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7/6/2009 - 7/12/2009

Scouting Foliar Diseases in a Cool Summer

by  XB Yang, Department of Plant Pathology

So far this season, weather in Iowa has been cool and wet.  NOAA’s mid-term weather forecast for the coming 30 days shows that Iowa will have cooler than normal temperatures with above normal precipitation. A cool and wet summer is favorable to soybean diseases, especially foliar diseases. If the weather trend holds, we should see more foliar soybean diseases late this season than a normal year. Fungicide is a means to controlling foliar diseases. To determine if a chemical control should be applied to manage the foliar diseases, scouting is critical.

White mold. This season’s cool and wet condition is ideal for white mold occurrence. The disease is caused by a fungus Sclerotinia scleortiorum which survives in the soil as sclerotia. The sclerotia will form a mushroom like structure which produces spores when soybean canopy is dense and close. For soybean fields which have a history of soybean white mold, the mushrooms will be produced around mid-July in Iowa with the disease showing up in early August. This season’s cool weather may promote an unusually early occurrence.

To control the disease one can use Cobra or other chemicals in a similar family. This measure is commonly used by many Iowa soybean producers in fields that have a high risk of soybean white mold. Some of these chemicals can induce disease resistance in soybean when applied. In early studies, applications were made during later vegetative growth stages to early flowering stage of soybean, a time frame before the fungus enters a soybean stem from senescent flowers.  Applications after early flower stage, especially this late (R3 stage), is unlikely to be beneficial. 

Foliar diseases. Frequent rains and cool weather also are good for several foliar diseases, such as brown spot, bacterial blight and frogeye leaf spot.

Occurrences of high levels of bacterial blight have been reported. The disease is caused by Pseudomonas syringae pv glycinea.  Young leaves are most susceptible to the bacterial infection; therefore, the disease is first noticed on the top of the plant. The angular lesions enlarge and merge to produce large, irregular lesions. The centers of old lesions often drop out, resulting in a ragged appearance of infected leaves. There is no treatment measure to control bacterial blight during a growing season. The only measure is to select a resistant variety pre-planting. If the disease is high this season, take a good note and avoid using the same variety in next season.

bacterial blight

Bacterial blight


Brown spot, a fungal disease, can be confused with bacterial blight in early soybean growth stage. Brown spot first becomes prevalent when the fall is wet and cool and severe in later season, as it primarily attacks aging leaves. Impact to yield from the two diseases depends on the final severity in the fall. In a normal season, soybean plants will grow out of this disease; this year’s situation is yet to see. 

brown spot

Brown spot


Frogeye leaf spot is a fungal disease with spots that look like frogeyes. The spot has a gray center with distinct reddish-brown margins. This disease infects younger leaves, so it first appears in the upper canopy. The disease occurs in mid- to late season, and is more common in fields nears river bottom fields.

frogeye leaf spot

Frogeye leaf spot


Fungal diseases can be controlled by using fungicides, but should be applied only if the disease risk is high. Fungicide applications will be economically effective when foliar diseases are severe. When disease is prevalent and severe in a season, application of fungicide is likely to increase yields.

Data from ISU Extension and Pioneer suggests that in a wet and cool season, more than 50 percent of the sprays will yield an economic return;over 70 percent of the sprays will provide a positive yield. The higher the foliar disease severity, the greater the return from the use of fungicides. 

If you have a high risk of foliar disease and decide to spray, keep in mind that application at R1 or earlier did not pay off. Application at R3 consistently produced highest yields.  Application twice in a season was no better than a single application at R3 in Iowa.




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

Corn: Uneven Early-Season Growth Can Mean Uneven Tasseling

By Roger Elmore and John Sawyer, Department of Agronomy 

Uneven growth reports on Iowa corn proliferated during the early part of the 2009 growing season.  As we transition into reproductive growth stages, expect to see variation within fields and hybrids as a result of various early-season stresses.

What occurred earlier this season?
Numerous potential reasons exist for uneven growth and development. Unfortunately, emergence issues and uneven growth do not disappear entirely during the growing season. Although the difference among plants may lessen, uneven tassel emergence will result because of these problems.  A review of articles posted in the ICM news reflects the early 2009 season and the types of stress encountered by the crop.

