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June 7, 2012
 

CORN

 

Stress-Induced K Deficiency in Corn

If there is one corn problem I have looked at most in the past 25 years in eastern Iowa, it is what we’ve come to call “stress-induced K deficiency.” Usually the fields look fine until the corn reaches the rapid-growth phase, then corn stops growing and the lower leaves turn yellow along the leaf edges. Good corn can be right next to extremely stunted corn with no apparent reason for it. The corn in these areas remain stunted and the lower leaves remain yellow, but they usually yield better than expected. There is usually no pattern to the problem, except in most fields the end-rows look better than the rest of the field. We used to call the problem “drought-induced K deficiency” because it was most widespread when we had hot, dry weather in the spring.  

 

Nodal roots emerge in the top inch of the soil. In my opinion, we see this problem when there is something in the surface inch of the soil which the nodal roots do not “like” at the time they are first developing soon after the emergence of the corn. If the surface inch is very dry, this leads to roots that do not function properly. We have certainly had those hot, dry conditions this spring and are now starting to see problems with K deficiency symptoms in corn in the area.      

 

When I look at corn fields showing K-deficiency symptoms, I always look at the end-rows first. If they look good and the rest of the field has areas that show the deficiency symptoms, it tells me that the problem is at least partly environmental. It is still a good idea to check the soil test levels of K, because the problem is going to tend to show up more in soils with low K levels. I think the end-rows may look better because of the different traffic pattern in the field causing the soil structure to be different. In this case a little surface soil compaction may actually alleviate the problem. I have noticed that many times the soil seems a little more firm in areas where the corn looks better. This is especially true on tilled ground where fluffy, lose soil tends to dry out more than firm soil.  

 

The picture shows the pattern where the end-rows look fine and the stunted yellow corn begins just inside of the end-rows. In this field the end-rows were actually a different hybrid, but that is another thing that often is seen is that there can be large hybrid differences, with some showing extreme stunting and others showing no symptoms.  Anything that restricts root growth during the initiation of the nodal root system can lead to the problem. Planting and/or soil settling or eroding after planting aggravates the problem. In many fields it is difficult to come up with an explanation why the problem is appearing. Antonio Mallarino will have an article in the ICM News regarding K deficiency symptoms in corn http://www.extension.iastate.edu/CropNews/.

 

 

 

 

Black Cutworms Still Causing Some Stand Losses

 

We need to continue scouting for black cutworms until the corn reaches V5. Usually the cutworms can no longer cut through the corn after that stage. Most of the damage now is from cutworms boring into the plants below the ground. As long as it stays dry, the cutworms will do their damage underground. Insecticides will have a limited impact on the cutworms until we get a rain to bring them above ground again. Rotary hoeing after a Lorsban application may improve the control under dry conditions, but the synthetic pyrethoids should not be incorporated.

 

SOYBEANS

 

Two-spotted Spider Mites

 

There have been several reports of two-spotted spider mites in soybean in the Highway 34 area. Usually rain promotes a fungal disease that destroys spider mite populations; hopefully the rain of last week end has that effect, but rainfall events of short duration followed by low humidity often do not. For details on scouting for and managing this pest go to http://www.extension.iastate.edu/Pages/eccrops/spidermite.html or pages 148 - 149 of the July 22, 2002 Integrated Crop Management Newsletter. If treatment for this pest is necessary, organophosphates (Dimethoate 400 or Lorsban 4E) are the most effective products. Dimethoate 400 will have less residual activity. If re-treatment is necessary, most, if not all, of the chlorpyrifos product (Lorsban and “generics”) labels state to not use chlorpyrifos twice in a row. However, chlorpyrifos and dimethoate are both Group 1B insecticides, so the benefit of rotating between those products is questionable. The Group 3A insecticides (pyrethroids) generally have little effect on spider mites and can actually cause spider mite populations to flare by killing predatory insects. The exception is products that contain bifenthrin, such as Bifenture EC, Brigade 2EC, Discipline 2EC, Fanfare 2EC, Hero, Sniper, and Tundra EC; these pyrethroids appear to have good activity against two-spotted spider mites when applied at the rate stated on the label for this pest.

 

Checking Soybean Stands

 

With last week’s rain, I would expect most soybeans that are going to emerge to be emerged in the next 2-3 days. Some soybeans emerged soon after planting, some seeds swelled but didn’t emerge, and some were in dry soil until last Thursday’s rain. The 0.5-1” of rain should be enough to allow most of the still viable seed (seed that hadn’t swelled) to germinate and emerge, so in the next few days would be the time to check stands to see if they are adequate. Fortunately soybeans can compensate for reduced stands, and little yield loss is usually seen even with half a stand. In a trial at Crawfordsville last year, we didn’t see any yield loss even with a harvest population of 35,000 plants per acre. I’m not quite ready to call 35,000 a keeper stand, but I usually don’t recommend re-planting when stands are around 50,000-60,000 or more in June.

 

A popular practice with reduced soybean stands is to try to “thicken up” the stand by planting or drilling some additional seed into reduced stands. The same study at Crawfordsville last year showed that planting additional seed into reduced stands of soybeans at VC (unifoliate emerging) and V2 (2nd trifoliate emerging) did not hurt anything, but also did not improve the yield. To see other results from this study see http://www.ag.iastate.edu/farms/11reports/Southeast/SoybeanReplantStudy.pdf. The later it gets, the more likely that the beans planted into existing stands may not contribute to the yield and may just become weeds to the soybeans already growing in the field. Many times the best decision is to accept the reduced stand, but make sure to control the weeds. Average yields with mid-June planted soybeans are about 60% of the yield of early May planted soybeans in central and northern Iowa. In southern Iowa the average yield reduction is closer to 20% for mid-June planted soybeans, however there is a much greater variability in yield the later the planting is. The following table can help in determining soybean stands:

 

APPROXIMATE NUMBER OF PLANTS PER               

FOOT OF ROW TO GIVE VARIOUS POPULATIONS

PER ACRE

                                      Population

Row Width

150,000

125,000

100,000

75,000

50,000

36 - 38

10.6

8.8

7.1

5.3

3.5

30

8.6

7.2

5.7

4.3

2.9

20

5.7

4.8

3.8

2.9

1.9

15

4.3

3.6

2.9

2.2

1.4

10

2.9

2.4

1.9

1.4

1.0

7

2.0

1.7

1.3

1.0

0.7

 

 

 

FOR YOUR CALENDAR

SOUTHEAST IOWA RESEARCH and DEMONSTRATION FARM, Crawfordsville

SPRING FIELD DAY and 25th Anniversary Celebration (afternoon) &

SPECIAL SESSION FOR CCAs (morning)

JUNE 21, 2012

 

CCAs can obtain 5 hours of credit (3.5 in Soil & Water) by attending the morning CCA session at 9:00 a.m., followed by the noon lunch and afternoon celebration. Attendance for the morning CCA session is limited to 45. Pre-Registration is Required (No Walk-ins). The Fee For CCAs is $50. Please send me an e-mail note by June 20 if you plan to attend (fawcett@iastate.edu).

 

Details are be posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.

 

NORTHEAST IOWA RESEARCH and DEMONSTRATION FARM, Nashua

SPRING FIELD DAY

JUNE 28, 2012, 1 – 4:30 p.m.

 

Details are posted at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.

 

TECHNIQUES and TECHNOLOGIES TO AVOID SPRAY DRIFT

July 17, 2012, Field Extension Education Laboratory (FEEL) near Boone, IA

 

Two half-day sessions (morning session repeated in the afternoon) (no cost to participants) will be conducted, focusing on:

- Nozzle selection/use with demonstration on spray table

- Balancing efficacy and drift

- Environmental factors, adjuvants and limitations, field demonstration, etc.

More information and registration will soon be available at http://www.aep.iastate.edu/.

 

Managing Corn Residue for Soybeans Planted With Different Tillage Systems

July 18 10:00 a.m.

 

Ben & John Olson Farm – about 5 miles NE of Van Horne on 21 Ave. Dr.

Free Lunch & 1.5 hours of CCA credit (S&W)

More Details Later

 

 

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

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Last Update: June 7, 2012
Contact: Jim Fawcett fawcett@iastate.edu


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