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4/14/2008 - 4/20/2008

Iowa 2008 Prediction for Stewart’s Disease of Corn

By Forrest W. Nutter, Jr., Department of Plant Pathology, Lu Liu, Sustainable Agriculture Program, Rich Pope, Department of Entomology, and Marlin E. Rice, Department of Entomology

Following an Iowa winter of ice, snow and cold temperatures, the predicted risk for Stewart’s disease of corn in 2008 is negligible throughout most of Iowa, with only the southeastern-most counties having a low risk.

Stewart’s disease (also known as Stewart’s wilt), is caused by the bacterium Pantoea stewartii.  In addition to the pathogen, an insect vector, the corn flea beetle, plays a critical role in the spread of this microorganism from plant-to-plant.

The corn flea beetle is important not only because it serves as a vector to acquire and transmit the causal bacterium, but because it also provides safe refuge for the bacterium (within the gut of hibernating corn flea beetles) to survive the winter. Thus, if temperatures are mild enough for the corn flea beetle to survive the winter, the pathogen (P. stewartii) will also survive. In the spring, surviving beetles infested with the bacterium will emerge from grassy areas near neighboring corn fields and will begin to transmit the pathogen to corn seedlings.

Stewart's disease can occur at any stage of plant development, but symptoms are almost always associated with flea beetle feeding. At the seedling stage, infected plants wilt rapidly from systemic infection, and seedling death is common. However, plants that do survive will be stunted and will serve as a source for next year’s overwintering, as well as future corn flea beetle generations to acquire and spread the pathogen.

Plant-to-plant spread by overwinterring corn flea beetles will continue until late May, when corn flea beetles lay their eggs at the base of corn plants. The overwintering generation of adult corn flea beetles then dies.  Beetle death begins a “beetle-free” period lasting 2 to 3 weeks (early to late June), which ends as the next generation of adult corn flea beetles begin to emerge.  During this “beetle-free” period, foliar insecticide sprays are not effective.

The first summer generation of adult corn flea beetles emerges in late June, and feeds on diseased corn plants. They can acquire the bacterium, and facilitate the further spread of the bacterium to healthy corn plants. Later in the growing season, usually after pollination, the leaf blight stage may occur. Diseased plants at this phase exhibit long, wavy streaks (lesions) that are initially water soaked, and then turn yellow and die. Corn flea beetle feeding scars are usually visible within the lesions. If the disease is severe, whole leaves may wilt and die.

A second summer generation of corn flea beetles will emerge about mid-August, but late growing season foliar insecticide sprays are not recommended to control this generation.
Mild winters during the past decade have resulted in an increased occurrence of Stewart's disease in Iowa. Two disease prediction models are available to predict the seasonal and county-level risk of Stewart’s disease. These models are: the Stevens-Boewe Index Model, and the recently-developed Iowa State Mean Monthly Temperature Model. Both models use the monthly mean winter temperatures that occur during in December, January and February to predict the degree to which the corn flea beetle population survived the winter.

The Stevens-Boewe Index predicts the severity (how much of the corn leaf tissue is infected) of the leaf blight stage of Stewart's disease in the late summer. The risk is calculated by summing the average monthly temperatures for December, January and February. A sum below 80 indicates a negligible risk, 80 to 95 is considered low, 85 to 90 indicates moderate risk, and greater than 90 is considered a severe risk. Stewart’s disease predictions based upon the Stevens-Boewe Index, for the late leaf blight phase of Stewart's disease throughout Iowa in 2008 (Figure 1).



Stevens-Boewe Index

Figure 1

The Iowa State Model predicts the prevalence (presence or absence) for Stewart's disease at the county-level. A high prevalence of Stewart's disease is predicted if the mean monthly air temperatures for December, January and February are each above 24 degrees F. For December 2007, the mean temperature was greater than 24 degrees F only in crop reporting district #9 (southeast Iowa). The mean monthly temperatures for January and February were all well below 24 degrees F, which strongly suggests that temperatures were not favorable for corn flea beetles to survive the 2008 winter.

However, continuous snow cover on the ground over most of central and northern Iowa from early December to early March functioned as an insulation blanket to protect beetles from subfreezing temperatures. What this means is that although the models predict a negligible risk for Stewart's disease to occur throughout most Iowa in 2008, and only a low disease risk for southeast Iowa (Figure 2), the incidence may be slightly higher than we expect because of better than anticipated beetle survival from the snow cover.