Early thunderstorms brought hail and strong winds destroying leaf area and killing plants.  Recent thunderstorms resulted in green snap and root-lodged corn. Twisted whorls slowed development of some plants across the state. First generation corn borer activity was high, relative to the early 2000s, increasing variation within fields. Because of cooler than normal temperatures and associated slower corn growth,  post-emergence herbicide applications negatively affected corn. Many observed seedling damage from ammonia - both fall and spring applied. Early season weather conditions resulted in wide ranges in corn plant color - from green to yellow-green to purple and some with stripes. Black cutworm activity and seedling disease incidence seemed above normal.  Armyworm outbreaks occurred in corn following cover crops with heavy residue. Intense weed competition occurred in some fields before post-emergence herbicide applications. Some replanting occurred because of emergence variability. Soil surface crusting and sidewall compaction impeded plant emergence and growth in some fields. Shallow planting depth resulted in cases of rootless corn.  Wet soils at planting and associated poor root and vegetative growth contributed to many of these problems.

In an apparent rerun of 2008, low areas flooded and problems associated with this included crazy top and nitrogen deficiency symptoms. In addition to all of this, wet weather delayed post-emergence nitrogen applications resulting in attempts to apply nitrogen using various foliar fertilization methods. 

In spite of all of these negative aspects of early corn growth, the USDA reported that as of July 6, 82 percent of Iowa’s crop rated good to excellent in stark contrast to the 57 percent rating during the same week in 2008. Most signs indicate Iowa’s corn crop looks good! Only four percent was rated poor or very poor –much better than the 13 percent of last year.  

As the corn crop hastens toward tasseling, expect to see variation within and among some fields directly resulting from one or more of the early-season stresses mentioned above.  If severity of the stress factor was high resulting in significant variation in plant development, expect lower yield potential.



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. John Sawyer is professor with research and extension responsibilities in soil fertility and nutrient management. Sawyer can be reached at or by calling (515) 294-7078.

SCN Females Are Now Apparent on Soybean Roots

By Greg Tylka, Department of Plant Pathology

Many Iowa soybean fields may be infested with soybean cyst nematode (SCN) but the infestations may not be known because SCN does not always cause obvious, above ground symptoms. Up to 40 percent yield loss can occur without the appearance of above ground symptoms.

A free, easy and reliable way to check fields for the presence of SCN is to dig roots of susceptible soybean varieties, then carefully crumble away much of the soil from the roots and look for adult SCN females on the roots. The SCN females are small, round, white objects on the roots and are about the size of a period at the end of a sentence.

SCN females are present now on the roots of soybeans in Iowa. They will be apparent on young roots of susceptible soybean plants through July, August and probably early September.   

It is easier to observe the nematode on soybean roots early in the season because the SCN females appear on new roots that can be easily dug from the soil surrounding the base of the stem of the plant.  Later in the season, adult SCN females appear on new roots that are located deeper down in the soil and farther laterally from the stem of the plant.

SCN is spread by the movement of infested soil, so checking roots of plants near the entrance of fields where farm equipment enters and along fence lines where wind-blown soil accumulates may increase the likelihood of finding SCN-infected plants.  Fields in which soybeans have been grown frequently in the past, and fields where soybean yields have declined over time for no apparent reason are logical places to start looking for SCN, too.

Additional information about the biology, sampling, and management of SCN can be found on the Web at


scn females

Adult SCN females (yellow arrows) on soybean root. The two larger round objects (orange arrows) are nitrogen-fixing nodules.



scn location

Areas of a field where SCN might be first discovered.


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.

Twisted Whorls - 2009

By Roger Elmore and Lori Abendroth, Department of Agronomy

Pale yellow upper corn leaves abound across Iowa as corn enters the teens of growth stages with some tasseling. These once sun-starved leaves entrapped in the whorl by more mature leaves will wave for a day or more above rapidly growing crop canopies before turning green.  Although incidences seem more numerous than in other years, the symptom is not unusual. We reported on it last year  occurring at about the same growth stages – tenth to twelfth leaf - and have an article posted on our website that discusses earlier-season situations.

As in other years, twisted whorl symptoms occur in different hybrids and fields at varying degrees of severity. Since its incidence is widespread in Iowa, environmental stress is the probable cause for most cases. Environmental stresses that can cause this are a wide range of temperatures, as experienced earlier this season, and hail or strong winds. Mid-season twisted whorls will follow the same pattern as their early-season counter parts; the majority should unfurl within a week. The resulting pale-yellow leaves will green as they accumulate chlorophyll.  If the deformity causes a delay in either growth or development it may reduce yield.  

