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7/11/2011 - 7/17/2011

Corn and “a Big Long Heat Wave on the Way”

By Roger Elmore and Elwynn Taylor, Department of Agronomy

As the quote in the headline suggests, high temperatures are approaching Iowa! The short-term forecast includes daily high temperatures above 90 F starting Saturday, July 16 through Thursday. Highs above 93 F are forecast for Sunday through Thursday. The long range forecast, both the 6 to 10, and the 8 to 14 day forecasts (see Figure 1), call for above average temperatures, and average or below average precipitation. Low daily temperatures will reach only into the mid-seventies. Corn will rapidly accumulate heat units, as many as 30 per day! How will corn fare?


Corn development stages

As of Monday,  July 11, USDA-NASS estimated 4 percent of Iowa’s corn tasseled and 1 percent silked, both considerably behind average. Late planting this spring, cool soils and close to average heat unit accumulation since is responsible for slow crop development. This report summarizes crop status through Sunday, July 10. Both silking and tasseling have since progressed rapidly.  Most fields are either very close to tasseling and silk, or those landmark processes have already started.


Water use, pollination and silking

Unfortunately, maximum corn water use occurs at tasseling, approaching 0.35 to 0.40 inches per day (See Figure 1 in the Univ. of Nebraska publication). Our better soils can hold two inches of crop available water per foot. Availability of pollen is usually not a problem with modern hybrids for a couple of reasons. 

First, at its peak a plant produces 500,000 pollen grains per day! There is usually more than enough pollen to go around. Secondly, most pollen shed occurs during the morning when temperatures are cooler and moisture stress less evident.

Unfortunately, when stress occurs, pollen shed is often not affected while silking is delayed. Breeding efforts over the last few decades though have improved stress-tolerance of hybrids significantly. The time between pollination and silking – also known as the anthesis-silk interval, ASI - is very short with modern hybrids; sometimes silking actually precedes pollen shed. The shorter ASI results in few barren plants. In older hybrids, silking always followed initial pollen shed by at least several days.

Stress during pollination and silking could result in shorter ears, increased tip back and fewer kernels per ear. All of these contribute to less yield potential.


Some positives

We have a few things going for the corn crop as heat approaches. Soil moisture conditions are excellent across the state: Near normal – in north central, northeast, and east central Iowa;  ‘Unusual moist’ in west central and central Iowa; “Very moist” in northwest Iowa; “Extremely moist” in south central and southeast Iowa. Likewise, the crop moisture index shows that all of Iowa sits at the midpoint, “Slightly dry/ Favorably moist.” A good share of our soils have high water holding capacity. As the heat spell continues, the differences in mid-afternoon corn leaf rolling between soils with better moisture holding capacities than others will be evident.

Figure 1. Temperature outlook forJuly  23-29 , 2011 by Climate Prediction Center.  Temperatures are likely to be in the warmest one-third of all years in the eastern three-fourths of the 48 states and very likely to be in the warmest one-third in the eastern Corn Belt. From:


Earlier records of high temperatures and drought

Iowa experienced 100 F temperatures May 6 and 7, 2011 in numerous locations. Another unusually hot period developed in June and the developing July heat appears to be another in the series of 30-day hot/cool cycling that began in October 2010. This type of temperature cycling is typical of strong La Niña conditions and may be the last extreme cycle as the La Niña event diminished to neutral conditions July 1. 

The 100 F temperatures in early May are reminiscent of the drought of 1988, but that year had consistently hot and dry conditions as opposed to the 30-day cycle this year. Accordingly the 1988 crop was severely damaged by early July, and fortunately that is not the case in 2011. A point of concern is the forecast distribution of the warm temperatures: warmer than usual in the East and cooler than usual in the West with the transition approximately at the Continental Divide. Such distributions tend to persist for up to six weeks and consistently result in below trend corn yield for the U.S. as was the case in 2010.


High temperature impacts on corn

The forecast heat wave may have a double impact on the crop. The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1 percent for every 12 hours of leaf rolling - except during the week of silking when the yield is cut 1 percent per 4 hours of leaf rolling. Unfortunately, most of our crop will be silking next week.

