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6/8/2009 - 6/14/2009

Seedling Diseases Update

By XB Yang, Department of Plant Pathology

This spring’s cool and wet weather, with accumulated degree days behind average, favors seedling diseases. Fungal pathogens have been reported to cause poor seedling stands in both corn and soybean fields .

Infected soybean seedlings are rotted and can easily be pulled from the soil. Infected corn roots are discolored, soft or mushy. Mesocotyles also may have a brown discoloration. Infected corn seedlings often appear yellow or stunted.

In cooler soils Pythium is considered the main cause of seedling blights; however, Phytophthora can infect seedlings in cooler soils as well. Phytophthora seedling blight has already been reported in soybean, even in cultivars that have the Rps1k resistance gene. Phytophthora actually prefer warmer temperatures, as do Rhizoctonia. These two seedling blights will likely become more prevalent as weather and soil temperatures warm in the next week or two.

Iron chlorosis is showing up in some early planted soybean fields which are at the V2-V3 stages. Plants with iron chlorosis usually first appear in low and wet areas of soybean fields. These fields often have high pH (greater than 7) and poor drainage. Symptoms are more pronounced when soil temperature is low and soil moisture is high.

Iron chlorosis symptoms include yellowing between veins of young leaves. Brown and necrotic spots may occur in leaf margins. In severe cases, soybeans turn yellow and can be killed. Plants with iron chlorosis are more prone to fungal root rot problems. If plants with iron chlorosis have root rot, their taproots have a dark brown or reddish brown discoloration. Resistant cultivars to iron chlorois can improve yield. If you have high incidence this spring, consider using iron chlorosis resistant cultivars in the future.  
Replanting soybeans may need to be considered for fields that have reduced stands from seedling blights. If seedling blight in soybean causes stand reduction so severe that replanting is needed, one should consider using a seed treatment to avoid damping off in replanted fields. This is especially important to seedling blight caused by Phytophthora or Rhizoctonia as soil temperatures warm. With warmer temperatures and more inoculum build-up, the risk of seedling blight in these fields is greater than early spring.

It is important to identify the cause of stand reduction before deciding to replant to determine if a seed treatment is needed. For help in identifying the cause of stand reduction, refer to our last issue of ICM article. If you cannot determine the cause of the seedling blight, send a sample to the ISU Plant and Insect Diagnostic Clinic, 327 Bessey Hall, Iowa State University, Ames, IA 50011.

If the seedling blight has not reduced stands enough to replant, it will still be beneficial to take disease notes for future use. Consider seed treatment to reduce the likelihood of a disease problem in future soybean crops. If Phytophthora damping-off occurs on a Phytophthora-resistant cultivar, resistance in your cultivars has been defeated by the fungus. Consider using a cultivar with different resistance genes for your next soybean crop.



XB Yang is a professor of plant pathology with research and extension responsibilities in crop diseases. Yang can be reached at (515) 294-8826 or by emailing

Soybean Rust Stirring in the South

By Daren Mueller, Department of Plant Pathology

Soybean rust is starting to catch the attention of plant pathologists this year. If you peruse the USDA ipmPIPE Soybean Rust Web site, the distribution of soybean rust may not appear much different than in years past. Like before, soybean rust has survived the winter in the South on kudzu, this year in Alabama, Florida, Georgia and Louisiana. Soybean rust also was reported in five municipalities in Mexico in January, but these have been destroyed or are no longer active, so there is currently no known soybean rust in Mexico.

Different soybean rust patterns this year
If you dig deeper into the information on the ipmPIPE Web site, details reveal two emerging differences. First, soybean rust has been reported on soybean in sentinel plots in Alabama and Louisiana. Both reports are several weeks ahead of previous first finds on soybean in each state. Second, unlike the past few years, there has been ample rain for soybean rust to increase inoculum (spores). Many of the plant pathologists in these southern states anticipate soybean rust to show up in new places because of these ideal weather conditions.

What does this mean for Iowa? It is still too early to tell, but these developments are certainly catching our attention. A few years ago we made a checklist of factors that could lead soybean rust to cause yield losses in Iowa. These included:

  • Successful rust overwintering in the South
  • Buildup of inoculum early in the spring in the South
  • Spread of inoculum to locations that can serve as a springboard for rust to get to Iowa (Arkansas, Texas, Missouri, Oklahoma)
  • Spores are carried up to Iowa – preferably under heavy cloud cover (as spores are sensitive to light)
  • Spores arrive in Iowa when soybean crop is before R5-R6 and when weather conditions are conducive for soybean rust infection

Since soybean rust was discovered in the United States, it has rarely passed the first checkpoint. Mostly because of dry weather, soybean rust never has established itself early in the spring. In fact, most seasons have not seen soybean rust spread until late in the season (typically after August).


soybean rust map new

Figure 1. Current distribution of soybean rust in 2009 (last updated June 12, 2009) (Access most recent map at


Spore movement forecast
According to Iowa State University and St. Louis University’s soybean rust spore movement predictive model, spore dispersal has been limited to the far southern United States, but has increased recently as the sources of inoculums have increased. For now, even with the predicted increased spore movement, the chances of spores getting to Iowa in the next month is very low because there are no strong sources of inoculum in Arkansas, Oklahoma, Texas or Mexico.
ISU Soybean Rust Web Site
A quick reminder that the Iowa State University Soybean Rust Web site was redesigned in 2008. The new site highlights weekly messages written throughout the growing season that provide up-to-date information on soybean rust. Individuals can sign up on the site to have these reports e-mailed to them. The Web site also provides the basics on management of soybean rust, as well as fast facts about soybean rust.



Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Mueller can be reached at or by calling (515) 460-8000.

Spray Drift Potential Increases during Warm Weather Applications

By Mark Hanna, Department of Agriculture and Biosystems Engineering and Kristine Schaefer, Pest Management and the Environment

Recent rains over much of the state are encouraging crop and weed growth and pesticide applications will be a primary focus as soon as fields are suitable.  In a May 15, 2009 ICM News article, management factors to reduce drift potential were reviewed, with special focus on the effect of droplet size and wind speed on physical spray drift. As daily high temperatures increase into the upper 70 to 80s degrees F range, mid-day relative humidity frequently drops below 50 percent. This article takes a look at the effect of temperature and humidity on pesticide drift. 

Temperature and humidity affect the evaporation of liquid droplets.  The higher the temperature and lower the humidity, the faster evaporation occurs.  As a result of evaporation, spray droplet size decreases. The smaller droplets are then more susceptible to moving off target with prevailing winds as they become entrained in ambient air currents (Figure 1).

spray drift fig 1

Figure 1.  Effect of temperature and relative humidity on droplet size and movement.


The adverse effects of high temperature, low relative humidity and increasing wind speeds on the distance liquid spray solutions drift are illustrated below (Figure 2).  Values in the figure are an indication of what might be expected when using a number four tip at 40 psi with a sensitive target 30 feet downwind.  Such conditions can be common for a mid-season insecticide or fungicide application using a medium spray droplet quality. 

At 50 degree temperatures with 80 percent relative humidity and an average wind speed of 10 miles per hour, about 1percent of spray volume will drift 30 feet away from the nozzle.  If the temperature increased to 86 degrees with a lower relative humidity of 40 percent, and the wind speed remains unchanged, twice as much material, about 2 percent will drift 30 feet away from the nozzle. With average wind speeds increased to 15 miles per hour under the 86 degree temperatures, 40 percent relative humidity conditions, the amount of spray drift moving 30 feet away from the nozzle is nearly quadrupled to 3.5 percent. 

new spray drift fig 2

Figure 2.  Effects of crosswind speed and air conditions (80 percent r.h. at 50 degrees; 40 percent r.h. at 86 degrees) on drift distance for percentage of spray volume exiting an 8004 flat-fan nozzle at 40 psi.


