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6/25/2012 - 7/1/2012

Soil Moisture Conditions and Crop Water Use

By Mahdi Al-Kaisi, Elwynn Taylor and Roger Elmore, Department of Agronomy

Soil moisture during the growing season is essential to obtain optimal yield. Optimal yield is affected by the availability of moisture as stored water in the soil profile or timely recharge during rain events. Soil texture, tillage practices, residue cover, drainage and weed control can play a significant role in soil moisture availability. Generally, the no-till system is the most effective practice in conserving soil moisture among other tillage systems, especially during dry periods in rain-fed agricultural areas.

Crops need water to uptake nutrients and most importantly to reduce heat stress through the transpiration process. That is where the plant releases water through the leaf openings called stomata. This is the  plant’s main cooling mechanism.

Evaporation of water from soil and plant surfaces is another source of water loss during the growing season. The two processes together are called evapotranspiration (ET) or crop water use. Crop water use represents both soil evaporation and plant transpiration used for crop  growth and cooling purposes.


Potential ET (PET) or Reference ET  (ETr) vs. Actual ET (ETa)

Actual Evapotranspiration (ETa) or actual crop water use can be estimated or determined if we know potential evapotranspiration (PET) and the correct crop coefficient (Kc) at a certain growth stage for a particular crop. Potential ET - sometimes called reference ETr - is defined as the rate of readily available water removal from wet soil and plant surface under well-watered conditions when moisture availability is not a limiting factor.

To calculate crop water use or ETa:
ETa =Kc x PET
Where, Kc is crop coefficient provided for each crop at different growth stages and PET is potential evapotranspiration provided from weather stations at different growth stages.

The first step in figuring crop water use is to determine PET for the locations of interest. Obtain PET in this way :

  1. Go to
  2. Roll down the page and click on “ISU AgClimate.”
  3. Click on “Request Daily Data” at the lower right.
  4. Choose a  weather station close to your area of interest.
  5. Select Daily Evapotranspiration (PET) (inch). Also, you can select precipitation for comparison. 
  6. Select date: For example, you can use June 1 to June 30, 2012, to obtain your PET values. Separate values by TABS. 
  7. Punch  “Get Data.”

Or to get PET from yesterday, select:

Once you have PET, use Table 1 to estimate corn water use based on the current corn growth stage matched with the present PET.


Table 1. Estimation of corn water use for Iowa based on potential daily evapotranspiration (PET) at different crop developmental stage (Dev. Stage).

† Kc = Iowa crop coefficient for corn estimated by Elwynn Taylor from previous research and experience.

‡  0.55  inch of water loss of Potential ET may result when there is no water limitation or availability concerns.  This high level may not be applicable to Iowa conditions since PET is affected by many factors and it is area specific.  It serves here as a reference point that can be used for calculating crop water use.


For more information on Crop Water Use, see:


The authors are professors in the Department of Agronmy. Mahdi Al-Kaisi, soil management, can be reached at 515-294-8304 or e-mail Elwynn Taylor, ag meterology, can be reached at 515-294-1923 or e-mail Roger Elmore, corn agronomy, can be reached at 515-294-6655 or e-mail  

Managing Corn Residue with Different Tillage Systems and N Applications Field Days

By Jim Fawcett and John Holmes, Extension Field Agronomists, and Greg Walston, Extension Program Director


Iowa State University Extension and Outreach will host two tours of residue management plots in July.

  • Wednesday, July 18, 10 a.m. to noon
    Ben and John Olson Farm,  (5 miles northeast of Van Horne on 21st Avenue Drive; lunch provided by the Benton County Cattlemen)
  • Thursday, July 26, 9 a.m. to noon
    NIACC campus

Dr. Mahdi Al-Kaisi and his graduate student, Lara Schenck, will explain their research looking at different approaches to managing corn residue prior to a soybean crop. Extension field agronomists will be on hand to discuss current crop problems. Local farmers and agronomists are encouraged to attend. Certified crop advisers can earn two soil and water credits by attending.

With greater corn yields and better insect and disease resistance, corn residue levels continue to increase. Farmers have applied liquid nitrogen in the fall to help breakdown corn residue. Dr. Al-Kaisi and Schenck are evaluating several tillage systems and two nitrogen rates to provide tips on managing fall corn stalk residue. During the field days, they will explain the treatments and share preliminary observations. 

The Iowa Soybean Association Research Committee has funded this research to help farmers learn how to manage corn residue more efficiently. In Benton County, Extension Program Director Greg Walston and local FFA students have helped collect emergence data and assisted with sampling and water infiltrations. NIACC agriculture classes have helped do tillage, apply treatments and collect information. 

For more information, contact your local ISU Extension and Outreach office. 