The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July, the crop does not have a measurable yield response to one day of temperatures between 93F to 98 F. However, the fourth consecutive day with a maximum temperature of 93 F or above results in a 1 percent yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2 percent loss; the sixth day an additional 4 percent loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of leaves then becomes likely and very large yield losses are incurred. 

Generally a six-day heat wave at silking time is sufficient to assure a yield not to exceed trend (Iowa trend yield is near 174 bushels per acre). Should warmer than usual nights continue for a six-week period the state is assured a below trend harvest. None of these three factors are assured but the possibility is very real.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or (515) 294-6655. Elwynn Taylor is extension climatologist and can be reached at or by calling (515) 294-1923.

Crop Damage Meeting July 19 at Marshall County Extension Office

In response to the recent storm damage to crops, Iowa State University Extension, Key Cooperative, and Centrol of Iowa will be holding a joint meeting at the ISU Extension - Marshall County office in Marshalltown, Iowa. The field event will begin at 9:30 a.m.

Mark Licht, ISU Extension field agronomist, and Ryan Risdal, Centrol of Iowa agronomist, will discuss agronomic considerations for storm damaged crops and future management decisions. Alison Robertson, ISU Extension plant pathologist, will be on hand to discuss the potential for corn diseases and fungicide options. Al Kiewiet, USDA FSA County Executive Director, will discuss crop disaster programs. Key Cooperative crop insurance specialists will be on hand to assist with questions.

The Marshall County extension office is located at 2608 S. 2nd Street, Marshalltown, below the USDA Service Center.

For more information about the program, please contact the Marshall County extension office, phone: 641-752-1551 or Mark Licht, ISU Extension field agronomist, phone: 712-790-7233 or

Managing Glyphosate Failures

By Bob Hartzler and Mike Owen, Department of Agronomy

In the past week we have received numerous calls from farmers, agricultural chemical dealers and industry representatives regarding waterhemp and horseweed/marestail surviving glyphosate applications made in late June and early July. Although there are numerous reasons why a herbicide application might fail at controlling weeds, we are certain that a significant percentage of these failures are due to the presence of glyphosate-resistant biotypes in the field.

The common question is what can be done to rescue the situation in the field. Unfortunately, at this time of the year there are few options. If glyphosate failed earlier to control the weeds, it is unlikely that a repeat application will do any good in controlling the surviving weeds. The PPO inhibitors (Reflex, Cobra, Phoenix, UltraBlazer, etc.) are the other postemergence option available for waterhemp in soybean. However, the label restrictions regarding weed size are long past and thus these herbicides are unlikely to provide affective control. There also is a potential for serious crop injury with the high temperature forecast for the coming week.  Furthermore, the harvest interval restrictions for each of these products should be reviewed.

Although not popular with the majority of growers, mechanical control is really the only available option to manage escaped and/or herbicide-resistant waterhemp and horseweed/marestail at this time. If only scattered plants are present in the field, hand-weeding the field would be worth the effort since this will slow the establishment and spread of resistance within the field. If the presence of surviving waterhemp and horseweed/marestail is more widespread, a trip back in time using a cultivator is really the only option to reduce the problems with these escapes. 


Bob Hartzler and Micheal Owen are professors of agronomy and weed science extension specialists with responsibilities in weed management and herbicide use.

The Effect of Spraying Fungicides at R1 or R3 on Soybean

By Nathan Bestor, Daren Mueller and Alison Robertson, Department of Plant Pathology and Microbiology

Across Iowa many growers are starting to ask if or when to apply fungicide on soybeans in 2011. There are essentially two options being discussed. The first is to apply fungicides at R1 (flowering). This application could be tank-mixed with the last application of glyphosate. The second option is to apply fungicide at R3 during pod formation. This application could be tank-mixed with an insecticide or applied alone. Which application timing is best? Which products best protect yield?

To answer these questions we conducted soybean fungicide trials at various locations in Iowa from 2008-2010. We evaluated the yield response of soybeans to applications of a strobilurin (Headline ®), a triazole (Flusilazole or Domark ®) and a premix of strobilurin + triazole (Stratego YLD ®). We also assessed foliar disease severity at R5. Finally we calculated the probabilities that an aerial application of a fungicide product would “break even” at a specific site year when prices of beans were estimated at $10, $13, and $16 per bushel. The cost of fungicides was estimated using information gathered from area cooperatives and field agronomists.