In addition to environmental stress, other factors may cause these symptoms such as an over-the-top application of a fertilizer, herbicide or insecticide. Early-season (V5-V6) twisted whorl symptoms are often, but not always, associated with herbicides. For example, growth regulators, like dicamba or 2,4-D,  can cause twisted whorls especially when applied after emergence (See information from Bob Nielsen).  Some hybrids are likely more prone to twisted whorl than other hybrids. 

Twisted whorls are not a symptom of a plant disease. However, injuries caused as a result of twisted whorls can increase smut infections (see photo in last-year’s ICM article). Likewise, twisted whorls are not symptomatic of nutrient deficiencies.


twiste whorls

Yellow leaves emerging from a twisted whorl.  Ames IA, 29 June 2009. R.W. Elmore. 



twisted whorls 2

Leaves will remain ‘wrinkled’ for the remainder of the growing season even though the yellow leaves will green.  Ames IA, 29 June 2009. R.W. Elmore. 



Roger Elmore is a 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. Elmore can be contacted by email at or (515) 294-6655; Abendroth can be contacted by email at or (515) 294-5692.

Considerations for Soybean Insecticides and Fungicides

By Alison Robertson, Daren Mueller and Nate Bestor, Department of Plant Pathology Matt O’Neal and Rebekah Ritson, Department of Entomology

Several Iowa agribusinesses are offering soybean growers pest management plans that include applications of fungicide and insecticide. Although combining an insecticide and fungicide may be convenient, the results from our 2008 Pesticide Stewardship trials suggest this is a convenience that may not pay off.

2008 Statewide Pesticide Stewardship Trials
In 2008, we conducted trials at six locations in Iowa to compare calendar based applications of pesticides with applications of pesticides based on insect or disease thresholds. Treatments evaluated were fungicide alone, insecticide alone, or a tank mix of fungicide plus insecticide at either R1 or R3. Data collected from the trials included seasonal exposure of soybean to aphids (measured in cumulative aphid days; CAD), foliar and stem disease severity, yield and much more.

Insecticides. Aphid pressure varied across locations from very high at the ISU Northwest Research Farm (Peak aphids = 4073 per plant on August 17) to low at the ISU Southeast Research farm (Peak aphids = 114 aphids per plant on August 20).   The best yield-protection was observed when insecticides were applied at or near the 250 per plant threshold.

When applied on a calendar basis, the highest yields came from plots treated with a foliar insecticide applied at R3 than R1. However, we did not see yield reduction at every location as aphid populations did not always reach damaging levels. Using insecticides when they are not needed is not an efficient use of this tool and may, under some circumstances, result in greater pest problems later in the season.

Last growing season there were numerous reports of growers who had applied an insecticide at R1, when there was no insect pressure, but still had to apply a second dose of insecticide later in the season when soybean aphid reached threshold.

Fungicides. In general, severity of brown spot and anthracnose stem blight were reduced by an application of fungicide at R3. Greater yields were achieved with fungicides applied at R3 compared to R1 applications at all locations. This was no surprise to us, since others have reported similar findings with regards to fungicide timing on soybean. Furthermore, disease pressure within the soybean canopy is usually very low at R1, and increases from growth stage R3 onwards. Therefore, an application of fungicide as a disease management tool makes more sense at R3.

We will continue this research in 2009, but so far we can conclude that applications of either a fungicide alone, an insecticide alone or a tank mix of fungicide plus insecticide at R1 has not been beneficial. Applications of both fungicide and/or insecticide may cause aphid populations to be higher than with no application.

Growers are advised to scout for soybean aphid and apply insecticide when thresholds are reached. Similarly, to improve chances of a significant yield response, fungicide should not be applied before growth stage R3, and fields should be scouted to determine the level of disease pressure before application.

Fungicide Product Update
There were no major changes in product availability over the past year. Two Section 18 labels remain for soybean rust – Punch and Topguard. Remember that the ONLY legal use of these fungicides in Iowa should be targeting soybean rust. Punch will not be available for use on soybean in 2010 so check with local agriculture dealers if product is still remaining at the end of the season. A decision by EPA on Topguard should come sometime in late 2009.



Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Matt O’Neal is a professor of entomology with extension and research responsibilities. Nate Bestor is a research associate in plant pathology; Rebekah Ritson is a graduate assistant in sustainable agriculture. Robertson may be reached at (515) 294-6708 or by email at; Mueller at (515) 460-8000 or by emailing; O’Neal at (515) 294-8622 or by emailing

July 6 Crop and Weather Report

By Doug Cooper, Extension Communications

Highlights of the weekly Iowa State University Extension crop and weather report with Doug Cooper, extension communications specialist.

Extension climatologist Elwynn Taylor says weather conditions should favor corn and soybean during the next 10 to 14 days. The Southern Oscillation Index (SOI) continues to move toward a return to La Niña.

Rich Pope, integrated pest management specialist, reports soybean aphids continue to be found in many of the state's soybean fields. Scouting of fields is an important management step.

Roger Elmore, extension corn agronomist, talks about uneven fields, weeds and lodging – all issues corn producers need to stay on top of as the crop enters the 'dog days of summer.'

Degree Days - Temperature Roller Coaster

By Rich Pope, Department of Plant Pathology

Iowa crops continue in good condition as we finish one of the coolest July 4 holidays on record.  Each of the last three weeks have alternated between above and below normal temperatures. As of July 5, we have accumulated 87 fewer degree days statewide since May 1 compared with the 30-year average. That is roughly equivalent to 3.5 average July 4 days, so we aren't in terrible shape. The cool-warm-cool pattern appears now to continue as the forecast is for above normal temperatures through the middle of July.

Degree day accumulation map May 1 through July 5, 2009

Soybean aphid has been reported in several fields in the northern half of Iowa, and scouting is now the watch word. Many cornfields, especially where corn is following corn, are still uneven but that may even out as we experience good growing conditions.   


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.

Getting to Know the Aphids in Alfalfa

By Erin Hodgson, Department of Entomology

Aphids are common insects to see in field crops, especially in alfalfa. In Iowa, there are at least four different aphid species that can develop on alfalfa (Table 1). A recent report of cowpea aphid near Gibson, Iowa from extension field agronomist Mark Carlton prompted me to write this article. Many of you are quite familiar with pea aphid, but other species occasionally show up alone or in combination. Learning to distinguish the aphids takes a little practice with a hand lens, but is worth knowing in order to make sound treatment decisions.

General Description.  In general, aphids are soft-bodied and pear-shaped insects with walking legs. The main diagnostic feature of aphids is a pair of cornicles that resemble tailpipes, located towards the end of the abdomen. Sometimes the cornicles are highly reduced, making identification more difficult (e.g., spotted alfalfa aphid). In addition, all aphids have a piercing-sucking stylet and feed on plant phloem. Aphids excrete a sugar-rich honeydew that can promote a sooty mold that reduces photosynthesis. Some aphids are capable of vectoring plant diseases via persistent or non-persistent (i.e., dirty needle) transmission. Those species that vector disease are considered more economically important because low aphid densities can reduce quality and yield. Heavily infested plants will be discolored and stunted.

Table 1. Common aphids in Iowa alfalfa.

table 1


Pea aphid.  The pea aphid is found throughout North America and is the most common species in Iowa alfalfa. Adults are one-fourth inches in length, and body color ranges from light green to yellow, or pale pink. In addition to their large size, pea aphids can be distinguished from other aphids by the dark bands of color on the antennae. Pea aphids are in alfalfa the entire summer, but reproduction is dramatically slowed down when temperatures exceed 90 degrees F. Colonies prefer to feed on stems and newly expanding leaves. Pea aphids may turn leaves yellow and stunt overall plant growth when present in moderate numbers (50-100 per stem).

pea aphid



Blue alfalfa aphid.  The blue alfalfa aphid can be found throughout the United States, but is not commonly found in Iowa alfalfa. Often blue alfalfa aphids and pea aphids are intermingled on plant stems, but can be distinguished with a few common characters. Adults are three-sixteenth inches in length and bluish-green in color. Blue alfalfa aphids can be dull or waxy and have uniformly dark antennae, compared to shiny pea aphids with dark antennal bands. Blue alfalfa aphids are most productive during spring and early summer due to mild conditions; these aphids begin to decline when temperatures exceed 90 degrees F. Colonies prefer to cluster and feed on newly expanding leaves, but will move down to stems as the leaves mature and become crowded. 

