Late Season Cotton/Peanut Disease Management

Below is an update from UGA Extension Plant Pathologist Bob Kemerait.  Tucker Price 507-8862

When to Terminate Peanut Fungicides – Bob Kemerait

NOTE:  A critical consideration late in the season is that pre-harvest intervals vary with some of the fungicides.  Growers must always check the label to make sure on all of these.

Below are some typical situations that peanut growers may find themselves in and suggestions for control:

1. Grower is 4 or more weeks away from harvest and currently has excellent disease control.
   1. Suggestion – I recommend the grower apply at least one more fungicide at least for leaf spot control.
   2. Suggestion – Given the low cost of tebuconazole, the grower may consider applying a tank-mix of tebuconazole  + chlorothalonil for added insurance of white mold and leaf spot.
   3. NOTE: If white mold is not an issue, then the grower should stick with a leaf spot spray only.
2. Grower is 4 or more weeks away from harvest and has disease problems in the field.

1. 1. If the problem is with leaf spot – Grower should insure that any fungicide applied has systemic/curative activity. If a grower wants to use chlorothalonil, then they would mix a product like thiophanate methyl (Topsin M) or cyproconazole (Alto), with the chlorothalonil. Others may consider applying Priaxor, if they have not already applied Priaxor twice earlier in the season.
   2. If the problem is white mold – Grower should continue with fungicide applications for management of white mold. If they have completed their regular white mold program, then they should extend the program, perhaps with a tebuconazole/chlorothalonil mix.  If the grower is unhappy with the level of control from their fungicide program, then we can offer alternative fungicides to apply.
   3. If the problem is underground white mold – Underground white mold is difficult to control.  Applying a white mold fungicide ahead of irrigation or rain, or applying at night, can help to increase management of this disease.
2. Grower is 3 or less weeks away from projected harvest and does not currently have a disease issue. Good news! This grower should be good-to-go for the remainder of the season and no more fungicides are required.  SEE NOTE BELOW ABOUT HURRICANES
3. Grower is 3 or less weeks away from harvest and has a problem with disease.

1. 1. If leaf spot is a problem and 2-3 weeks away from harvest, a last leaf spot fungicide application may be beneficial. If leaf spot is too severe, then a last application will not help.  Tank mixing chlorothalonil with a systemic fungicide, like thiophanate methyl or other appropriate systemic fungicide, could be beneficial.
   2. If white mold is a problem and harvest is 3 weeks away, then it is likely beneficial to apply a final white mold fungicide. If harvest is 2 weeks or less away, then it is unlikely that a fungicide will be of any benefit.
   3. NOTE:  If harvest is likely to be delayed by threat from a hurricane or tropical storm, then the grower may reconsider recommendations for end-of-season fungicide applications.

Cotton Areolate or “False” Mildew

Areolate or “False” Mildew is a fungal disease of cotton caused by a fungal pathogen Ramularia.  Areolate mildew has been particularly severe in the eastern part of our cotton production region.  Symptoms look a lot like “powdery mildew” on other crops. The white sporulation is typically on the bottom of the leaf and the disease may cause a distinct “rolling” of the severely affected leaves followed by significant and rapid defoliation.

We have very little data, but from trials conducted in 2005 in Appling County, I know that fungicides azoxystrobin (Quadris) and pyraclostrobin (Headline) protected against premature defoliation and also improved yields slightly (numerical but not statistical).

Recommendations for use of a fungicide for areolate mildew are difficult to make.  Here are my considerations.

1.  How much of the crop is already defoliated from disease? If more than about 20-25% I don’t think a fungicide offers any benefit.
2. How long until the grower wants to defoliate?  I believe that with favorable conditions like we have now, areolate mildew can cause significant defoliation in 7-10 days.  If predicted time of grower crop defoliation is a month or more out, then a use of a fungicide may become more important.
3. What portion of the bolls are already open?  More than a few percent and use of a fungicide becomes less important.
4. How mature are the bolls in the top of the plant?  If they are already firm and hard, then a fungicide is less important.  However if bolls the grower expects to pick are still soft, then keeping leaves on longer can be important.

A difficult call to use a fungicide for areolate mildew, but there are times it could help.