Our results are summarized in Table 1. 

  • In all years, disease severity was low. 
  • Brown spot was the most common disease and was found virtually only in the lower canopy. 
  • The yield response of soybeans was greater with an R3 application rather than an R1 application. So, an R3 application of a fungicide was more likely to break even than an R1 application
  • The chance that an R3 application of a fungicide would break even ranged from 7 to 99 percent.
  • Products that contained a strobilurin as one of the active ingredients were more likely to  break even compared to a product containing only a triazole. 

Spray when disease could impact yield

Although these data suggest that nearly 60-70 percent of R3 applications of a fungicide containing a strobilurin at least break even, spraying when disease is present will further increase your chances of getting your money back. If brown spot remains in the lowest third of the soybean canopy – it probably will not impact yield. If the disease moves into the middle third of the canopy, brown spot can start to reduce yield. We saw this at one of our locations in 2010. At that location, the mean yield response due to a fungicide application at R3 was nearly double the responses at other locations (5-6 bu/ac vs 2-4 bu/ac).  

Resistance to fungicides reported in 2010

A caution when spraying fungicides – scout fields after applying the fungicides and look for applications that do not appear to affect disease severity. In 2010, resistance to the strobilurin fungicides (Headline ®, Quadris ®, one of two active ingredients in Stratego YLD ® and Quilt ®) in the fungus that causes frogeye leaf spot was reported in Tennessee, Kentucky and Illinois [University of Illinois].

Funding for this project was provided by the Iowa Soybean Association, Bayer CropScience, DuPont, Valent and BASF.



Nathan Bestor is a graduate assistant in the Department of Plant Pathology and Microbiology. He can be reached at 515-294-1741 or Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at 515- 460-8000 or by email at Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at or phone 515-294-6708.

Crop Minute - Scout For Soybean Aphids

Erin Hodgson, ISU Extension field crop entomologist, talks about the statewideneed to scout fields for soybean aphid during this week's Crop Minute. High numbers of aphids have shown up in central Minnesota, and soybean aphids have been detected in almost every part of Iowa, except the southeast corner. Now is the time to scout fields.

Thoughts on Spraying Downed Corn with a Fungicide

By Alison Robertson, Department of Plant Pathology and Microbiology

Severe thunderstorms early Monday, July 11, flattened over 100, 000 acres of corn from southern Story County to Dubuque County in east central Iowa. Much of the corn was at late vegetative stages, which are critical stages of development. I have had several questions about possibly applying fungicide to these fields. I don’t know of any data to reference, but since one of our fungicide trials was extensively damaged in the storm, maybe this time next year we will have some data since we intend to carry on with the trial. Anyway, these are my thoughts:

  • As ISU field agronomist Jim Fawcett points out in his weekly newsletter, the first thing to do is determine how much green snap has occurred and how much damage is root lodging. 
  • Roger Elmore also summarized the types of damage that can occur to corn and potential yield losses 
  • Both Elmore and Fawcett refer to research done in Wisconsin, which is summarized in an ICM newsletter article that found yield losses of about 10-20 percent when corn was totally flattened at V14-V18.  
  • Flattened corn may provide a better microclimate for disease development because leaves are closer to inoculum source, less wind movement through canopy, higher humidity and longer periods of leaf wetness that may favor infection.
  • Physical damage to the plants may lead to increased bacterial disease, which a fungicide would not control, or smut for which no products are labeled.
  • The chances of a fungicide application breaking even or being profitable is reduced in fields with lower yield potential. Fungicide applications should be targeted to fields with high yield potential.
  • Fungicide applications prior to tasseling can affect ear formation particularly when an adjuvant is used.
  • Fungicide coverage of flattened corn will be more sporadic because penetration through this altered canopy will not be as thorough.
  • Strobilurin fungicides have been reported to improve host plant tolerance to yield-robbing environmental stresses, such as drought, heat, cold temperatures and ozone damage. Does flattened corn count as such a stress? I honestly don’t know.
  • Since lodging reduces the ability of the plant to take up water and nutrients, these fields may be more at risk for stalk rot issues towards the growing season. Improved stalk quality with fungicides is more common when foliar disease severity is high; although there are a few reports of reduced stalk rot with a fungicide application in the absence of severe foliar disease.  Applying a fungicide to flattened corn to reduce stalk rots later in the season is likely a high risk decision. 


Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at or phone 515-294-6708.

Storm Damage Meeting Near Vinton

By Jim Fawcett, ISU Extension field agronomist

I never thought I’d live to see a storm that rivals the 1998 storm that had 100 mph winds in a 20-mile swath over 100 miles long, but the storm Monday morning, July 11 apparently had straight line winds of up to 120 mph in a swath of 20 miles or so wide and traveled from south of Ames up through Tama, Benton, Linn and Jones counties and into Dubuque County. There is likely over 100,000 acres of corn that is flattened, in addition to thousands of trees snapped off, grain bins blown over and farm buildings destroyed.

The good news is it looks like the vast majority of the corn is flat because of root lodging and not green snap. I’ve looked at fields from Martelle in Jones County over through Vinton to Toledo, and every flattened field I looked at had very little green snap. With the 1998 storm there was a tremendous amount of green snap, which resulted in large yield losses. The 1998 storm occurred on June 29, so much more of the corn was in the more vulnerable V10-V12 stage when stalks are more brittle. Most of the corn is in the V14-V18 stage now, so less subject to green snap. The heavy downpours that occurred at the same time may have also helped by saturating the topsoil and allowing roots to shift rather than breaking stalks.

Crop Storm Meeting

There will be a crop storm damage meeting Thursday, July 14 at 3 p.m. at the John Olson farm site, 2.5 miles south of Vinton at the intersection of Highway 218 and 63rd Street, northwest corner. There will be signs. The meeting will cover what to expect from recent storm damage to crops, management decisions, livestock feed options and Farm Service Agency programs. Speakers at the meeting will be Jim Fawcett, ISU Extension field agronomist, Jim Jensen, ISU Extension management specialist, Denise Schwab, ISU beef program specialist and Pat Derdzinski, Benton County Farm Service Agency director.

There is also a lot of seed corn affected in the storm damage area. In general, the lodging is not as extensive in the seed fields, because the corn is shorter, but unfortunately there are many seed fields where the lodging is great enough so that it is difficult to impossible to walk through the fields. This means they cannot be detasseled, so they are a total loss. Many seed fields may be disked up.

The main thing to do at this point is check with your insurance to see if wind damage is covered. Many policies do not cover wind, or if they do it is only covered if there is green snap.


Jim Fawcett is an Iowa State University Extension field agronomist serving eastern Iowa. He can be reached at 319-337-2145 or by emailing

Updated Soybean Aphid Field Guide Available

By Erin Hodgson, Department of Entomology

Iowa State University (ISU) Extension and the Iowa Soybean Association (ISA) have released a collaborative second edition of the Soybean Aphid Field Guide. The guide’s authors are Erin Hodgson, ISU Extension entomologist, and Matt O’Neal, ISU research entomologist. 

Every year more is learned about the biology of soybean aphids and new ways to prevent this pest from causing yield loss. These insights have been made possible through funding from the soybean checkoff program. These discoveries and tools are incorporated into the revised edition of the guide in an effort to return this checkoff-funded research back to soybean farmers.

The second edition of the Soybean Aphid Field Guide is expanded in every section. Research from around the region is incorporated and a more comprehensive management program has been developed. Growers are encouraged to use multiple tools to protect soybean yield against the soybean aphid.

One of the biggest additions to the toolbox of aphid management strategies is aphid-resistant soybeans. The guide includes a review of the research that led to aphid resistance becoming a common component of soybean genetics for many current and future commercial soybean varieties.

Funding for printing and distribution of the guide was provided by the Iowa soybean checkoff and ISU Extension. Printed copies of the Soybean Aphid Field Guide can be ordered from the ISU Extension Online Store at or by calling 515-294-5247; or obtained by contacting the Iowa Soybean Association at 800-383-1423 or viewed online.


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847.