In addition to making nozzle size and type, boom height, sprayer pressure and speed adjustments, drift retardant additives may also be considered. The addition of deposition aids or drift reducing adjuvants to a spray application can help in some situations to reduce off site movement and help to deposit and retain more of the pesticide product on target. There are many drift reducing products available. Some are formulated with AMS and other surfactants. Careful considerations of the pesticide products being applied, the label requirements in terms of additives, the composition and qualities of the drift retardant product, and the type of nozzles and sprayer setup are important before selecting and using a drift retardant. 



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.  Kristine Schaefer is a program specialist in the Department of Entomology serving on the Pesticide Management and the Environment team. Schaefer can be reached by email at or by phone at (515) 294-4286.

Corn Response to Postemergence Treatments

By Bob Hartzler, Department of  Agronomy

Much of the state has experienced below normal temperatures along with wet soils the past few weeks.  These conditions not only slow the growth of corn, it also alters plant development. 

Corn is likely to be more responsive to herbicides under these conditions.  We are seeing an increased response to postemergence herbicides. This is likely due to changes in the cuticle that allow greater movement of chemicals from the leaf surface into the leaf. 

In some situations, the response appears to be primarily due to the additives in the spray mix rather than actual herbicide. Symptoms associated with additives (surfactants, AMS, etc.) include chlorotic mottling or necrosis of leaves, and are likely to be short-lived with no negative impact on crop development. Herbicides with a lower margin of crop safety are more likely to induce a negative response under these weather conditions. 

On the positive side, activity on weeds should be excellent.  In nearly all situations the risk of crop injury is less than the benefits achieved with timely weed control.

leaf burn

An example of non-specific leaf burn.



Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164.

Corn Seedling Damage from Ammonia

By John Sawyer, Department of Agronomy

There have been several reports of corn seedlings being damaged from ammonia, even fall applied anhydrous ammonia. Ammonia injury has been noted in past springs and more frequently occurs with shallow placed ammonia, ammonia applied near the time of planting, urea placed near the seed, and with dry soil conditions.

In some cases the plant is killed, and in other cases the early root and plant growth can be impacted slightly or severely. Damage, potential re-growth, and season-long plant development rate can be aggravated by other stresses. For more information and pictures of this type of damage, please read a more in-depth article on the topic, Corn Seedling Injury from Ammonia.

damaged corn seedling


ammonia damaged corn seedling

Two examples of corn seedling damage from ammonia.


John Sawyer is professor with research and extension responsibilities in soil fertility and nutrient management.

Degree Days - Cool is cool

By Rich Pope, Corn and Soybean Inititative

The month of June opened with a cool week, with about 80 percent of the normal heat accumulated between May 31 and June 7.  Although a bit warmer weather would accelerate crop development, Iowa crops are progressing nicely in most areas.

Statewide, we have received about 90 percent of the heat for corn and soybean growth as in an average year. We are now 44 base-50 degree days behind average.Degree day accumulations as of June 7, 2009

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at or by calling (515)294-5899.

Size Restrictions for Postemergence Herbicides in Corn

by Bob Hartzler, Department of Agronomy

While most farmers and applicators have good intentions to apply herbicides in a timely fashion, it isn’t uncommon for equipment limitations or weather to result in delayed applications. Nearly all postemergence herbicides have restrictions on how late in the season they can be delayed. 

The most common reason for the restriction is the reduction in crop tolerance to the herbicide as corn size increases. For some products the application timing is restricted to prevent herbicide residues from exceeding tolerance levels in the grain. Regardless of the reason, it is important to follow the restriction.

Timely applications not only avoid problems with label restrictions, but they also provide more effective control due to treating weeds at a more susceptible stage and at a time when the crop canopy does not interfere with coverage, and  also are more effective at protecting yields from early-season competition than late applications.

corn size restrictions


Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164.

This article was published originally on 6/15/2009 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.