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Steps to protect crop: Late-season disease harms soybeans

USDA's October crop report predicted Mississippi's state average soybean yield would be 36 bushels per acre. That would be our best ever, but I believe we have done even better — a 38- to 40-bushel state average.

Preliminary yield information is available on the Web at We are sorting through yield information to make varietal decisions for next year. We plan to make all our varietal decisions by Dec. 1 for our verification fields.

We want to complete this task so producers participating in our verification program can secure seed for the upcoming growing season. The top performing varieties will be in high demand.

Several disease concerns — particularly late-season cercospora and stem canker — surfaced this year.

Late-season cercospora seems to have increased the last couple of years, but that doesn't mean it will be a problem every year, nor has it been a major problem in the past. It was more evident this year than ever before.

Varietal resistance to late-season cercospora is not what it used to be. Many of the varieties we grow today seem to be more susceptible than we'd like. Keep in mind, however, this lack of resistance has allowed us to achieve yield levels most only dreamed about in years past.

Late-season cercospora has not been a traditional major yield-robber, but the lack of resistance and the reduction of some other diseases may be allowing it to get a better foothold.

Quadris will not control late-season cercospora at the rates we are using in soybeans. We have not tried higher rates, but cercospora appears to be one of Quadris' weaknesses.

Some have described Quadris as a cercospora fertilizer. If you are not certain you have late-season cercospora, you will have no doubt about its presence following a Quadris application. It appears that as other diseases, such as frogeye leafspot, anthracnose, and pod and stem blight, are reduced, the severity of cercospora increases — probably because the competition is reduced.

We are currently evaluating other products and tank mixes with Quadris, and I hope we can get some additional compounds labeled in the near future to help with control.

Look for varieties that have resistance or tolerance to late-season cercospora. Continue to utilize early-maturing varieties and the early-planting system. Given this disease's inconsistency, it will behoove you to pick the best variety for your farm, regardless of resistance.

Manage the crop to the best of your ability and scout for this problem. I admit it was quite widespread this year, but that does not mean it will occur next year, nor has this disease been consistent in comparison to other diseases.

An additional consideration regarding the increased incidence of foliar disease — in particular late-season cercospora — is plant health.

Under scenarios of high fertility levels, the effects of various diseases can be minimized. This is not control, but a healthy plant is less susceptible or better able to combat disease pressure.

I have no doubt that many fields over the last few years would have experienced less yield loss if higher levels of phosphorus and potassium, mainly potassium, were available.

Several years ago Fred Jones and Normie Buehring conducted some research looking at the effects of varying fertility levels and foliar fungicides and their effects on yields. The amazing thing was where potash levels were high they observed less late-season cercospora versus medium levels.

In addition, they were able to pick up 6- to 7-bushel yield differences when the plants exhibited no fertility deficiency symptoms. (These differences were only detected through tissue testing.) Plants grown in a high fertility environment were overall healthier and produced higher yields, even though visual deficiencies were not observed between high and medium testing soils.

Last year in our SMART program, over half the fields sampled needed a maintenance application of phosphorus or potassium or both. Fertility isn't a major problem, but after years of continuous cropping and the removal of several big crops, we need to give this area some attention.

We often find it is not whole fields, but areas in a field — so smaller sampling size and variable rate applications will play a major role in correcting problems. We have been blessed with a wonderful fall so if you have not yet pulled soil samples, please take time to do so. In the next article, I will discuss one of the most devastating diseases we have, stem canker.

Alan Blaine is the Mississippi Extension soybean specialist. e-mail:

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