blue alfalfa aphid



Spotted alfalfa aphid.  The spotted alfalfa aphid is found throughout the United States and is occasionally found in Iowa alfalfa. The spotted alfalfa aphid is smaller than most aphids in alfalfa, reaching one-eighth inches in length. This aphid is also distinguished because it is pale yellow with dark spots covering the abdomen. Unlike the pea aphid or blue alfalfa aphid, the spotted alfalfa aphid can successfully reproduce in warm temperatures (above 90 degrees F). Colonies prefer to feed on the lower portions of alfalfa, including stems, petioles and leaves. Spotted alfalfa aphids transmit a plant toxin while feeding and can cause early leaf drop, distinctive vein-banding or chlorosis.

spotted alfalfa aphid



Cowpea aphid.  The cowpea aphid is common throughout the United States and Mexico and is becoming more common in Iowa alfalfa. This small aphid is less than one-eighth inches in length, and is easily distinguished from other aphids in alfalfa because adults are shiny black and nymphs are dull grey. The base of cowpea aphid antennae are white, but gradually darken towards the tip, and the legs are white with dark “feet.” Colonies prefer feeding on newly expanding leaves, but cluster on leaves, blooms and stems. These aphids are most successful during early spring or late fall, and begin to decline when temperatures exceed 75 degrees F. Cowpea aphids transmit a plant toxin while feeding; moderate infestations can cause wilting and discoloration, and heavy infestations (more than100 per stem) can cause severe stunting, dieback or death.

cow pea aphid


Scouting and Thresholds.  Although aphids are considered secondary pests in alfalfa, sometimes they surpass treatment guidelines (Table 2). Scouting for aphids in alfalfa is relatively easy, and can be estimated by sweep netting or direct stem counts. Fields should be scouted weekly, especially in the spring and early summer. Count aphids on at least 30 stems or take at least 20 sweeps per field, and average the number of aphids per stem or per sweep. For large fields, consider sampling multiple areas to ensure coverage.

Aphid Management.  There are options to consider before using insecticides. Biological control, the use of resistant cultivars, and harvesting will often minimize aphids to tolerable levels in most cases. Fortunately, there are many different natural enemies to aphids. For those fields with consistent aphids, consider cultivars with at least moderate resistance to pea aphid.

Besides cost, there are two negative consequences for the overuse of insecticides on aphids: resurgence and resistance. Resurgence happens as a result of killing the primary insect pest (e.g., alfalfa weevil or potato leafhopper) along with the biological control agents in the field. Aphids on the undersides of leaves and lower stems will survive and the colony thrives without predators or other competition. Historically, aphids can build up genetic resistance to insecticides when complete coverage is not achieved over multiple applications. Any aphid survivors and their offspring are considered genetically resistant to that class of insecticides at any dosage. Because aphids have multiple generations with clonal reproduction, genetic resistance can build up faster than in other insects.

Insecticides should only be applied if they exceed treatment guidelines (Table 2). Several products are registered in Iowa for aphid control in alfalfa: beta-cyfluthrin, chlorpyrifos, cyfluthrin, gamma-cyhalothrin, lambda-cyhalothrin, methomyl, permethrin, pymetrozine, and zeta-cypermethrin. Use sufficient volume and pressure to ensure contact with aphids on the lower parts of the plant.

Table 2. Treatment guidelines for aphids (per stem) in alfalfa.

table 2



Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847.

Signs of Crazy Top in Corn

By Fanny Iriarte and Alison Robertson, Department of Plant Pathology

The Plant Disease and Insect Diagnostic Clinic recently received some corn samples with symptoms and signs of crazy top, a disease caused by Sclerophthora macrospora

Symptoms of this disease vary greatly with time of infection and degree of host colonization by the pathogen.  Generally, excessive tillering, rolling, and twisting of the upper leaves appear first.  Later, infected plants produce a “leafy” tassel. 

Crazy top develops when soils have been flooded shortly after planting or before plants are in the four to five leaf stage.  Pooling of soil and water in the whorl of small plants can also lead to infection. Saturation for 24-48 hours is sufficient for infection to occur, since it allows the overwintering soilborne oospores to germinate and produce zoospores that swim in the water to infection sites on corn.  

Diseased grasses may also be a source of inoculum.  According the the Compendium of Corn Diseases infection occurs at a wide range of soil temperatures.  Although seed transmission of the pathogen has been demonstrated from freshly harvested seeds, it is generally considered unimportant in the spread of the pathogen. 

