Crops Bulletin
July 17, 2013
Issue 8

Prepared by

Paul Kassel

Extension Field Agronomist


Serving Clay, Buena Vista, Dickinson, Emmet, Hancock, Kossuth, Palo Alto, Pocahontas, Sac and Winnebago Counties


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Waterhemp: Out of control waterhemp has dominated many conversations this week. Many waterhemp plants appear to be glyphosate resistant - either by biological resistance or glyphosate uptake issues.


Herbicide options are limited. The diphenyl ether (DPE) products like Cobra, Flexstar and Ultra Blazer may help. These products burn the leaves off of susceptible weeds. The DPE products work better when weeds are small and when the leaf burning action can affect all weed leaves. However susceptible weeds can usually recover when weeds are large and they have lots of growing points.


Soybean plants will respond when the DPEs are applied mid-season. This crop injury is usually not serious but does reduce the ability of the soybean plant to compete/shade out the remaining weeds.


Fungicide Use on Corn: The conservative approach to fungicide use on corn is usually recommended in the following situations.

  •  Corn following corn rotation.
  •  Disease susceptible hybrids.
  •  Leaf diseases  - gray leaf spot, common   rust,  eyespot - are present.      

Yield benefits have been obtained from fungicide application on corn when the above factors have not been present. However, predicting this yield response without the presence of disease is more difficult. Hybrid response, weather conditions and yield potential are all factors - but are difficult to measure and predict. However, fields that are uniform in growth and development may be good candidates for fungicide treatments if yield potential looks good at the R1-R2 stage.


Fungicide Use on Soybean: Fungicide applications for soybean should be applied at the R3 stage  (3/16 inch pod at one of the four uppermost nodes). Foliar diseases of soybean are at low levels this year.   However, fungicide applications to soybean have increased soybean yields when foliar disease levels are low. Research by ISU in northwest Iowa has shown a fairly consistent 2-3 bu/a yield benefit with fungicide applications on soybean.


Asian Soybean Rust (ASR): There have been reports of ASR in Mississippi, Arkansas and Florida this season. Some of these reports are earlier than normal. Soybean planting in the southern states - especially in doubled cropped soybean - has been delayed. Therefore, there is legitimate concern about ASR in the southern states. It should not be a concern for Iowa (again) unless the disease builds up quickly over the next month.


Prepared by Paul Kassel, Extension Field Agronomist

Phone: (712) 262-2264, Email: