Mid-August Row Crop Disease Update
Dr. Bob Kemerait, University of Georgia Extension pathologist, gives a mid-August row crop disease update below:
Much of the corn is being harvested now; it has been a year filled with Northern corn leaf blight, southern rust, and even some charcoal rot (Macon County..) The current weather patterns coupled with associated delays in harvest are likely to result in increased levels of mycotoxins(aflatoxin, fumonosins, etc) from infection by specific fungi. For growers with late planted, irrigated, good yield potential corn: It is important to protect the corn with a fungicide against SOUTHERN CORN RUST and NORTHERN CORN LEAF BLIGHT. Although I believe that “VT/first tassel” is an appropriate time to begin fungicide applications on corn planted in the spring, tassel is likely LATE in many cases for late-planted corn. I would tend to focus on the V5-V6 growth stages based upon current studies we have in the field. We have a number of good fungicides labeled on corn.
Asian soybean rust (www.sbrusa.net) has been found across the Coastal Plain in kudzu and in Decatur County and Tift Counties in soybeans. Soybean rust is also confirmed to be progressing across much of Alabama and approaching the border with central-western Georgia. Bottom line is that weather conditions are EXCELLENT for development and spread of rust and the crop is at susceptible growth stages. Any soybean growers with beans that have reached the late bloom-early pod development stages (R2-R3) should strongly consider applying a fungicide to the crop; fungicides are typically being applied with dimilin and boron and I have not heard of any problems adding an insecticide for kudzu bugs to the mix (though I have not done this myself).
White mold. White mold. White mold. Although we are starting to pick up leaf spot (early and late) in our fungicide plots, the real story now is white mold. Hopefully, most growers are using an appropriate fungicide program. Growers should remember that in a year like 2012, it is impossible to eliminate white mold from a field, no matter how good the fungicide program may be. I am asked daily about “the BEST white mold fungicide program”. There are many strong programs; the most important thing is to stick with a program that you know is effective. Given current conditions, growers may want to protect their crop against white mold until late in the season.
Target spot/Corynespora leaf spot is becoming increasingly widespread along the Coastal Plain of Georgia. Although it is most severe in southwestern Georgia, it is also bad in areas to include Colquitt County, Crisp County, Turner County, and I have seen outbreaks in east Georgia (e.g., Wayne County). If anyone still have doubts about the importance of target spot, or the need for fungicides, I would love to show them some of our fungicide studies. In one instance, target spot that was hard to find on one date had caused 50% defoliation in untreated plots two weeks later. I strongly encourage agents to visit cotton fields, especially those with the following risk factors: cotton-behind-cotton, conservation tillage, irrigated, rank growth, a history of target spot, to determine if there is a problem.
Research this season has CLEARLY shown both the benefits of applying fungicides to reduce defoliation and importance of timing. Fungicide applications that are too early (this year, first square) or too late (later weeks of bloom) have much reduced impact on disease management than did applications times around the early weeks of bloom. The exact timing of application will be a “moving target”, at least for now; however it gives us a starting place. It is my observation that Headline (6 fl oz/A) is a very good fungicide to use, Twinline(8.5 fl oz/A) is also effective, but I like Headline better. Quadrisis also an effective fungicide but I have less experience with it. I am beginning to have my doubts about Tebucoanzole…
The BIG QUESTION NOW!!!! “Bob, should I spray my cotton with fungicides???” I wish I had some better answers, but let’s start with the easiest- A) I would NOT spray if any of the following apply- more than 20% defoliation already and spots throughout the canopy (read: too late!), dryland cotton (f there is any this season…) with poor canopy development or yield potential, well-rotated cotton where very little target spot is found and the disease does not seem to be developing.
Where I MIGHT spray at this time: Grower field is at risk- based upon conditions above and defoliation and disease are present but not to advanced in the field, cotton is within the first 4-6 weeks of bloom, and cotton is still 4-6 weeks away from defoliation. While I cannot ensure that an application of fungicide now will increase yields, I believe that the potential for explosive disease in the field coupled with resulting losses justifies the use of a fungicide.
Boll rots are also severe this year (based largely upon good cotton growth and rainfall). Much of our boll rots this season are “diplodia boll rot” and also some “anthracnose boll rot”.