Wind and Corn

By Roger Elmore, Department of Agronomy
Straight line – derecho – winds sometimes exceeding 100 miles per hour ravaged central, eastern and north-eastern Iowa corn fields early Monday morning, July 11. The storm followed its destructive path into Wisconsin and northern Illinois. Winds in Iowa leveled corn in the worst affected areas – to less than knee high. This follows glowing reports of how well the crop looked, before the wind came along. The lodging was not likely related to corn rootworm feeding.

Crop and soil conditions

Monday’s USDA-NASS Crops and Weather report listed 82 percent of Iowa’s corn in the ‘Good’ to ‘Excellent’ category;  only four percent of our crop had tassels and 1 percent silked. This differs from the  average of 25 percent tasseled and 13 percent silked, and dramatically contrasts with that of last year when 47 percent was tasseled and 24 percent silked. By the end of this week, tassels and silks will emerge on a good share of our acres.

According to the current USDA- NASS report, top- and sub-soils across most of Iowa had ‘Adequate’ to ‘Surplus’ soil moisture. In the areas with the worst wind damage, central and east central Iowa, 83 to 84 percent of the top soils were listed as having either ‘Adequate’ or  ‘Surplus’ and 83 percent soil moisture and 92 to 93 percent of the sub-soils ranked in those highest categories too. Last year though, 99 to 100 percent of central and east central top- and sub- soils ranked in that category for the same week.

Types of damage

Corn is susceptible to root lodging, pinching and greensnap prior to silking. All three likely happened to varying degrees in the damaged area although reports are still coming in at this time. Let’s talk briefly about each one:

Root lodging

 “Roots act as guy ropes and props that anchor corn plants against lodging. Initially both windward and leeward roots play a role with slow wind speeds, however, as wind speeds increase, the role of the windward and leeward roots change. During high wind events, windward roots are pulled from the soil while leeward roots are pushed into the soil. Although it might make sense that lodging comes from windward roots that fail to hold fast to the soil, the fragile link in rooting structures is the weakness in compression of the leeward corn roots from bearing large downward loads. A rotation of 10 degrees is enough to cause the leeward roots to buckle and the plant to lodge.

Root mass reaches its maximum at silking (R1). Brace roots provide support to the stalk and are of considerable importance in "resurrecting" plants root lodged by strong winds. Fortunately, plants root lodged before R1/R2 [silking and blister stages] are somewhat able to compensate for the canopy disruption caused by the lodging. After a couple of days, the upper portions of these plants resume a vertical growth pattern, ‘goosenecking.’ Although this rearrangement of the crop canopy may limit potential yield losses, it does make harvesting slower and increases the potential for ear loss during harvest.” (Taken from an Agronomy Extension Corn Production web article.)

Corn in a University of Wisconsin study with moist soil was lodged by hand at different growth stages. Lodged corn in the V17 to R1 (silking) stages reduced yields by 12 to 31 percent. My colleague, Joe Lauer, University of Wisconsin corn agronomist, summarized this data as well as the greensnap data in an Agronomy Advice article yesterday.

corn lodging 
Corn root lodging in eastern Iowa field.


“Corn is most susceptible to greensnap prior to tasseling, when it is rapidly growing….We've learned from previous greensnap events in Iowa and Nebraska that yield loss is directly related to the amount of stalk breakage that occurred. In other words, yield loss from broken plants is directly related to stand loss. If 10 percent of plants are broken this will result in a 10 percent yield reduction. This is due to a lack of compensation ability to this reduced plant competition by the corn plant this time of year. The maximum of what that ear is capable of has already been largely determined.

Hybrids vary dramatically in their tolerance to greensnap. Several companies provide growers with greensnap ratings that may prove useful in selecting less susceptible hybrids. Stage of growth affects breakage too as mentioned above. Factors that increase early season growth tend to increase breakage susceptibility, such as high N, P and K rates; spring-applied N; tillage; and high organic matter.” (Taken from an Agronomy Extension Corn Production web article.)

Most of this previous research was with breakage below the primary ear node.

