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7/20/2009 - 7/26/2009

Notes on Soybean Foliar Fungicide Applications in a Cool, Wet Year

By  XB Yang, Department of Plant Pathology

Last week I wrote an article on scouting soybean foliar diseases in this unusual summer, which has been cool and wet. As the weather trend of cool and wet continues, there are questions about the use of soybean fungicides to manage the risk of soybean foliar diseases. Now plants in many soybean fields are approaching the R3 growth stage which is critical to the effective use of foliar fungicide sprays. This report addresses a few questions on fungicide applications.

What is happening in the fields? At this week’s teleconference, many ISU agronomists reported observations of foliar diseases on corn or soybean. Soybean brown spot and frogeye leaf spot were reported with some reports of high levels of prevalence. My own field scouting also indicates the level of these diseases is higher than in a normal season, which is a not surprising due to the cooler and wetter than normal weather.

When to spray? R3 is time to pull the trigger if foliar disease risk is high in your soybean fields. Application at R1 or earlier did not pay off. Application at R3 consistently produces the highest yields across the region.  R3 is the growth stage when soybean starts to set pods.

How to assess the risk? Even when the season is cool across Iowa, the disease risk is not evenly spread. This year the summer temperatures are cooler than those during the past four seasons. Different fungal pathogens like different temperatures. There is little quantitative information available on how to assess soybean foliar disease risk under unusually cool temperatures, especially for frogeye leaf spot, brown spot, and Cercospora leaf spot. Here are few points that may be helpful:

1) If you consistently have positive spray results in your fields in the past four years, the chance is higher that you will have positive results this year.

2) Data from different sectors suggests that in the last four years in Iowa when precipitation was plentiful, more than 50 percent of the sprays yielded economical return and more than 70 percent of the sprays provide positive yield. 

3) If your fields are in high ground and never had foliar disease problems in the past, the risk would be low.

4) If you had white mold in the past and your soybean had a dense canopy in mid-July or early, white mold risk is high.

What will not work? Many report the occurrence of bacterial blight, a disease caused by a bacterium. The disease likes cool and wet weather and is mostly seen on the top canopy of the plants. Any fungicides will not be effective to reduce bacterial blight. Another disease that most fungicides have little effect on is soybean white mold. Additionally, it is too late to control white mold at R3 growth stage. Effective spray for white mold should be carried out at R1 growth stage because white mold fungus infects soybean plants through dead flowers.


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

Further Considerations for Foliar Fungicides on Corn and Soybean

By Daren Mueller and Alison Robertson, Department of Plant Pathology

We continue to receive e-mails and phone calls regarding the use of foliar fungicides on corn and soybean. Grain prices are down, fungicide prices are up, and this growing season, economics is likely playing a bigger role in the decision to apply a fungicide to either corn or soybean. We would like to bring up a couple more considerations for the decision making process, and also reiterate a couple of points made in previous articles

Iowa Soybean Rust Sentinel Plot Network
Soybean rust scouting in sentinel plots across the United States creates information for the real-time USDA ipmPIPE Soybean Rust Web site. The sentinel plots system is sponsored by the North Central Soybean Research Program, the United Soybean Board, and the United States Department of Agriculture. These plots of soybean or kudzu stretch from Florida to Texas and up through the Midwest. In 2008, sentinel plots were established in Mexico for the first time.

This year, 13 sentinel plots have been established across Iowa. We have been monitoring fields in these counties with the help of several ISU Extension field agronomists and staff at nine of the Iowa State University Research farms. Sampling began in the middle of June and will continue through September. To date, we have seen frogeye leaf spot, brown spot and more recently bacterial blight.

1. Fungicides control fungal diseases. Speaking of bacterial blight – after the heavy storms from a few weeks ago, this disease has been seen in many fields. Remember, this disease will NOT be affected by fungicides so be sure to properly identify the foliar diseases.

2. Timing of application. Currently, brown spot and frogeye leaf spot are the only two foliar diseases seen in Iowa that would be affected by a foliar fungicide application. Research done at ISU has shown the best time to apply a foliar fungicide to manage either of this diseases is R3. At this time, disease severity starts to rapidly increase. Research is underway to develop disease thresholds to aid the decision-making process.


1. Decisions should be made on a field by field basis. In some fields in southwest Iowa, gray leaf spot levels are at threshold levels, and further north one might argue eyespot is also at threshold. Scouting is important to determine in which fields disease pressure exists since many fields are not at threshold.

2. Fungicide applications may be profitable when disease pressure is high. Cost of a fungicide application is approximately $26 to $28 per acre and with grain prices hovering around $3.25 per bushel; some may choose not to spray. Greg Shaner (Purdue University) reported a mean yield response of 7.5 bu/acre when gray leaf spot disease severity on the ear leaf was greater than 5 percent at R5 to R6 (Figure 1). Therefore when disease pressure is high and a hybrid appears susceptible to disease, a foliar application of a fungicide can be profitable.