Jim Fawcett is an ISU Extension and Outreach field agronomist. He can be reached at (319) 337-2145 or e-mail John Holmes is an ISU Extension and Outreach field agronomist. He can be reached at 515-532-3453 or e-mail Greg Walston is a program director for ISU Extension and Outreach in Benton County. He can be reached at 319-472-4739 or e-mail

Tips for Diagnosing Goss's Wilt and Leaf Blight

By Alison Robertson, Department of Plant Pathology and Microbiology


In the past 5 years, Goss’s wilt and leaf blight has become increasingly prevalent in Iowa. This growing season, the disease has already been reported from several fields. Consequently many agronomists and scouts have been checking fields and some agronomists have been making use of the immunostrip test from AgDia.


What symptoms should you be looking for?

Freckles! Dark spots that resemble freckles and occur in the outside edge of a developing lesion are the most characteristic symptom of Goss’s (Figure 1).

Figure 1. Freckles are diagnostic for Goss's wilt and leaf blight.


If you see freckles, you can be confident you’re looking at this disease. Freckles may be confused with black colonies of saprophytic fungi that colonize dead leaf tissue. A neat trick we have learned is that if you hold a suspected lesion to the light, freckles appear transparent (Figure 2), while colonies of saprophytic fungi are dark (Figure 3). On systemically infected wilted plants, freckles are also evident on the leaves.

Figure 2. When backlit, freckles appear transparent.


Figure 3. Colonies of saprophytic fungi remain dark when backlit.


Other characteristics of Goss’s leaf blight are very large red to grey lesions that usually start from the leaf tip or leaf margins and extend down the leaf (Figure 4). The lesions are usually a blend of dead tissue, grey-green water soaked tissue and yellowed tissue. You may also notice the lesions are sticky and have shiny patches on the surface. This is the Goss’s bacterium that has oozed out of the diseased tissue and dried on the surface of the leaf.

Figure 4. Lesions of Goss's leaf blight are large and usually start from the leaf tip and extend downward.


What if I get a positive with an immunostrip test?

The immunostrip test is a useful test to confirm Goss’s, but it must be used carefully. The test does give false positives. We know, for example, that purple leaf sheath (Figure 5) will give a positive with the strip test from AgDia. Purple blotches on the leaf sheath of corn plants are not disease, but are caused by saprophytic organisms feeding on dust, pollen, etc., that has collected behind the leaf sheath.

Figure 5. Characteristic symptoms of purple leaf sheath.


Other tips

Other evidence to consider for a positive Goss’s diagnosis in addition to the leaf symptoms and the immunostrip test include:

  • Hybrid – Is the hybrid rated susceptible to Goss’s?
  • Field – Does the field have a history of Goss’s ? Was corn grown in the field in the previous year and is surface residue present?
  • Weather – Has severe weather occurred in the area within the past few weeks?

As with all diseases, correct diagnosis is important to enable appropriate management practices to be followed. For Goss’s, tolerant hybrids is by far the best management tool we have at present. There are numerous foliar applied products being marketed for Goss’s management. Several field trials are being done here in Iowa, and also neighboring states, to evaluate the efficacy of these products.


Alison Robertson is an associate professor of plant pathology and microbiology with extension and research responsibilities. You can reach her at 515-294-6708 or e-mail

New Fungicide Questions Creeping into the Conversation

By Daren Mueller and Alison Robertson, Department of Plant Pathology and Microbiology

Across Iowa many farmers and agronomists are starting to ask if or when to apply fungicide on corn and soybeans. Like the past few years, there have been very little early season foliar problems on both crops. In soybean, frogeye leaf spot, Cercospora leaf blight, and brown spot have been reported on seedlings. Brown spot is really the only foliar disease still being reported, although soybeans usually grow out of any early season infection. Likewise in corn, very few foliar diseases have been found prior to tasseling.

We have written several previous articles, including 2011 summaries for corn and soybean, fungicide application timing for soybean and a summary of responses on corn. This year we have had questions about spraying fungicides on drought stressed crops and applying half rates of fungicides. We will address both in this article.

The greatest responses to foliar fungicides we have seen over the past five years in Iowa have been when disease could impact yield of corn or soybean. Similar data have been reported from the north central region and even nationally.

We have found however, that when applying foliar fungicides in absence of significant disease, selecting the right product may make a small difference. In a four-year study funded through the Iowa Soybean Association and soybean checkoff involving hundreds of treatments on soybean across more than 20 total locations, we found that products containing the strobilurin fungicide had a greater response in the absence of disease (Figure 1). When disease was present, there were no differences between products containing a strobilurin, a triazole, premixes of a strobilurin and a triazole, or a carboximide.