2008 Iowa State Stewart Disease Prediction

Figure 2

Insect Economic Thresholds
Stewart’s disease can be controlled on susceptible corn by controlling the corn flea beetle with a foliar-applied insecticide, but timing of the application is critical. Seed treatments also may provide a better approach to control. A 2000 study at the University of Illinois demonstrated that two insecticides, imidacloprid (Gaucho®) and thiamethoxam (Cruiser®), applied to sweet corn seed reduced the incidence of Stewart's wilt by 50 to 85 percent under field conditions with naturally occurring populations of corn flea beetles. These seed treatment insecticides controlled Stewart's wilt during the very early growth of corn plants when applications of conventional, foliar insecticides were ineffective, according to the researchers. The full article may be found at

Use the following thresholds for rescue treatments in corn:
Field corn--prior to stage V5, 50 percent of plants with severe feeding injury and five or more beetles per plant.
Seed corn--on susceptible inbreds, 10 percent of the plants with severe feeding injury and two or more beetles per plant.

Labeled insecticides include, but are not limited to, Asana XL, Capture 2 EC, Lorsban 4 E, Pounce 3.2 EC, and Warrior. See manufacturer's labels for use rates and restrictions.

Forrest W. Nutter, Jr. is a professor in the Department of Plant Pathology working on disease risk models for improved disease management. Lu Liu is a graduate student in the ISU Graduate Program in Sustainable Agriculture. Rich Pope is an Extension program specialist working in the Corn and Soybean Initiative. Marlin E. Rice is a professor of entomology with extension and research responsibilities.

Calculating Degree Days

By Rich Pope, Department of Entomology

Warm-blooded animals including humans generate their own internal heat and have regulatory systems to hold maintain body temperature in an operational range. These systems provide insulation from fluctuations of temperature in the environment and allow growth and development based on the passage of time in minutes, hours, weeks, months and years.

In contrast, most of the creatures we manage in agriculture don't have an internal heat regulatory system and depend on environmental temperatures to drive their development. For plants, disease organisms, insects and other “cold-blooded” creatures, development is dependent on the temperature around them. So if we understand the key temperatures needed for a given species, we often can monitor and predict development based on measuring how much heat each species accumulates from the environment, relative to its functioning temperature range.

Readers of the Integrated Crop Management News are likely familiar with the weekly crop development degree day-postings, and also with the insect-specific degree-day models, including those for black cutworm, stalk borer and bean leaf beetle.

Scientists have estimated a temperature that approximates the coldest temperature where effective development occurs for many species. That is the lowest cardinal temperature. For some species, there is also a high cardinal temperature, which is a point where growth and development are at their peaks. A list below shows some of the cardinal temperatures frequently used in Iowa crop management.

Regardless of the base (lower cardinal temperature), the process to calculate degree days is similar. To model crop or pest growth, we estimate the accumulation of heat on a daily basis. We look at each day as a provider of heat that leads to development. Let's go through this process step by step for a given day and degree-day base, as follows:

  • collect the daily high and low temperatures for a site, or average highs and lows across a region that occur that day.
  • average the high and low temperatures to estimate the average heat gained for that day, with the following adjustments:
    • temperatures below the base temperature contribute nothing to development; therefore, whenever the actual low temperature is lower than the base you should artificially reset the low to the base temperature, to estimate the heat received more closely. Adjust the high temperature the same way, if actual highs are greater than the maximum cardinal temperatures.
  • subtract the averaged temperature from the base temperature and, voila!, you have the accumulated degree days for that organism on that date.
  • calculate the heat-units gained for subsequent days and add them to estimate the accumulated degree days over a time period.

Here is an example. Let's calculate how many base 50 degrees F degree days (for example for black cutworm development) accumulated on two days in May for a farm somewhere in Iowa.

Day       Low    High
May 4   38        69
May 5   55        75
May 4
First, adjust the low temperature to the base (50 degrees F) because no development occurs below 50 degrees F. That means we average the high (69) and the adjusted low (50), which comes out to 59.5. Subtract the base, 50 degrees F, and that means there are 9.5 degree days for May 4. (if the average is below the base, do not subtract, rather you simply gain 0 degree days that day.)

