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Late-season problems for soybeans

Most of the recent calls about soybeans I have received relate to end of the year “late- season” issues. Late-season problems include green bean syndrome, uneven dry-down, excessive stink bug damage, cercospora leaf blight, pod and stem blight, drought stress (caused by either not irrigating or Mother Nature) and defoliation issues.

Regarding beans that have been harvested this year, we have not had any significant problems with green bean syndrome. What is occurring, however, is that some fields are not drying down as they should, compared to other fields that had essentially the same growing conditions.

What are the causes of these problems? In some cases, it is very difficult to determine what the primary cause is because late in the season several problems manifest themselves.

Yields that have been reported across Louisiana are exceptional thus far. The reasons are twofold. Because of soybean rust, more attention was given to soybeans than ever before, and we had good growing conditions with few insects and little disease until late in the season.

USDA has projected yield for Louisiana soybean production in 2005 to be 35 bushels per acre. If this projected yield estimate holds, it will be a new state record. The late-season beans will determine our final yields. The later-season beans have done very well, but need rain.

Stink bug pressure has been lighter than usual, and Orthene has been a great product to combat this pest. We do want to think about some chemistry rotation next year, especially late in the season when the numbers are fewer.

Cercospora leaf blight, in my opinion, is Louisiana’s number one disease nemesis. It causes more problems for us than any other disease, but shows up late in the season. This year there was much confusion regarding fungicide efficacy and which ones did the job. In a previous article I encouraged growers to ask for help if they needed it. I hope that you will, because I witnessed a great deal of fungicide applications wasted by producers who thought they were doing the right thing.

I have some strong opinions about cercospora leaf blight south of Alexandria. It is bad, and the pressure is severe in most years. This year the pressure happened to be lighter than we are accustomed to. Because of this, Topsin-M or the generic thiophanate-methyl should be used south of Alexandria to suppress this disease.

In north Louisiana, the pressure is less severe, but still there and causing problems. Producers can sometimes get by without the Topsin-M application.

Pod and stem blight was evident in more fields this year than is normal, which also facilitated some late-season seed quality issues.

Regarding defoliation, I am a big advocate of using a desiccant such as paraquat to aid in the defoliation of a soybean crop. Many folks are still confused as to why this helps dry the crop. The herbicide itself has no drying effect on the seed. It desiccates the plant leaves and petioles, allowing more air movement and sunlight penetration in the canopy and increasing the dry-down rate of the crop.

Perhaps the largest benefit of a desiccant at harvest is increased combine efficiency, which is gained by having a more uniformly dried crop and killing the weeds in the field.

On early-planted fields, I suggest that growers budget a paraquat application into their production costs, especially if they are really pushing the crop to be harvested by a specific day.

The number one question that I get with desiccation is: How long do I have to wait to harvest? In my opinion, a four-day minimum is best. I feel that it really gives the crop ample time to turn and allows the wind to knock some of the leaves and petioles off the plant.

After R5 to R6, with the exception of a possible insect spray, irrigation or desiccant, soybeans really do not need much attention. The problems that show up late in the season are those missed during the growing season. After R6, you really start noticing flat pods, disfigured pods, pods not filling, disease incidence and severity increasing.

At this point, there is really nothing that you can do to fix any of these problems. This is why it is so important to stay abreast of the changing needs of the crop during the growing season.

On an unrelated note, we are in full swing of harvesting our verification fields and our demonstrations across the state. In the coming weeks, we will process the data and report it in articles as well as post results online. If you want to access the demonstration data, please log on to and click on the crops and livestock link and then select the appropriate commodity. On the right hand side of your respective commodity choice will be a link for Extension Demonstrations. Click it and all of the data is there categorized by parish.

David Y. Lanclos is the soybean, corn and grain sorghum specialist at LSU AgCenter. e-mail: [email protected]

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