Crazy top is not a severe disease but it can cause significant losses in low-lying areas of fields that are prone to flooding, since infected plants are often barren. This disease is best managed by improving field drainage or by avoiding low, wet areas in fields. Rotation may be helpful if care is taken to keep rotational fields free of grassy weeds. 

Therefore, there’s not much that can be done to manage this disease in season.  Pulling up and discarding infected plants can reduce buildup of inoculum in the field, and thus reduce the risk of disease in subsequent years. 

crazy top



Fanny Iriarte is a plant pathologist at the Plant Disease and Insect Diagonostic Clinic. Alison Robertson is an assistant professors of plant pathology with research and extension responsibilities in field crop diseases. Iriarte may be contacted at (515)294-5374 or by emailing Robertson may be contacted at (515)294-6708 or by emailing

Decision for Fungicide Applications on Corn Looming

By Alison Robertson and Daren Mueller, Department of Plant Pathology

Even after two full years of multiple trials, applying fungicides to corn is still not a clear-cut decision for growers. Many growers have experienced yield increases; some have not seen yield increases and even have seen yield losses from fungicides. At a cost of $24 an acre, this is not a decision to take lightly.

What the research shows
Yield responses of hybrid corn to foliar fungicide application vary widely. In Iowa in 2008, the mean yield response in small plot trials was 3.5 bu/acre (Robertson et al, 2008).  Similarly, the mean yield response of replicated strip on-farm trials conducted by Iowa Soybean Association On-Farm Network was 3.5 bu/acre. Greg Shaner at Purdue University compiled data from 13 universities in 2008 across the U.S. and reported a mean yield response of 3.6 bu/acre. However, an average yield increase of 7.4 bu/acre was reported by Pioneer (Jeschke, 2009) in their evaluation trials across the country and industry has reported 12-16 bu/acre yield increases

While university and industry reports differ in the magnitude of yield responses, one thing we agree on is that yield response is greater when significant disease pressure is present (Table 1). 

       corn fungicide table


Therefore, scouting corn fields prior to an application of a foliar fungicide application can be a worthwhile investment. Fields should be scouted near or at tasseling. A minimum of 100 leaves should be examined per field. At several places within the field, examine the ear leaf and leaves below of several plants to determine if disease is present. If foliar disease is present on the majority of these leaves, a foliar fungicide application may be warranted. 

Factors that may affect disease development, and thus the decision to apply a fungicide, include hybrid susceptibility to disease, current and predicted weather conditions during grain fill, disease history of the field and previous crop. 

Some basic economics that should be considered before applying fungicides are:
• cost of the fungicide, plus application
• price of corn
• drying costs (corn sprayed with a fungicide often is wetter at harvest)
• harvestability (corn sprayed with a fungicide may have better standability than unsprayed corn)

All these factors affect the economics of a fungicide application.

Other considerations
Field guides
- Two new resources are available for identifying corn diseases. The most common foliar diseases in Iowa are gray leaf spot, common rust and eyespot.  Other diseases include Northern corn leaf blight and southern rust. A Corn Field Guide (CSI 001), which includes all diseases, insects and disorders, and a pocket card titled ‘Scouting for corn diseases’ (CSI 0002) are now available from ISU extension.

Application timing - One thing we have consistently seen over the past few years in several of our on-farm and small plot research treatments is a negative yield response from a portion of the trials. While we have not identified any specific reason for these negative responses, one possible cause this past year may have been the uneven crop caused by flooding. Applications prior to tasseling may affect ear development, specifically when an NIS or crop oil is used (Nafzinger, 2008; Nielsen et al, 2008). This season, many fields once again have uneven corn, so if a fungicide is to be applied, we advise waiting until the whole field has tasseled.

Next year’s crop - Lastly, many growers are considering no-till soybeans after corn.  We have heard reports from the field that residue of corn to which a fungicide has been applied takes longer to breakdown. This has made planting soybean difficult the following year. 


Jeschke, M. 2009.  Foliar fungicide effects on corn yield.  Crop Insights 18:1-5

Shaner, G. 2008. Regional Corn Fungicide Trials – 2008. Presented at the 2008 Corn Disease Management Working Group Meeting, Chicago, IL.

Robertson, A.E., Shriver, J. and Peckinovsky, K. 2008.  Increasing the odds of a profitable yield response to foliar fungicide application on corn.  Proceedings of the 20th Annual Integrated Crop Management Conference, Ames, IA


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


This article was published originally on 7/13/2009 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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