May 5
Repeat the process for May 5. However, because the low is above 50, the low temperature needs no adjustment. As with May 4, we average the high (75 degrees F) and the adjusted low (55), which comes out to 65. Subtract the base, 50, and that means there are 15 degree days for May 5. The two-day accumulation is 9.5 plus 15, or 24.5 base-50 degree days.

Although there are some assumptions involved in making development models based on degree-days, the information allows pest management efforts to be well timed and effective.

Minimum and maximum cardinal temperatures in crop and pest management and use of information.

Crop or pest



information use




crop development




crop development

Black cutworm



300 DD from egg to cutting

Stalk borer



predicting migration

Bean leaf beetle



2nd generation emergence

Seedcorn maggot



seed treatment on replant

Alfalfa weevil



larval presence in fields

Western bean cutworm



adult emergence/ egglaying

Rich Pope is an Extension program specialist working in the Corn and Soybean Initiative.

Too Early to Worry about Planting Corn!

By  Roger Elmore and Lori Abendroth, Department of Agronomy

Rain is forecast again for most of Iowa later this week. Little if any spring work has occurred because of cold weather and already saturated soils. More rain right now is not welcome for most of us corn growers!

We nervously look at the calendar and wonder how we can plant all our corn this year. We need to remember that although we are likely planting one million acres less in Iowa than we did in 2007, it is still more acreage than we’ve planted since 1992. And nationwide, we have to go back to 1949 to find more corn in the U.S., except again for 2007.

Hybrid cold tolerance, seed treatments, tillage systems, herbicide options and planter systems are technological improvements allowing early planting when conditions are good. For example, we had half of our 2006 corn planted nearly two weeks earlier than we did in the late 1970’s.

Our early planting trend was spoiled by wet cold weather in 2007 with half of the corn planted after the first week of May. We essentially had three planting dates last year in Iowa. One thing in common with all three sets of 2007 planting date was that it was wet every time! Yet we still had the third highest yield in the history of Iowa in 2007, 171 bushels per acre!  Planting date is one of many management practices that relate to yield but it is not always the main factor.

Since 1981, the most corn we’ve ever planted in Iowa by mid-April was 6 percent; this was in 2006. In most years we have only had 3 percent or less of the corn acres in by then. By the end of the third week in April, only one in four years do we have more than 10 percent of Iowa’s corn planted (see Figure 1). In 2006 we again had the most with more than 25 percent planted. Usually we’ve had less than 5 percent of our corn planted by the third week of April.

April Corn Planting Dates in Iowa
Figure 1.

Don’t worry!  There is still plenty of time to plant corn. Remember that our best data shows that Iowa corn yield responses are relatively flat during April and only begin to decrease significantly as planting is delayed past May 10th. In addition, we can plant corn faster than ever before once the weather breaks.

We do not want to plant that precious seed into cold wet soils if there is no short-term promise that better days lie ahead. All sorts of problems arise such as poor germination, slow early-season growth, side-wall compaction and rootless corn.

The goal is to have all the corn come up at the same time with every plant looking like every other plant. It is far better to plant into cool, dry soils if the 7-day forecast calls for warming than to plant in warm soils knowing that temperatures will be dropping soon.

Patience is important when it comes to planting corn in April!

Roger Elmore is professor of agronomy with research and extension responsibilities in corn production. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production.

You Couldn’t Apply Phosphorus and Potassium Last Fall and Now It’s Too Wet

By Antonio Mallarino, Department of Agronomy


Winter came early last fall and perhaps you couldn’t make planned phosphorus and potassium applications. And, you couldn’t make applications during winter because of heavy snow cover. Now soils are wet and there is little time until corn planting begins. Also, fertilizers are in short supply and at high prices. So what should you do? There are a few viable options depending mainly on soil-test values and the particular crop. Most likely there is time for application if the crop is soybean or pasture, because either you applied the needed P and K before last year’s corn or have a few more days to fertilize this spring. You may have a tough situation for other crops such as oats, corn, or spring wheat, especially for oats and spring wheat because you should already have them planted.