5 Percent Gray Leaf Spot

Figure 1. Image of gray leaf spot of corn created using Severity.Pro software representing 5 percent disease severity.

3. Increased foliar disease severity increases the risk for stalk rot diseases. Thus sufficient foliar disease pressure can result in stalk quality and standability issues at harvest. Carl Bradley at University of Illinois reported data supporting this last week (see Effect of Foliar Fungicides on Corn Stalk Quality).

4. Disease triangle. Environment is critical for further disease development. If the weather dries up, foliar disease progress will slow or stop, which would minimize the effect of disease.

5. Fungicides control fungal diseases. Goss’s wilt and Holcus leaf spot have both been reported in Iowa. Since both are caused by bacteria, a fungicide is not effective against these diseases

6. Grain moisture and quality issues. Grain moisture of corn sprayed with a fungicide may be one half to a couple of percent higher than unsprayed corn. Not only does this extra moisture result in higher drying costs but grain quality issues are more likely (see Corn Quality Issues Continue).



Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. He may be reached at at (515) 460-8000, Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. She may be reached at (515) 294-6708,

Eyespot and Gray Leaf Spot Severity Continue to Increase in Iowa

By Alison Robertson, Department of Plant Pathology

Gray Leaf Spot at Threshold Levels

Gray leaf spot (GLS) has reached threshold levels in some corn fields in southwestern Iowa and a fungicide application should be considered for these fields. As many as 5-20 gray leaf spot lesions are present on the ear leaf and, in some fields GLS lesions are also present on the leaf above the ear leaf. Approximately one in two plants are infected, and corn is at growth stage VT/R1. A couple of weeks ago, I summarized data from Greg Shaner at Purdue University, that showed fungicide applications can be profitable when disease pressure is high and the infected hybrid is susceptible to gray leaf spot.

The disease threshold developed for GLS in the mid 1990s was ‘lesions on the third leaf below the ear leaf or higher on 50 percent of the plants at tasseling’. The threshold was based on observations and results from fungicide trials. Since hybrids have changed and new fungicide products have become available, research to re-evaluate this threshold is in progress. However for now, this threshold stands. Scouting corn fields in southwest Iowa and throughout the state is highly recommended. Disease pressure does vary considerably from field to field, and most fields are not at threshold for fungicide application. High risk fields included continuous corn fields, particularly those with high surface residue, and river bottom areas. Hybrid ratings for GLS resistance should also be taken into account.

Risk of stalk rot increased

Severe infections of GLS may reduce grain fill by up to 50 percent. Furthermore, severe infections of GLS increase the risk of stalk rot, which may result in standability issues at harvest. Thus managing GLS with a fungicide may also help with stalk quality (See article by Carl Bradley, Extension Pathologist at the University of Illinois, “Effects of Foliar Fungicides on Corn Stalk Quality.”)


Further north, eyespot continues to make its presence known in some fields. In some fields disease severity is 1-2 percent (Figure 1) on the 2nd and 3rd leaf below the ear leaf, with scattered lesions occurring further up the plant. I know of no disease threshold for eyespot; however, scouting is encouraged in fields where the disease is present to keep an eye on disease progress. A fungicide application may be necessary to reduce eyespot severity and risk of stalk rot.

Eyespot Computer Illustration

Figure 1. Image of eyespot of corn created using Severity.Pro software representing 1 percent disease severity.

Other diseases occurring across the state include:
Goss’s wilt has been reported in Carroll, Sac, Audubon and Guthrie counties. Large gray to reddish or yellow lesions that extend down the leaf veins are characteristic of this disease (Figure 2). Dark green to black “freckles,” evident within the lesions are diagnostic. Goss’s wilt was reported across Iowa last growing season (see Goss’s Wilt Prevalent in Western Iowa for more detailed information).

Goss's Wilt Lesions

Figure 2. Large grey to reddish or yellow lesions that extend down the leaf veins resulting in extensive leaf blight are characteristic of Goss’s wilt.

Physoderma leaf spot has again been found in southwest and southeast Iowa. Symptoms of the disease are numerous very small (approximately 1/4” in diameter) round to oval spots that are yellowish to brown in color and usually occur in broad bands across the leaf (Figure 3). Dark purplish to black oval spots also occur on the midrib of the leaf. Physoderma brown spot is often misdiagnosed as eyespot or southern rust. Eyespot lesions have a light, almost translucent center while Physoderma do not. Pustules of southern rust produce thousands of orange spores than can be wiped off the upper leaf surface with your finger. Although there some fungicides are registered for Physoderma management, in the Midwest, this disease rarely impacts yield.

