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Disease part of green bean problem

In a previous article, I discussed the impact stinkbugs can have regarding soybean plants remaining green at harvest.

This phenomenon is probably a combination of factors. In addition to stinkbugs, aerial blight, a high incidence of late-season cercospora, the use of foliar fungicides, and high numbers of three-cornered alfalfa hoppers were all observed this season.

We scout fields in our SMART program weekly in an attempt to head off potential problems. Once we start picking up insects or diseases and during peak irrigation, we try to scout twice a week.

This year it was obvious that with aerial blight and stinkbugs, once-a-week scouting was not enough. In several fields that were pest-free one week, the next week thresholds were exceeded and needed spraying as soon as possible.

Aerial blight — a disease that will affect seed set — thrives under hot, humid conditions. It is worse behind rice because it is caused by the same organism that causes sheath blight in rice. We did not have a widespread problem with aerial blight this year, but it did show up.

It is most damaging as the plant begins to fruit. It causes abortion of blooms and small pods. Once pods become fully elongated, they seem to be more tolerant to aerial blight. Under hot, humid conditions, aerial blight will cause the plant to continue to abort fruit as the disease moves up the plant.

Unfortunately, aerial blight usually is not observed until affected leaves are seen in the top of the canopy. Then it is too late to implement management practices.

Begin scouting fields at bloom to help avoid this problem. Look for a web-like mass in the lower canopy and dried up or aborted fruit.

The risk is higher if conditions are hot and humid at bloom initiation. Any break in these conditions (temperature or humidity) will slow or halt the disease.

Aerial blight can be controlled through variety selection or with the use of a foliar fungicide. A product that offers good protection is Quadris. The effectiveness of Quadris for aerial blight prevention is the reason we applied for an emergency use label in 2000.

Louisiana often experiences a condition known as “green bean syndrome.” This is not what occurred in Mississippi this year. Fields that remained green had some leaves and green stems, but pods matured. Most fields had an excellent yield potential, but due to the green appearance many went unharvested for an extended period of time.

Fields that are affected by green bean syndrome remain green and have little fruit on the plant. Anything that affects fruit set: aerial blight, stinkbugs, anthracnose, pod and stem blight, and three-cornered alfalfa hoppers can contribute to the problem.

Late-season cercospora has also been identified in fields where pods matured and plants retained green leaves and stems. Physiologically these plants were mixed up. Cercospora was observed on mature pods after leaf drop. The disease was heavy enough that the plant perceived the seed in these affected pods to be nonviable.

Plants remained green in an attempt to compensate for the perceived fruit loss. These seed were mature, but physiologically the plant was fooled into thinking no seed were present due to the high incidence of cercospora on the pods. The green condition is somewhat similar to cotton taking on second growth. Even with good yields, this condition greatly slows harvesting.

There are numerous variables which can not be controlled, but with timely scouting many can be controlled.

High yields can be obtained on your farm, but it may require different management strategies from year to year. Focus on those things you can control. For instance, scouting fields regularly and making timely management discussions.

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

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