For drought-stressed corn and soybean crops, there is limited scientifically valid data indicating if foliar fungicides will improve water use efficiency and increase yields. One published report on gravevine shows that strobilurins have no effect on water use efficiency (Diaz-Espejo, 2012). Considering the biology of pathogens and the plants, drought stress will reduce the chances of foliar fungal pathogens becoming established. Drought-stressed plants will also have lower yield potential. Both of these facts give us pause in recommending fungicides for drought-stressed crops.

Figure 1. Yield response of soybeans in Iowa to fungicides in different classes from 2008-2011. 


Resistance to fungicides

A caution when spraying fungicides – scout fields after applying the fungicides and look for applications that do not appear to affect disease severity. Resistance to the strobilurin fungicides (Headline ®, Quadris ®, one of two active ingredients in Stratego YLD ® and Quilt ®) in the fungus that causes frogeye leaf spot was reported in Tennessee, Kentucky and Illinois in 2010. Resistant strains of the fungus were again found in 2011 in the same states and also Missouri. Reduced sensitivity to strobilurin fungicides has been reported for more than 30 pathogens and is often detected within a few years of the product being used on a crop.

There is a reason why chemical companies recommend the rates that are on labels. Research indicates that the recommended rate is most effective against targeted pathogens, but risk management also is strongly considered.

This leads to our second topic – half rate of fungicides. When half rates of fungicides are used, this increases the chances of fungi being able to survive these applications, and consequently increases the chances of fungicide-resistant strains of the fungus developing and becoming established. We encourage all farmers and agronomists to follow label rates to ensure that these highly effective disease management tools will be around for a long time.



Diaz-Espejo, A., Cuevas, M.V., Ribas-Carbo, M., Flexas, J., Martorell, S., Fernandez, J.E. 2012. The effect of strobilurins on leaf gas exchange, water use efficiency and ABA content in grapevine under field conditions. J Plant Physiol. 169(4):379-86.


Daren Mueller is an assistant professor in the Department of Plant Pathology and Microbiology. He can be reached at 515-460-8000 or e-mail Alison Robertson is an associate professor with extension and research responsibilities in the Department of Plant Pathology and Microbiology. She can be reached at 515-294-6708 or e-mail

Western Bean Cutworm Scouting Update 2012

By Adam Sisson, Integrated Pest Management; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology

Western bean cutworm (WBC) moths have been reported in several Iowa counties. The first reported moth of 2012 was captured in Benton County, located in the east central part of the state, on June 18th.  With the higher than average accumulated heat units, moths may have emerged prior to this date, before traps were in place.

Trap data by participating counties can be viewed by the public at the North Central (NC) ipmPIPE webpage. At the NC ipmPIPE webpage, click on a highlighted county to see the number of recorded moths in that county. If captures occur on consecutive days in a trap and moth numbers are increasing, this is the signal to begin scouting at a location. Mills County, in the southwest part of the state, experienced this around June 20th. The presence of adult moths in traps indicates only that scouting efforts should begin in an area.

Adult emergence can also be predicted using a degree day (DD) model developed in Nebraska. This DD model is based on the accumulation of DD (base 50°F) from May 1. Scouting should begin at 25 percent adult emergence, which is predicted at 1,319 DD. Fifty percent adult emergence (peak) is predicted at 1,422 DD, and scouting should continue for 7 to 10 days afterwards. The map (Figure 1) shows the predicted dates of approximately 25 and 50 percent adult emergence based on the DD model.

Figure 1. May 1 –21 accumulated DDs (base 50 F) and normals are used to predict when approximately 25 percent (top) and 50 percent adult western bean cutworm adult emergence occur.


When scouting for WBC, examine 20 successive plants in five different areas of a field. On these plants, check for the presence of eggs or young larvae (Figures 2, 3) on the top three to four leaves. Management options and descriptions of WBC are outlined in a previous ICM News article, Use Treatment Thresholds for Western Bean Cutworm.

For field corn, if 5 to 8 percent of plants have eggs or larvae, an insecticide treatment may be warranted. For sweet corn, the threshold is reduced to 4 percent for the processing market and 1 percent for the fresh market. Insecticide application must be timed correctly, before larvae enter the ear to feed. The suggested application timing is 90 to 95 percent tassel emergence, or 70 to 90 percent hatch if tassels have extended.

Figure 2. Western bean cutworm eggs. Frank Peairs, Colorado State University,


Figure 3. Western bean cutworm larvae that have just emerged. Frank Peairs, Colorado State University,


Adam Sisson, Integrated Pest Management, can be reached at 515-294-5899 or e-mail  Laura Jesse, Plant and Insect Diagnostic Clinic, can be reached at 515-294-0581 or e-mail Erin Hodgson, Department of Entomology, can be reached at 515-294-2847 or e-mail

This article was published originally on 7/2/2012 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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