Physoderma Brown Spot

Figure 3. Bands of numerous small yellow to orange-brown spots across the leaf laminar and dark purplish spots on the midrib are characteristic symptoms of Physoderma brown spot.

Common rust is occurring across the state, particularly in southeast Iowa. Also northern corn leaf blight (large, tan, cigar-shaped lesions) and crazy top have also been found in in southeast Iowa. Both of these diseases are present at very low levels and do not currently require management.


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. She may be contacted at (515) 294-6708,

Late-Season Herbicide Applications in Soybean

By Bob Hartzler, Department of Agronomy

Slow canopy development and ample soil moisture have led to many fields being reinfested with weeds following postemergence herbicide applications.  While late-emerging weeds are much less competitive than weeds that emerge early in the season, at high densities they can impact yields, and regardless of density, these weeds can produce significant quantities of weed seed.  Thus, it is likely that many soybean fields will be treated in the next few weeks. 

There are several important considerations prior to making these applications:  1) application restrictions based on soybean stage, harvest intervals or crop rotation, 2) for glyphosate, limitations on the amount that may be applied per season, and 3) the ability of the late-season treatment to control the weeds.

Most products have label restrictions that might limit late-season use.  Glyphosate can be applied up to the R2 stage in Roundup Ready (RR) soybean.  A significant percentage of soybean fields in the state reached this stage by July 20; thus the option for glyphosate is rapidly coming to an end.  Flexstar/Reflex has a 10 month rotational restriction for corn, thus ruling out this option.  Other products have preharvest intervals that may restrict their use (Table 1).

Table 1.  Preharvest intervals for several postemergence herbicides in soybean


Preharvest interval

Assure II 










Poast Plus 








Select/Select Max 


The total amount of glyphosate that can be applied to RR soybean is limited.  The maximum amount allowed for postemergence applications per season is 2.2 lb a.e./A (equivalent to 2 qts of a 4.5 lb a.e./gal product (e.g. Roundup Original Max).  The maximum amount for burndown plus postemergence applications is 3.7 lb ae/A (3.3 qts of a 4.5 lb ae/gal product).  The rate limits on glyphosate are the same regardless of formulation, and are clearly stated on the product label.

The effectiveness of late-season applications is highly variable and difficult to predict.  It frequently is difficult to obtain good coverage of late-emerging weeds due to height differential between the weeds and soybean.  Weeds that escaped earlier applications may be too large for consistent control, or they may be hardened off from the initial application and less susceptible to the herbicide.  Unless there is an apparent reason for the failure of the first application, a second application is unlikely to provide control and is likely to enhance the selection of resistant biotypes.

In conclusion, weeds continue to be a problem in many fields across the state.  However, prior to making a late-season application, refer to the herbicide label for any restrictions that may limit the products use and consider the potential for the product providing effective control.


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.

Crops Put In a 4-Day Work Week

By Rich Pope, Department of Plant Pathology

For organisms, heat drives development.  If you can regulate your own heat like we humans do every day is a balmy 98.6 or so, so development every day is the same and can be measured by the calendar.  But for our field crops and most of the pests they face, development is based on the heat they get from the environment.  And, less heat means slower growth.  For the week of July 12--19 alone, Iowa normally accumulates 200 base-50 degree days  as a statewide average.  Last week that accumulation was 133,  or about four and a half "normal" days for the week. So, for the season since May 1, we are now 143 to 205 degree days behind normal. 

Iowa degree day accumulations through July 19, 2009

So what does that mean? Here are just a few fine points for discussion at field edges and coffeeshops:

  • Will soybeans be damaged by the cold during flowering?

Pod set is determined in part by biomass accumulation, which is slowed by the cold. But soybean has a great ability to flower over long periods, and warmth in the next 2--3 weeks should allow for ample pod set. We will start to have more concern if it gets to August and we are still unusually cool, but even 80-degree highs and 60-degree lows will help. 

  • What diseases might become issues?

Some diseases are more prevalent in the heat, some in cooler weather. Conditions may favor downy mildew on soybean in lowlying areas, and eyespot on corn is a concern in some areas, while southwest and south central Iowa have cornfields with significant gray leaf spot infections.

  • What about all the weeds?

Soybeans are growing slowly, and late to canopy. That allows light penetration and encourages weed growth. Add that to fields where weed control programs were delayed and weed pressures have built up, means that 2009 may be remembered as a year of the weeds.

  • It is an odd year, so does that mean soybean aphids are a huge concern?

So far, soybean aphids have been reported in some fields in the northern half of Iowa, but not genearlly at threshold levels. Soybean aphids bear watching because populations can grow rapidly with favorable conditions, which includes cool temperatures.

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.

This article was published originally on 7/27/2009 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.