UGA Extension Cook County

Cotton,Peanut, Corn, Soybean Disease Update

Row Crop Disease Update from UGA Plant Pathologist Bob Kemerait:

PEANUTS: Getting lots of questions these days about late-season peanut disease problems.  Just a few thoughts.    Three weeks to go until you dig the peanuts and little-or-no disease in the field?  I wouldn’t put out any more fungicides unless there is threat of a hurricane or tropical storm.  If three of more weeks out and on your last spray and you are seeing some leaf spot develop, applying a pint of chlorothalonil tank-mixed with 7.2 fl oz of  tebuconazole or 5.5 fl oz of Alto or 5 fl oz of Topsin or 2.5 fl oz of Domark.  If time for your last spray and very little leaf spot is present, then 1.5 pints of chlorothalonil may be all you need.

If white mold is popping up in your field late in the season and is confined to individual plants scattered across the field, then you may want to mix tebuconazole with your last leaf spot spray.  If the disease is more severe, or you are really worried about it, then you might consider using 16  fl oz or Convoy rather than tebuconazole.

It is generally advisable to wait to dig the peanuts until they are “ready” based upon the hull-scrape test.  This is true even if there is significant tomato spotted wilt in the field or some white mold.  HOWEVER:  if there is significant defoliation from leaf spot or significant white mold in the field, it often best to dig the peanuts earlier than planned to avoid excessive digging losses.

Cotton: We have three diseases of significant importance in the field right now.    Boll rot.  The rain and heavy vegetative growth we have seen this year has created perfect conditions for fungal boll rot.  We are seeing a lot of it.  We are not seeing a lot of bacterial boll rot, though some is certainly there.  Fungal boll rot is most severe in lower bolls deep in the canopy or where insect damage also occurs.  Fungicides are not an effective treatment; only opening the canopy up to increase airflow and reduce humidity can help reduce boll rot.  2.  Areolate Mildew.  I know that there is great concern and I have heard growers complaining that there fungicide applications did not stop the disease.  Here are some thoughts.  For the second year in a row, Areolate mildew is early and widespread.  Areolate mildew can cause significant premature defoliation.  I do know that fungicides like Headline and Quadris and certainly Priaxor can slow the spread of the disease, though not necessarily stop it, especially when it is well established in a field.  It is not clear how much yield is at risk or that can be protected; but it is believed that significant premature defoliation is not a good thing, unless one is trying to open the canopy up to slow boll rot.  Here are my recommendations, though they have not been proven with any hard data.  If a grower is within 4 weeks of defoliating the crop anyway, save the money and don’t spray.  If the grower is more than 4 weeks of defoliating and the areolate mildew is not too severe (i.e. already causing significant leaf drop) then there may be a benefit to treating with a fungicide.  This may not stop the disease but will slow its development.  3.  Target Spot.  Target spot has been severe and widespread in this rainy season.  I believe well-timed fungicides have been helpful this year.  I don’t believe there is any benefit to a fungicide application after the 6th week of bloom.  Either there is too much disease already to stop it or there is not enough time for disease to develop.  In this season, a second fungicide application 2-3 weeks after the first application is something to consider.

Late-planted corn: Southern corn rust is now commonly observed on older corn across the Coastal Plain, corn that is too late for it to matter.  However, southern corn rust does pose a threat to younger corn and preventative protection with a fungicide is something to consider, especially as the crop approaches the tassel growth stage.  Also, I am receiving numerous reports of young corn affected by northern corn leaf spot (Bipolaris zeicola) which produces numerous, small-to-medium sized red/brown spots, sometimes with appearance of concentric rings.  Typically, corn is most severely affected by northern corn leaf spot early in the season and then grows out of it; however I cannot be sure that this will always be the case.  I have no data on fungicides for management of northern corn leaf spot, but as it is closely related to northern and southern corn leaf blights, I am confident that mixed mode of action products we already use will be helpful for the “spot” disease.  If a grower does spray, applications as early as V6-V8 would be appropriate. But again, I just don’t know if it matters.

Soybeans: Asian soybean rust is still lightly scattered from Decatur County to Appling County, but has only been found so far in KUDZU.  Soybean rust is certainly not a major problem at the moment; however it could become so.  Management options are to protect the crop with a fungicide sometime between the R1 (early bloom) and R3 (early pod set) stages.  Such timings may correspond well with other disease and insect control measures.  Some “Cercospora leaf blight” is being reported; this disease causes much of the upper foliage to take on a “bronzed” cast and then leaves drop prematurely leaving the “bony” petioles like skeleton fingers to the sky.  Cercospora leaf blight also causes purple seed stain.  Fungicide applications at pod set (R3) can help manage this disease.