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6/4/2012 - 6/10/2012

New Approach to Assessing Possible Nematode Damage to Corn

By Greg Tylka, Department of Plant Pathology and Microbiology


Many different species of plant-parasitic nematodes occur in cornfields throughout the Midwest. Most of these microscopic, parasitic worms must reach a damage threshold number before they cause yield loss in corn. The damage threshold numbers for most nematodes that feed on corn in Iowa (see Common Corn Nematode Characteristics, ICM News, Aug. 3, 2009) are 100 or more worms per 100 cm3 , which is a little less than a half-cup of soil. The exceptions are the needle and sting nematodes, which damage corn even if only one or two worms per 100 cm3 soil are present. Fortunately, needle and sting nematodes only occur in soil with at least 70 percent sand and, therefore, do not occur in most Iowa fields.


A more comprehensive approach to addressing a complicated situation

Nematode damage threshold numbers currently used for corn were established in the 1980s. There are no combined or cumulative damage threshold numbers for cornfields infested with multiple nematode species.  Damage threshold can vary depending on conditions in the field – things such as soil texture, cropping history, tillage, and other factors.  Using a single number as a damage threshold for individual nematode species is an oversimplified approach to assessing the possibility of yield loss due to nematodes feeding on corn.

In September 2011, industry personnel hosted a meeting with university personnel to discuss and develop alternatives to the current single-species damage thresholds for nematodes that feed on corn.  Nematologists, plant diagnosticians and agronomists from Iowa State University, University of Nebraska, Kansas State University, University of Missouri, University of Arkansas, University of Wisconsin, Southern Illinois University, Purdue University, and Ohio State University were involved in the discussions. 

Standard recommendations for collecting samples to diagnose possible nematode damage to corn were developed, and those recommendations were presented in Stunted, Yellowing or Wilted Corn: Could Nematodes Be the Cause?, ICM News, May 29, 2012. Also, a comprehensive approach was developed for assessing the risk of damage to corn caused by multiple species of nematodes, as described below.


Considering combined nematode damage and risk factors of fields

A value called the “total nematode damage risk index” is used to assess the potential damage from all plant-parasitic nematodes that feed on corn that are identified in a sample.  A “site sensitivity index” is calculated using background information provided about the field from which the sample was collected. Factors that determine if a field is more or less vulnerable to nematode damage on corn include:

  • number of years that corn has been grown
  • predominate soil texture
  • availability of irrigation
  • use of conservation tillage
  • occurrence of stand establishment and/or compaction problems

The “total nematode damage risk index” and the “site sensitivity index” values are used to assess overall likelihood or risk of damage from nematodes feeding on corn.


New ISU Plant and Insect Diagnostic Clinic Nematode Sample Submission Form

The Iowa State University Plant and Insect Diagnostic Clinic analyzes soil and root samples for plant-parasitic nematodes that feed on corn. In order to allow clinic personnel to calculate a “site sensitivity index” for nematode samples from cornfields, the Plant Nematode Sample Submission Form (PD 32) was recently revised. The lower half of the form (see figure) asks questions about the site sensitivity factors mentioned above for the fields from which the samples were collected. Persons submitting samples for nematode testing from cornfields should provide complete information in this section of the form in order to facilitate an accurate assessment of the risk.

Section of the new ISU Plant Nematode Sample Submission Form requesting information about site sensitivity factors for nematodes that feed on corn.


Greg Tylka is a professor with extension and research responsibilities in management of plant-parasitic nematode in the Department of Plant Pathology and Microbiology at Iowa State University. He can be reached at or 515-294-3021.

Early Confirmation of Twospotted Spider Mite

By Adam Varenhorst, Matt O'Neal and Erin Hodgson, Department of Entomology


Twospotted spider mite is an occasional pest of both corn and soybean in Iowa that is exacerbated by dry conditions. Most parts of Iowa are considered abnormally dry right now, according to the National Drought Mitigation Center in Lincoln, Neb. Soil moisture conditions this spring are similar to the last drought that hit Iowa in 1988; the same year Iowa experienced a statewide outbreak of spider mites.

Spider mites generally reach economically damaging levels in late July or early August when conditions are favorable for their growth. However, twospotted spider mite can start building populations in June during years with early-season temperatures greater than 85°F, humidity less than 90 percent, and low moisture levels. These are ideal conditions for the twospotted spider mite, and populations are capable of increasing very rapidly. Twospotted spider mites have been reported by several crop consultants in southwestern Iowa this week.

A hand magnifying lens is recommended to scout for the minute (< 1/60 inches long) twospotted spider mites. They can be mistaken for specks of dirt to the naked eye (Photo 1). Twospotted spider mite larvae have six legs and nymphs and adults have eight legs. Mites can be removed from collected leaves by shaking the leaves onto a white piece of paper and then looking for moving mites. Twospotted spider mites are typically a cream or green color when feeding on corn or soybean. They can also be an orange to red color when conditions are unfavorable for their growth.

Photo 1. Twospotted spider mites have two dark spots on the top of the body, regardless of body color. Photo by Frank Peairs, Colorado State University.


Twospotted spider mites begin feeding on the bottom of the plant and move to the top as the plant’s health deteriorates. Although they lack wings, twospotted spider mites disperse with the wind to move from dying plants to areas with healthy plants. Therefore, it is important to scout healthy areas of an infested field that are downwind from damaged areas. Early symptoms of twospotted spider mite damage will appear as small yellow dots or stipples on the lower leaves of the plants. Prolonged feeding will cause the infested leaves to turn completely yellow, then brown, and eventually the leaf will die and fall from the plant. The webbing is visible on the edges and underside of leaves and is an indication of prolonged colony feeding (Photo 2).

Photo 2. Plants heavily infested with spider mites are typically covered in webbing. Photo by David Cappaert.


There are not established economic thresholds for twospotted spider mites in corn and soybean, but scouting for initial infestations is very important to avoid yield loss. Twospotted spider mite is capable of reducing soybean yield by 40 to 60 percent when left untreated; drought-stressed plants could experience even more yield loss.

Organophosphates are the recommended insecticidal chemistry for twospotted spider mite control. Examples include dimethoate and chlorpyrifos. These products may not kill the eggs, thus a treated field should be scouted 7 to 10 days after application to determine if a second application is necessary. As always, refer to the label for the appropriate rates and re-entry intervals. Pyrethroid insecticides should not be used to control twospotted spider mites as they are not as effective and can actually flare populations. Under dry conditions, foliar treatments are recommended when plants have substantial stippling or leaf-yellowing and spider mites are active (Photo 3). Because a naturally-occurring fungus can control populations, treatment of twospotted spider mites may not be required when temperatures drop below 85°F and humidity levels are greater than 90 percent for an extended time. Mites that are infected by the fungus will appear brown and will not move on the piece of paper used for scouting.

Photo 3. Foliar insecticide treatments should be made to fields before prolonged spider mite feeding causes yellowing or leaf drop. Photo taken by Tracy Cameron on June 6, 2012, in Richardson County, Neb.





Adam Varenhorst is an entomology graduate student. Matt O’Neal is an associate professor of entomology with research and teaching responsibilities. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact Erin at or phone 515-294-2847.

Twisted whorls and yellow leafs

By Roger Elmore, Department of Agronomy, and Mark Licht, ISU Extension and Outreach


Fast-growing corn in the last week has resulted in a few scattered plants in some fields - at least in central and west-central Iowa - displaying bright yellow leaves waving above the canopy (see photo).  Corn with these symptoms is around the V5 to V8 growth stage. The yellow upper corn leaves are sun-starved, entrapped earlier in the whorl by more mature leaves. They will wave for a day or more above rapidly growing crop canopies before turning green.

Plants exhibiting effects of twisted whorls. Leaves will remain "wrinkled" for the remainder of the growing season. June 7, 2012. R. Elmore photo.


The symptom is not unusual; we reported it in the ICM News in both 2008 and 2009 occurring at the same growth stages, as well as late as the tenth to twelfth leaf stage. An article posted on the ISU Extension and Outreach corn website also addressed the subject.

Most of the leaves should unfurl within a few days. The pale-yellow leaves will green as they accumulate chlorophyll. If the deformity causes a delay in either growth or development, it may reduce yield on a plant basis, but the effect - if not widespread - likely will not influence overall yield on a field basis.

Why are potassium deficiency symptoms showing now in corn and soybeans?

By Antonio P. Mallarino, Department of Agronomy

Some corn and soybean fields, mainly in eastern and southern Iowa, are showing potassium (K) deficiency symptoms, even when growth stages range from V3 to V8. The symptom for both crops is yellowing of the leaf margins of the older leaves that usually begins at the leaf tip and extends down the margins toward the leaf base. With severe deficiency the leaf edges may become brown and necrotic, although the newest leaves usually have normal coloration. For further information of symptoms, see Corn leaf potassium deficiency symptoms (ICM 7/1/2002), Is it iron or potassium deficiency? (ICM 7/1/2002), and ISU Extension and Outreach publication Nutrient Deficiencies and Application Injuries in Field Crops  (IPM 42).

The current symptoms may reflect low soil-test K and deficient pre-plant fertilization. In several fields the soil-test K level and pre-plant fertilization were adequate, however, so the deficiency most likely is caused by dry topsoil due to low rainfall during late May and early June. Any soil factor that limits root growth and water uptake can limit K uptake and induce a deficiency even with adequate soil K levels. For example, these include dry, too loose, or compacted soil; root pruning by insects or infection by diseases; and seed furrow sidewall compaction. Plants in the end-rows often don't show symptoms, mainly in no-till fields where the end-rows area is tilled. This is a clear indication that the problem may be an induced deficiency and not a low soil-test K level. Observation of plant nodal roots, physical soil conditions, and soil sampling and testing for K in adjacent areas with or without symptoms can provide clues about the reason for the deficiency and possible action. If the induced deficiency is not extreme, plants often recover when normal rainfall resumes, showing no deficiencies in new leaves and with little or no yield loss.


What can be done for the current crop?

Unfortunately, there is no certain economically effective corrective treatment for this year's crop, because the best way to prevent K deficiency is to apply adequate amounts of fertilizer before planting and avoid the soil conditions that induce deficiency even with adequate soil-test K levels. Also, deficiencies often occur in small and isolated field areas, which limit the cost-effectiveness of treatment.

Iowa research in many fields has shown that foliar fertilization with fluid fertilizer containing K can increase soybean yield in some conditions. Insufficient data is available for corn, but research in other states has shown that a response also is possible. Use of low-salt fluid fertilizers is safer (with no potassium chloride or potassium sulfate, for example) because the K concentration can be higher and the rates applied can be higher than with other products. Application rates for corn or soybean using these types of fluid fertilizers of up to 12 lb K2O/acre did not damage foliage, although good responses also were observed with about one-half this rate.

The post-planting soil application of fluid or granulated K fertilizer broadcast or banded between the rows as a rescue treatment may, or may not, be more effective than foliar fertilization. This application will be inefficient when rainfall continues to be deficient or when a K deficiency is induced by the soil and root problems mentioned above. On the other hand, higher rates than with a foliar spray can be applied, and if the deficiency is due to low soil-test K the applied K will begin to increase soil-test K levels for the next crop. For the current crop, however, an application to the soil will not be effective if sufficient rainfall is not received soon.


How can you prevent this type of deficiency for future crops?

If you see deficiency symptoms this spring, you need to think about how to prevent deficiency in future crops.

If the reason for deficiency symptoms is low soil-test K, the deficient areas can be targeted for post-harvest soil sampling, testing, and appropriate fertilization. Deficient areas can be easily marked with hand-held global positioning devices, and this information can be provided to a dealer having variable-rate fertilization capability. Deep-band K placement for the next crop, mainly with ridge-till, strip-till, or no-till, can go a long way toward alleviating K deficiencies. A planter-band (starter) fertilizer treatment at planting also will help, but in-furrow K starter is not recommended for soybean and low rates should be applied for corn. For further information see Use new potassium soil test and fertilizer recommendations (ICM 10/20/2003), Iowa State University Extension and Outreach publications Take a Good Soil Sample to Help Make Good Decisions (PM 287), and General Guide for Crop Nutrient and Limestone Recommendations in Iowa  (PM 1688).

If the deficiency symptoms are induced by soil or root issues other than low soil-test K, your area extension Field Agronomist or local crop consultant can provide additional suggestions about soil or crop management practices to help alleviate future problems.


Antonio P. Mallarino is nutrient management professor in the Department of Agronomy. He can be reached at 515-294-6200 or by e-mailing

No-Till Corn-on-Corn Field Day Near Waverly

Iowa Learning Farms (ILF) will host a field day (2012 Field Day Guide) at the Mark Mueller farm near Waverly on Thursday, June 14, beginning at 11 a.m. the field day will focus on no-till conservation farming and crop residue management for optimum results.

Bremer County farmer Mark Mueller began using no-till several years ago by planting soybean into standing cornstalks to control soil erosion and save labor. In 2010, he began no-till planing corn following corn. Many of Mark's acres are on a two-year corn/one-year soybean rotation, with some acres planted to a three-year corn/one-year soybean rotation. Mueller will talk about his planting equipment and yield successes; ILF farmer partner Collin Jensen will also be on hand to offer insights on successful no-till continuous corn production. Jensen farms near West Union in Fayette County. NRCS soil conservationist Shaffer Ridgeway will discuss cost-share incentives available to farmers interested in trying no-till on their farm.

The field day includes a complimentary lunch served by the Bremer County Pork Producers. The event is free and the public is invited to attend. The field day site is located just northwest of the Waverly airport. Take exit 205 from Highway 218 west of Waverly, travel east on 210th Street (business Highway 218) toward Waverly. Parking is available at the intersection of 210th Street and Caspar Avenue.

ILF takes a grassroots approach offering innovative ways to help all Iowans have an active role in keeping our state's natural resources healthy and not take them for granted. A goal of ILF is to build a culture of conservation, encouraging the adoption of residue management and conservation practices. Farmers, researchers and ILF team members are working together to identify and implement the best in-field management practices that increase water and soil quality while remaining profitable.


John Lundvall is the Iowa Learning Farms field coordinator. He can be reached at 515-294-8912 or by e-mailing

Now is the Time to Look for SCN Females on Soybean Roots

By Greg Tylka, Department of Plant Pathology and Microbiology

The soybean cyst nematode (SCN) is one of the most serious soil-borne pathogens of soybean in Iowa and throughout the Midwest.  Juveniles of this microscopic worm hatch from eggs in the spring, then burrow into soybean roots, where they attach to the vascular tissue of the plant and feed (Figure 1).  Developing SCN females get progressively larger as they mature, until their fully expanded, lemon-shaped bodies rupture out of the root and become visible on the root surface. 


Figure 1. Hatched SCN juveniles (stained purple) penetrating soybean root on way to vascular tissue for feeding.


Soybean cyst nematode females are round, white, and large enough to see with the unaided eye (Figure 2).  It takes four to six weeks or more for the first SCN females of the season to develop sufficiently to rupture out of and become apparent on the surface of soybean roots. 

Figure 2. Swollen, white SCN females visible on soybean roots with the unaided eye.


The first generation of adult SCN females are appearing now.  So the next six to eight weeks (through July) are prime time to dig roots and check for SCN females in Iowa. 

Plants should not be pulled from the soil because the young roots with the SCN females attached will be stripped off.  Instead, roots should be dug with a shovel or spade, and soil should be carefully removed from the roots.


Two reasons to check roots for SCN females

Observing SCN females on roots of susceptible soybean varieties is a quick and easy way to check the presence of this pathogen in a field, which is the first step towards successful management of SCN. 

Checking the roots of resistant soybeans for SCN females in fields that are infested is a good way to monitor the effectiveness of the resistant varieties. 

More information about the biology and management of SCN can be found at



Greg Tylka is a professor with extension and research responsibilities in management of plant-parasitic nematode in the Department of Plant Pathology and Microbiology at Iowa State University. He can be reached at or 515-294-3021.

2012 Weed Science Field Day

By Mike Owen, Department of Agronomy

The Iowa State University Weed Science Field Day will be held on June 28 at the Curtiss Farm on South State Street in Ames. Registration for the event begins at 8 a.m. with brief remarks and a self-guided tour following. Registration is $20 and includes refreshments and a field book that details the demonstrations and research at the Curtiss Farm location, as well as the other locations throughout Iowa. Those in the agricultural chemical Industry, growers, and seed dealers are welcome and will find much that will be of interest.

The Curtiss Farm is easily accessible from Highway 30; exit on University Avenue, go south to the four-way stop, turn west to the next stop, turn north on South State Street, and the Curtiss Farm is on the west. Follow the cinder road west through the farm to the registration tent. If you have questions please contact Mike Owen at or (515) 294-5936.


Mike Owen is associate chair and professor in the Department of Agronomy; contact him at or phone 515-294-5936.

How much can volunteer corn affect corn yields?

By Bob Hartzler, Department of Agronomy

Many continuous corn fields across Iowa have significant volunteer corn problems this spring. Unless this year’s corn possesses a herbicide trait not present in the prior corn crop, there are no selective control practices available, other than cultivation. The lack of control options reinforces the value of adjusting combines to minimize harvest losses.

As with any weed, the competitiveness of the volunteer corn is highly variable depending upon the specific situation. Research in South Dakota found that volunteer corn densities of 800 to 14,000 plants per acre caused yield losses of 0 to 13 percent.   Iowa State research found that 1,700 plants per acre (1 plant/10 feet of 30-inch row) caused 1.3 percent yield loss.  If dry conditions continue, the impact of volunteer corn on yields will increase. 

Volunteer corn can cause large yield losses.


Bob Hartzler is a professor of agronomy and extension weed specialist; contact him at or phone 515-294-1923.

This article was published originally on 6/11/2012 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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