics may not be evident until you check inside the canopy.

Growing degree days May 1 - September 12, 2011
GDD base 56
2709 (est)
Rocky Mount
Elizabeth City

Degree days can be found in daily peanut disease advisories for these locations.

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