Over the last several weeks I have received questions about late-season problems — predominantly foliar diseases and insects — in Mississippi soybeans.
Foliar diseases have been more prevalent this season, but you must keep several points in mind when considering control. For instance, varietal resistance is the best approach, especially for diseases such as stem canker and SDS (sudden death syndrome).
You cannot control all foliar diseases with a fungicide, and once they are established it is usually too late to benefit from a fungicide application.
Disease control must be approached from a preventative standpoint, not after the fact. I recognize this is a difficult approach for many, but to take yields to the next level will require an increase in management.
Foliar fungicides will play a role, and other late-season management options may be needed.
We have the potential for an excellent soybean crop. As of early September we were approximately one-third harvested and yields had been well above average. If we do not experience any harvest problems, this will be the best crop ever grown in Mississippi. I fully expect the state average yield to approach 40 bushels per acre. This crop could have been even better, but it would have required additional finetuning on the tail end.
I have had lots of calls in the last three weeks regarding the use of Quadris. In most situations it was obvious that producers were starting to see some disease and wanting to do something about it. In most situations an application was too late.
Quadris is effective in reducing losses from anthracnose and pod and stem blight, with its strong suit being aerial blight. It will help minimize frogeye when applied before the disease occurs.
However, most have been asking about the yellow/bronze discoloration they are seeing on the foliage late in the season. This is late-season cercospora — the same disease that causes the purple stain observed on seed from time to time. Quadris is not effective on late-season cercospora.
As other diseases are controlled, late-season cercospora often appears to become more prevalent. Our best option for preventing losses from cercospora is Topsin M; although it is similar to Benlate, it does not appear to do as good a job and it is expensive.
Currently we are looking at tank-mixes of Quadris and Topsin M to reduce cost and broaden the spectrum, but we hope some better options will be available soon.
In addition to late-season cercospora, we have encountered more frogeye leafspot than we have observed in several years. Frogeye can be controlled, but applications need to be made prior to the onset of disease — a preventative approach.
In our SMART verification program, we applied Quadris and Dimlin to all our Group V fields. These were all irrigated fields. We applied 4 ounces of Quadris plus 2 ounces of Dimilin per acre. Applications began in mid-July and were complete by early August. Fields this year are scattered from Tunica, Miss., to Valley Park, Miss.
As you look at these dates, you can see that we made this application early — before problems started showing up. Soybeans look good, and we anxiously await harvest.
Some may be skeptical of making this application because we do not have the same disease pressure every year. Think preventative, however, to achieve the full benefits. I believe if you irrigate you should consider an application such as this every year. Research work has shown yield increases as high as 14 bushels per acre. There are years the response is minimal, but I believe it will at least pay for itself. In other words, it may not make you anything, but it will not cost you either.
I often have growers tell me, “Beans are too cheap. I cannot afford to do these things.” I ask you to consider do you need to make higher yields: when prices are high or when prices are low. You have to give to get. If we do not take a preventative approach to disease control, we will continue to experience yield losses from time to time.
Pressure from loopers have been widespread but erratic. Many fields have escaped the need for spraying loopers due to one of two things: (1) a timely application of Dimilin suppressed loopers enough to allow the crop to progress and (2) earlier planting dates.
Stink bug numbers began picking up Labor Day weekend and pressure has been greater in the Group Vs than in the Group IVs. Keep a couple of things in mind about stink bugs. They can damage a green pecan, so they can damage beans until close to harvest. Although damage may be minimal late in the season, a lot depends on how long populations are allowed to persist in the field.
You may not reach threshold levels in some fields. Keep an eye on nymphs and adults and on how long they have been present. Stage of the crop, surrounding crops and time of planting all play major roles in the potential for damage. When considering thresholds late in the season (nine per 25 sweeps), I would rather have nine feed for one week than allow six to feed for two weeks. Most farmers would not spray, but this is when you have to consider other factors than just thresholds. Thresholds serve as a guide, but they are only one part of the equation.
I ran a sample of Group IVs from the north Delta that were not treated for stink bugs. After looking at the results of germination and TZ tests, I observed 50 percent damage. It is quite easy to pick out damage from harvested seed. I had an elevator operator tell me he was getting pretty good at looking at a sample and knowing whether or not it had been treated. In addition to visual damage, test weights are also lower. Pay attention to this pest late season.
Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected].