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5/4/2009 - 5/10/2009

Spring Sampling Not Recommended for Most Corn Nematodes

By Greg Tylka, Department of Plant Pathology

Most Iowa crop producers and agronomists are familiar with the soybean cyst nematode (SCN), but plant-parasitic nematodes also can damage corn. Corn nematodes are not one organism, but rather are a collection of numerous different species that can feed upon and sometimes damage corn. Each year, several instances of nematode damage to corn are discovered in Iowa. Numerous quick facts about corn nematodes were recently reviewed in the Integrated Crop Management News, April 28, 2009.

The symptoms of nematode damage to corn are very general or nondescript – namely stunting of plants, yellowing of leaves, and poor ear and kernel development.  So to determine if nematodes are damaging corn, one must find out what nematode species are present in a field and what their population densities (numbers) are when the numbers are at their highest.

The population densities of most corn nematodes increase during the first half of the growing season, so samples should be collected mid season. Sampling mid season allows for comparison of nematode numbers to damage thresholds established for corn. Spring sampling is not recommended for most corn nematode species because if numbers are low, there is no accurate way to know or predict if numbers will increase to damaging levels.

The information presented above does not apply to two corn nematode species – the needle nematode and the sting nematode. These two nematodes migrate down into the soil in the middle of summer, when soils are warmest, and they may be missed in mid-season soil samples.

Needle nematodes and sting nematodes only occur in sandy soils (70 percent or greater sand content). So if needle or sting nematode is suspected in a sandy field, collect 20 or more 12-inch-deep soil cores in the spring. Sampling can be done prior to planting or once seedlings begin to emerge from the soil. If sampling is done after crop emergence and damage to the young crop is seen, collect the soil cores from the root zone of corn plants within the area being damaged.

Soil cores should be mixed well and placed in a moisture-proof bag, then kept cool and submitted for processing as soon as possible.

Samples for nematode diagnosis can be sent to the ISU Plant and Insect Diagnostic Clinic, 327 Bessey Hall, Iowa State University, Ames, IA 50011. The test for all corn nematodes, including needle and sting nematodes, is called a complete nematode count.

Samples sent to ISU should be accompanied by a completed Plant Nematode Sample Submission Form - PD 32, available from the Extension online store at, and a check for the $30 per sample processing fee.

The Extension publication “Nematodes That Attack Corn in Iowa” – PM 1027 has more information about corn nematodes and is available from the Extension online store as a downloadable document. 

 nematode damage to corn

Young corn crop damaged by nematodes.



Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at or by calling (515) 294-3021.

Spring is Time to Check Alfalfa for Foliar Diseases

By XB Yang, Department of Plant Pathology

Early May is the time to check for alfalfa diseases. When spring has cool temperatures and frequent rains, the weather will promote the development of leaf diseases in some alfalfa fields. Knowing the occurrence of alfalfa diseases in early May helps make decisions about the first cutting. High levels of alfalfa foliar diseases such as spring back stem can cause early defoliation before you make the first cut, resulting in yield reduction.

Reports early this week suggest occurrence of low level alfalfa foliar diseases with some foliar diseases found in fields. When you scout, pay attention to second and third year alfalfa fields as they have greater disease risk than first year alfalfa fields. The diseases may occur in the past year and the pathogenic fungi can build up over time. Fields in lower spots with high soil moisture have higher disease risk. Keep in mind that there are differences in disease tolerance between varieties. 

If disease level is high, early cutting generally is recommended so that defoliation can be avoided. Cutting as early as mid-bud stage to avoid severe defoliation may be necessary when the disease is severe. In Iowa, three diseases are most common in spring – spring  black stem, downy mildew, and Leptosphaerulina leaf spot. Below is how to identify the three diseases.

Spring black stem produces numerous small, dark brown-to-black spots that first occur on the lower leaves and petioles, and appearing later on the stems. Irregularly shaped lesions on leaves increase in size and coalesce. Lesions on stems and petioles enlarge and may blacken large areas near the base of the plant. The fungus that causes this disease is dispersed by splashing rain. This disease is very common in Iowa and severe infection can result in defoliation.

Downy mildew is caused by the fungus Peronospora trifoliorum. This fungus infects alfalfa in spring when temperature is low and moisture is high. The weather conditions this spring are ideal for downy mildew. Severe disease was observed in 1993 when the spring was wet and cold. Symptoms of this disease are chlorotic blotches on the upper leaf surface and a white-to-gray mold on the lower leaf surface. Sometimes, the color may be pale. The fungus survives in shoots over the summer and spreads in the fall. If the disease is a problem in your field, consider planting a resistant variety in your next planting.

Leptosphaerulina leaf spot mainly attacks leaves. Both young and old leaves are susceptible to infection. Lesions often start as small black spots and remain as “pepper spots” or enlarge to “eyespots.” The lesions have light brown-to-tan centers with darker brown borders and are often surrounded by a chlorotic area. This disease is dispersed similarly to spring black stem, by splashing rain.

When you scout a field, check places where alfalfa grows well or the canopy is dense. Diseases are likely to start in dense canopy.  Also because these diseases start by attacking low leaves of plants, look for diseased leaves and stems in lower portions of the canopy. These diseases progress from lower portion of plants to the top. Pay special attention to fields that had disease problems last year because they have greater risk than other fields for inoculum carryover from the winter.

alfalfa blackstem lesion

Lesion of alfalfa black stem.



alfalfa black stem leaf

Leaf with alfalfa black stem.



alfalfa downey mildew
 Alfalfa downy mildew.



XB Yang is a professor of plant pathology with responsibility in research and extension. Yang can be contacted by email at or by phone at (515) 294-8826.

A Harsh Winter for Bean Leaf Beetles

Erin Hodgson, Department of Entomology and Rich Pope, Corn and Soybean Initiative

Although native to the U.S., bean leaf beetles have been only been considered an economic pest of soybean since the late 1980s. Adults feed on snap beans, dry beans, peas, alfalfa, clover and even corn, but strongly prefer soybean. Bean leaf beetle adults will feed on leaves, cotyledons, and pods, while the larvae feed on the roots; adult leaf-feeding injury is usually round or oval holes about the diameter of a pencil.

Populations of first and second generations of bean leaf beetle in Iowa have varied through the years, with widespread severe outbreaks seen in 2002 and 2005. Mild winters and persistent snow cover shelter overwintering adults, and earlier planting dates favor their egg-laying success. However, Iowa had an exceptionally harsh winter in 2008-2009, and predictive models suggest bean leaf beetles were hit hard. Based on accumulating subfreezing degrees, at least 73 percent of the adults could not survive regardless of the protective snow cover, as shown on the map below.

Over winter mortality of bean leaf beetles by region

Bean leaf beetle adults can noticeably defoliate soybean seedlings, but rarely cause economic losses. Unfortunately, overwintering and first generation adults can spread bean pod mottle virus (BPMV), and lead to secondary infections on the developing pod. Bean leaf beetle management options include delayed planting to discourage heavy adult feeding and early-season virus transmission. Food-grade soybean and seed fields should be closely monitored for adults as soon as the plants emerge. Areas with high BPMV incidence should follow an integrated approach to reduce pod damage and improve seed quality (archived article here).


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. Rich Pope is an extension program specialist in the Integrated Pest Management program.

Winter Annuals, Giant Ragweed, Foxtail Barley, and Reach Back

By Micheal Owen, Department of Agronomy

Winter annual weeds have really begun to grow with a vengeance and the recent wet conditions have kept most of the field work to a minimum. Given that most of the corn is already planted, I am hopeful (albeit not confident) that these weeds have been effectively eliminated from the fields. If not, do not delay as they become considerably more difficult to control once they get some growth momentum. In particular, marestail (aka.Horseweed) is extremely tough to manage once it grows beyond the rosette stage. Other winter annuals are also more difficult to manage as they mature.

Simple perennials such as dandelions are also doing well this year. If you have simple perennieal or winter annual weed problems, move quickly and move effectively. I suspect that many of the fields that are intended for soybeans are getting extremely overgrown. Do yourself a favor and manage these weeds before you plant and do not presume to wait, as two things are likely to happen if you take this recourse; 1) you will lose yield potential and 2) your weed control will not please you. Act soon in order to best protect your crop yield potential and manage these weeds most effectively.

Giant ragweed is becoming a more frequent visitor to Iowa fields. This weed is one of the first to germinate (in some of our plots, it is already pretty large) and becomes very difficult to control with most POST products with the exception of 2,4-D and other auxinic herbicides.  POST HPPD inhibitor herbicides and triazines will do a nice job IF the giant ragweeds are small. If you plan to use glyphosate, do not skimp on the rate and apply sooner rather than later.

Foxtail barley is also becoming an occasional problem in many no-tillage fields. This perennial bunchgrass is very difficult to control in the spring. Generally the best you can do is to set it back with whatever treatment you spray (glyphosate and some HPPD inhibitors have activity on the weed) but I am not aware of any good spring-applied control tactic. The best treatment is glyphosate in the fall or aggressive tillage.

Reach back is a term used to describe residual herbicides that fail to control weeds when they emerge but with subsequent rain will move into the roots and ultimately kill the weeds as the herbicide moves upwards in the xylem (water pipes of the plant). While this phenomenon does actually occur, I strongly suggest that you not count on it. There are too many factors that must come together at the same time for “reach back” to work. If weeds emerge through a soil-applied herbicide program, do not delay by waiting for the reach back effect; move aggressively to manage the escaped weeds as quickly as you can. In this case, time actually is money – the longer the weeds grow with the crop, the more yield you lose, never to be regained – regardless of how dead you kill the weeds. 



Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at or by phone at (515) 294-5936.

Corn Emergence in 2009

By Roger Elmore, Department of Agronomy

Corn spikes through fertile soil of early-planted fields in most parts of Iowa as I write.  According to the May 4, 2009 USDA-NASS estimates, 2 percent of Iowa’s corn is emerged compared to none last year and the five year average of 7 percent. 

Amount planted varies widely in the state. Slightly more than a third of the corn is planted in East Central, South Central, and Southeast Iowa cropping districts; whereas, the remaining districts have over half of the corn planted with the most reported in Northwest, 78 percent.  Some producers have completed planting.

Meanwhile, 40 percent of Iowa's corn crop remains unplanted. We are far ahead of last year, 84 percent unplanted; similar to 2005; but lag behind unplanted acreage estimates at this date in both 2004, 23 percent, and 2006, 29 percent. We are far ahead of states to our east. For example, 95 percent of Illinois' crop remains unplanted!

corn emerging 2

Emergence and early-season growth signal if the growing season has gotten off to a good start or not. Iowa’s top corn producers walk their fields calculating plant populations, evaluating plant spacing variability, and examining seedling vigor and health much like a physician does during a newborn child’s first physical. These early benchmarks serve for the rest of the corn growing season. 

Evaluate crop stands as soon as possible, especially in fields where seedbed conditions were marginal at planting. If problems exist, often there is little chance of correcting many of the situations you find. But changes can still be made for the not yet planted crop – if any – and certainly it will produce good fodder for improving next year’s planting season.

We can’t always avoid planting into marginal conditions which result in poor stands, wild spacing variability, and poor plant vigor. Numerous causes exist. We summarized several of these in the Integrated Crop Management News, May 23, 2008.  Some of these issues will undoubtedly reoccur this year.

Every growing season is different but one thing remains in common: high yields most likely occur when seed is planted into a good seedbed. Work at achieving this. Be aware that planting into marginal conditions brings about marginal returns. The goal for maximum yield is for every plant to look like every plant.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Elmore can be contacted by email at or (515) 294-6655.

Degree Days - Off on a Muddy Track

By Rich Pope, Corn and Soybean Initiative

Today's article is the first weekly post that monitors accumulation of degree days through the 2009 growing season. The map below shows the base 50 degree F days that have accumulated in each of Iowa's nine crop reporting districts, and the departure in accumulations from long-term averages.

Accumulated degree days May1 through May 4, 2009

As of the morning of May 4, approximately 70 percent of corn has been planted in Iowa. Most of this corn was planted prior to the heavy rains that struck central and eastern Iowa the weekend of April 25.  Just like the Kentucky Derby, many acres started on a muddy track. Although that isn't itself a major problem, farmers are advised to watch fields where corn is emerging for successful stand establishment, particularly where driving rains may have caused crusts to form.  Soybean planting was estimated at less than 5 percent complete for the state.

For plantings prior to May 1, the map below shows degree day accumulations prior to May 1.  As an example, for a field that was planted on April 10 in central Iowa, 135 degree days accumulated for crop development in addition to those listed since May 1.  So for that field, as of May 4, there would be 35+135 or 170 degree days currently accumulated.

2009 degree days accumulated prior to May 1

Several ISU field agronomists are seeing corn emergence. A general rule of thumb for degree days required between planting and emergence is from 100 to150, and several fields have passed 150 with no emergence. Remember that the degree days posted here are based on air temperatures, and that until emergence, the corn plant is underground. Wet soils warm more slowly, especially when there are cloudy days that limit solar warming. That said, with warmer and sunny days forecast this week, many fields planted in mid-April should be emerging this week.


Rich Pope is a program specialist with responsibilities with Integrated Pest Management.

May 4 Iowa Crop and Weather Report

This week’s crop and weather report includes interviews with Elwynn Taylor, Iowa State University Extension climatologist; integrated pest management specialist Rich Pope; soybean agronomist Palle Pedersen; and soybean entomologist Erin Hodgson.

It appears that La Niña is dead—effective Monday, May 4, 2009, according to Taylor. He doesn’t guarantee ideal growing season weather, but says it should eliminate extremes in temperature and precipitation.

A wet week slowed corn planting except in Northwest Iowa—where nearly all of the corn has now been planted, reports Pope. He says crusting is being reported in parts of Iowa due to heavy rainfall a week ago. Pope also introduces Erin Hodgson, the new extension soybean entomologist. Hodgson comes to ISU from Utah State.

Palle Pedersen says he is optimistic that 2009 will be a good year for soybean production in Iowa – especially now that this spring’s weather has been much better than the past couple of years.

Survey Participants Respond Favorably to ICM News

By Sorrel Brown, Ag and Natural Resources Extension


ICM News subscribers, earlier this year, we asked you to respond to an online survey on the Integrated Crop Management News to find out how you value the research information that is provided.  We are pleased to say 624 subscribers responded. The breakdown by profession showed that 76 percent of you indicated producer and agribusiness agronomist as your primary occupations.  Other respondents included government employees, educators and independent crop consultants. The majority of respondents were from Iowa, with some from surrounding states and other countries.
Key points from the survey include:


  • 95.5 percent said ICM News made them aware of methods to improve their or a client’s farming operation.
  • 96 percent reported that ICM News made them more confident in making agronomic decisions or recommendations.
  • 89 percent indicated ICM News influenced them to take action in making or recommending a change in farming practices.
  • 83.7 percent said ICM News prompted them to seek out additional resources on a topic.
  • 89.1 percent indicated they had shared ICM News information with as many as 50 other people.
  • 89 percent said they found enough value in ICM News that they would recommend to others.


A sampling of practices that have been modified includes:

  • Using the ISU Nitrogen Rate Calculator to fine-tune corn nitrogen management
  • Increasing plant densities
  • Using economic thresholds in insect scouting to determine if/when to spray
  • Harvesting late soybeans as grain rather than forage (decision paid off)
  • Encouraging the adaptation of strip-till
  • Not spraying for soybean aphid until needed
  • Using more moderate temperatures to dry specialty corn
  • Not entering a field early to avoid compaction problems
  • Planting fuller season soybeans in late season (with favorable results)
  • Better manure application management on corn
  • Eliminating fall herbicide application
  • Reducing tillage on flooded ground
  • Testing for soybean cyst nematode


The implications of the survey are that the applied research on crop production and protection that is presented in the Integrated Crop Management News is highly valued and plays an important part in the decision-making that occurs in Iowa row crop farming operations.  Better informed decisions are more likely to have positive economic, emotional, and environmental consequences.




Sorrel Brown is a program evaluator for Ag & Natural Resources Extension and Agricultural Education & Studies at Iowa State University.

Pay Attention to Soil Crusting After Heavy Rain Events

By Mahdi Al-Kaisi, Department of Agronomy and Mark Hanna, Department of Agriculture and Biosystems Engineering

Recent rain brings another challenge that farmers may need to deal with, especially in conventionally tilled fields that were planted recently to corn or soybean.  In addition to the immediate effect on soil erosion and potential damages to newly seeded waterways, there are after effects of the rain when weather conditions improve and the soil surface starts to dry. The potential problem is soil crust.

soil crusting

This could occur especially in intensively tilled fields where residue cover is not adequate, as well as with fine texture soils, and soils with low organic matter content. These conditions could increase the potential for soil crust formation. Residue cover plays a significant role in reducing soil crust by absorbing the impact of rain drops that destroy soil surface structure.  The destruction of soil structure impacts plant germination and seedling emergence for both corn and soybean. 

Soil crusting can also result in poor growing conditions and reduced water infiltration. Soybean seedling emergence can be a problem if a dense surface crust forms. In this situation, hypocotyl is broken when pushing up against a solid crust. Monitor high-risk fields for soil crusting, especially where plant emergence has not yet occurred, in order to avoid damage to seedlings.

Rotary Hoe
The quick-relief solution to such a problem is the use of a rotary hoe. This tool is commonly used in treating soil crusting to improve seedling emergence. However, the timing is critical in order to achieve the intended results and prevent seedling damage.  The rotary hoe is a potentially good tool to use to break up soil crust, but make sure you've got a crust that is actually sealing the soil surface before using it. 

To minimize the damages to the seedlings and to increase success, rotary hoe at a time when the soil surface is at the right moisture conditions. This will require frequent field scouting to ensure that soil surface moisture is just above field capacity. Field capacity is the point when a handful of soil will crumple easily in your hand under minimum pressure, leaving a trace of moisture on your palm.  This moisture condition will ensure less damage to emerging seedlings and less soil compaction during the hoeing process. 

Rotary hoe at high field speeds (8 to 10 miles per hour) unless safety is a concern.  However, if soybeans are the crop emerging, make sure both cotyledons aren't broken off by the hoe. Corn will likely be the crop emerging from rains this past weekend.  Expect a minor stand loss (approximately 1 to 2 percent) from hoeing, but this should be insignificant if corn is truly having difficulty breaking through a crust.  Getting off the tractor and checking for stand loss is a good idea when starting a field.  If loss seems excessive (greater than 3 to 5 percent), you may want to slow your travel speed to be less aggressive with the tool.  

It is very important to check early-planted fields periodically, especially those conventionally tilled with fine soil texture and low organic matter.  Timing is important to manage soil crust at the proper moisture conditions.


Mahdi Al-Kaisi is an associate professor in agronomy with research and extension responsibilities in soil management and environmental soil science. He can be reached at or (515) 294-8304.
Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery. Hanna can be reached at or (515) 294-0468.

This article was published originally on 5/11/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.