Your best option is  clear if most of a field tests in the High or Very High soil-test category established by Iowa State University (see ISU Extension publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa). For soil series with low subsoil P and K (most in Iowa), the High category for P determined with the Bray-1 or Mehlich-3 P methods is 21 to 30 ppm and for K by the Ammonium Acetate or Mehlich-3 methods is 171 to 200 ppm. No P and K fertilizer is recommended for any grain crop in these categories because the probability of a yield response to fertilization is very small for High and almost zero for Very High. Only starter for corn is recommended for specific situations, for example with no-till and thick residue cover and heavy-textured, wet and cold soils (which may be the case in many fields this spring). Therefore, and especially with the currently high fertilizer prices, you could save considerable input costs and application complications by not applying any P or K this spring to high-testing soils except for starter in some situations.


If soil tests are in the Very Low or Low categories, you should definitely apply needed P and K. The potential yield loss from not making these applications for these two soil-test categories is large and the likelihood of yield increase is very high. The probability of large economic return to investment in fertilizer is large because crop prices are high, despite the high prevailing fertilizer prices. Therefore, a few days delay in planting in order to apply fertilizer is warranted. Applying a common starter rate for corn, especially the usually low rates placed in the seed furrow, will seldom result in the same yield response as from applying the full recommended nutrient amount in these test categories. Broadcast, band or injected P or K after crop planting is not a recommended practice as a rescue application.


When most of a field tests in the Optimum soil-test category there is no single best option as to apply or not apply P and K fertilizer this spring. Recommended P and K rates in publication PM 1688 are provided to maintain soil-test levels in this category, and because the application reduces the risk of yield loss due to nutrient deficiency and is a profitable practice in the long term. However, Iowa research shows that the probability of a yield increase is 25 percent or less in this category and, furthermore, application rates lower than the maintenance rates often result in the same yield response from one crop.


Therefore, if your corn planter is equipped to apply starter (for application in furrow or 2 inches beside and below the seed) you can skip a broadcast application this spring. You should resume maintenance fertilization next fall. Soil-test values will not decrease much due to this year’s crop harvest, with research on average indicating a decrease of 1 to 2 ppm for P and 2 to 5 ppm for K. It would be wise to test your soil again in the fall.


For more information see ISU Extension publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, the article “How do uncertain prices influence phosphorus and potassium fertilization this fall?” in The Integrated Crop Management Newsletter IC-498(24), or the Iowa State University Soil Fertility Web page at


Antonio Mallarino is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

Soil Fertility Web Site Updated

By John Sawyer, and Antonio Mallarino,  Department of Agronomy


The Iowa State University Agronomy Extension Soil Fertility web site has undergone a recent redesign. While the site ( has been a resource for several years, it was in need of an overhaul. Part of the update was to improve flow and access to the different areas of the site.


While the original nutrient topic areas are still there, such as lime and soil pH, nitrogen, phosphorus, potassium, manure nutrients and secondary and micronutrients, other areas have been added including nutrients and water quality. In each nutrient topic section, information is available that is pertinent to that topic, including Extension publications, newsletter articles, conference proceedings and reports, photos, presentations and links to Web sites.


Other sections of the site have been expanded, including more photos in the photo gallery and direct links from the home page for popular soil fertility tools such as the Corn Nitrogen Rate Calculator and the Compendium of Nontraditional Materials, and to other tools such as common conversion factors and soil temperatures. New search tools have been added, and in the future a search of just the soil fertility site will be added. In addition, for fun a “Did you know?” soil fertility fact is posted on the home page.


A significant component of the site overhaul was implementation of a new section located on the home page called Current Topic. In this section we will write articles about current soil fertility and nutrient management issues in Iowa crop production. The most recent article will be posted on the home page, as well a link to past articles. To read the full article (PDF format), simply click on the Read on link. The Past Current Topics is a link that goes to a listing of all past Current Topic articles. We hope you find these articles interesting and useful. Please check back often to find new articles and to look for other information related to soil fertility and nutrient management.


We hope you will use the ISU Agronomy Extension Soil Fertility Web Site as your source of soil fertility information and your initial stop when accessing the Web.


John Sawyer is an associate professor of agronomy and Antonio Mallarino is a professor of agronomy, both with research and extension responsibilities in soil fertility and nutrient management.

This article was published originally on 4/21/2008 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.