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East-Central and Southeast Iowa Crop Information

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8/11/2010

SOYBEAN

INSECTS

The third generation of green cloverworm is now showing up in some soybean fields. In the past the cloverworms that hatch in August have not been very destructive because they are killed early by disease and predators, so hopefully that will be the case this year also. Now that most soybeans are in the R5 stage (beginning seed), it may pay to spray an insecticide if 10% defoliation is reached. According to hale charts, 10% defoliation at R5 can result in a 4% yield loss, which would be 2 bu/A for 50 bushel beans. Japanese beetle numbers appear to still be lower than past years, but fields that have both Japanese beetles and green cloverworms may reach the 10% defoliation threshold

Soybean aphids are still at low levels in the area. Brian Lang has reported that some fields in NE Iowa have now exceeded the 250 aphid/plant threshold, although the soybeans are nearing the R5.5 stage when usually insecticide treatments do not pay.

DISEASES

 

Soybean Sudden Death Syndrome

 

            Sudden Death Syndrome (SDS)  is widespread again this year and now easy to see from the road in many fields. The typical leaf symptoms are that the leaf veins say green and the tissue between the veins turns yellow and brown:

 

The leaf symptoms of SDS are similar to the leaf symptoms of brown stem rot. To distinguish between the two, split the lower stem longitudinally. The center (pith) will be brown or rotted out with brown stem rot and will still be white with SDS. SDS first shows up anywhere there is poor drainage or surface compaction, so traffic patterns, especially along field edges, are easy to see. The fungus infects the soybeans soon after germination, even though the injury symptoms do not appear until late July or August. Cool, wet soils, which were widespread in May, favor the infection. In most years the earliest planted beans to have more problems with SDS, because the soils tend to be cooler and wetter early in the spring. Fields with a history of SDS should be planted last. Also tile drainage can reduce the disease incidence. Usually fields with SDS also have soybean cyst nematode. If the field has not been tested for nematodes, send soil samples in this fall and use nematode resistant varieties if the disease is confirmed. Planting nematode  resistant beans will not eliminate SDS, but may reduce the severity of the disease. There are no varieties totally  resistant to SDS, but there are differences among varieties in susceptibility, so variety selection can help manage the problem in the future. Hopefully some day we will have a seed treatment for SDS, but none are available yet.

 

Frogeye Leafspot

 

            Frogeye leafspot is also fairly widespread in the area. It causes circular gray spots surrounded by a reddish-brown border on the middle to upper leaves in the soybean canopy. The disease usually doesn’t show up until early August. Before soybean rust was present in the country, this was one of the primary reasons that soybeans were sprayed with a fungicide in the southern states. There may be some fields that could still benefit from a fungicide application to prevent the disease, although it is likely too late for most fields. In severe cases it can cause early leaf drop and premature death. It can also cause seed quality problems by turning the seedcoat gray on infected seed. Since it can be seedborne, the soybeans should not be saved for seed. The picture below shows frogeye leafspot, as well as a couple of Japanese beetles that wanted in the picture.

 

 

White Mold

 

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I have yet to receive a call about or be in a field with White Mold, although Brian Lang recently reported it is now common in NE Iowa.  While the evidence of the infections may show up at this time of year, the infections took place shortly after the beginning of flowering in late June.  The infection itself is no longer spreading, but the evidence of the infection gives the appearance of the disease spreading as more plants show the symptoms of the disease. There most likely will be little positive effect of a fungicide application at this time because of the advanced progress of the disease.

 

The most important thing for growers to do at this time is to note the presence of white mold in the field and then select for varieties with lower susceptibility or higher tolerance for white mold the next time soybeans are grown in the field. One thing to consider in fields with a history of white mold is to introduce the natural pesticide “Contans”, which is a pathogen of the white mold fungus. It has provided some benefit in other crops with white mold problems. It is best sprayed in the fall. The recommended rate is 1-4 lb/A. It won’t be a quick fix for the problem, but can help to reduce the level of fungus down to a more manageable level.

 

 

 

 

 

 

 

 

 

 

CORN

 

Tipping Back and Premature Denting

 

Some corn is showing considerable “tipping back.”  (If there is no “tipping back,” that usually means that a higher population would have resulted in higher yield.) Some corn is also already showing denting of the kernels, but the kernels are only in the early dough stage.

Once the ears reach the R2 (“blister”) stage of development, if the plant is stressed and starts to “become less optimistic,” kernel abortion will occur from the tip back.  Kernel abortion will continue to the end of R3 (“milk”) if stress is present. Once the plants are into the R4 (“dough”) stage of development, the only adjustment the plant can make is to the kernel size; the early denting is an indicator that the plants are continuing to be “less optimistic” and are adjusting kernel size down, which will result in lighter kernels at harvest.

What is causing this stress?  It may be the heat, but if the plants are running out of nutrients (nitrogen?) that is a definite possibility.  However, if roots are very poor, rotted, or damaged by insects, that, too, can cause the plants to experience lack of water and/or nutrients and cause the plants to “retrench.”  In a different year, drought could also cause this. Yields can still be very good in fields with tipping back and premature denting, but the top of the yield has been lost.

 

 

Late Glyphosate Applications and Pollination Problems

 

            There have been some reports of late applications of glyphosate on RR corn (usually on corn taller than 4 feet) leading to some pollination problems. There are scattered missing kernels and what some have called “bubble” kernels – kernels that pollinate but do not develop. Hopefully this is not a widespread problem, but it is something we may hear more about as the combines get into the field.

 

FOR YOUR CALENDAR

 

Northeast Iowa Research and Demonstration Farm, Nashua

Fall Field Day on August 26, 2010, 1:00-4:00 p.m.

 

Program will include Grain Marketing by Chad Hart; Corn Growth & Development and Yield Prediction Methods by Roger Elmore; Combine Settings to Reduce Harvest Loss by Greg Brenneman; A “Hot Topic” Session on ISU research farm results with Ken Pecinovsky, Brian Lang and Greg Brenneman on topics including narrow-row corn, land rolling soybeans, strip tillage, and other topics.  The Field Day is free and open to the public.  Pre-registration is not required.  3 CCA credits will be available for $20.  For questions, contact Brian Lang at 563-382-2949.

 

Southeast Iowa Research and Demonstration Farm, Crawfordsville

Fall Field Day on September 15, 2010

 

Tentatively there will be a manure application field day in the morning followed by a “more traditional” field day in the afternoon.  More details will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.

 

If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: August 11, 2010
Contact: Jim Fawcett fawcett@iastate.edu


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