However, new data indicates that the impact may not be as severe as a 1:1 loss.  Melanie Knaak - in her creative component for her Distance Masters Degree in Agronomy at Iowa State University, 2011 - found that yields resulting from breakage below the ear were not as severe as we previously thought. In the two years of work in Minnesota, the percent broken:percent yield reduction ranged from about 1:0.50  to 1:0.73, considerably better than the older reports of a 1:1 loss. Breakage above the ear had even less negative yield consequences. This new information suggests that standing plants compensate to some degree for the loss of their neighboring broken plants.

In the worst case situation, yield reduction from greensnap may range up to a 1:1 percent broken : yield loss. It is possible that these losses will be as low as 1:0.73 or even 1:0.50 losses.

Little data exists to my knowledge on the outcome of pinched plants. They will certainly remain alive and attempt to gooseneck so that the top of the plant will be upright. I would expect yield losses somewhere between that of root-lodged plants and those of plants broken by the winds - greensnap.

A thought on seed production fields
The research summarized above relates to grain production fields. Unfortunately, the area affected by the winds also contains large areas of seed production fields. In addition, to the yield reductions mentioned above, rogueing, detasseling,  field inspection and other operations will be dramatically affected because of the disruption in the canopy including position of the female rows  and their orientation.

A few things to be thankful for….
Belief it or not, yield reductions from the derecho winds could have been even more severe. Let me explain:

1) Soils at the time of the derecho were wet as discussed above. This is why root lodging occurred. If soils were drier, we would have experienced more greensnap and thus higher yield losses.

2) Also as mentioned above, crop development is behind normal this year because of the wet spring and delayed planting. If more of our crop was tasseling or silking at the time the derecho hit, our losses would have been much greater.


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

Corn Leaf Diseases and Soybean Stem Diseases Prevalent Throughout Iowa

By Alison Robertson, Department of Plant Pathology and Microbiology

If you are not out scouting corn fields for leaf diseases, and soybeans for disease, you should be. I have seen and had several reports and emails of various diseases occurring throughout the state. Most diseases on corn are hybrid specific, so scouting can be targeted to those hybrids that are rated on the susceptible end of each company’s rating scale.

Northern leaf blight (NLB) and gray leaf spot (GLS) can each be found in the lower canopy of corn in southern and central Iowa depending on the hybrid. Infection by the NLB fungus is favored by frequent rains and overcast conditions, like those we had in late June. Warm and very humid conditions favor GLS development.

Our threshold for application of a fungicide is: corn foliar disease (not including common rust) observed on the third leaf below the ear or higher on 50 percent of the plants. We have a fungicide trial evaluating this threshold in Ames. This same trial is being done in by colleagues in Wisconsin, Illinois and Ohio and is part of a research project funded by the USDA. We have inoculated the trial with the NLB and GLS fungi. Severe NLB disease has developed in the inoculated plots.

Holcus leaf spot has also been reported but at very low incidence and severity – a few plants with as many spots. Holcus is a bacterial disease.  Warm (75-85 F), wet, windy conditions early in the season favor infection and the development of Holcus leaf spot. Symptoms are light tan (sometimes almost white), round, oval spots, which may appear water soaked at the margins or have a light brown border occur on the lower leaves. The spots are initially about 1/4 inch in diameter, but sometimes grow larger and coalesce into irregular spots and streaks of dead tissue. Later the lesions dry out, turn light brown and have a papery texture. 

Holcus spot symptoms can resemble those of eyespot. Eyespot symptoms are smaller, round spots with a distinct brown border and a yellow halo. Eyespot was reported earlier in the season on specific hybrids, but I suspect the warm weather we have had since the beginning of July has stopped further disease development. Eyespot is favored by cool (70s), wet conditions.
I continue to receive reports of Goss’s wilt; once again it is very hybrid-specific. Physoderma brown spot is also prevalent.


Soybean stem diseases

Soybean stem diseases, Phytophthora root rot and northern stem canker have also been reported. Both diseases cause brown lesions to develop on the outside stem of plants. Infected plants wilt and die. Phytophthora is usually more prevalent in low wet spots in a field that are prone to flooding. The pathogen, an oomycete, infects the roots of plants via swimming zoospores, thus the brown lesion on the stem extends from the tap root for several nodes above the soil level (Figure 1). Infection by the stem canker fungus usually occurs at the second or third node of the plant, and the canker (brown lesion) extends up the plant. So, to distinguish between these two stem diseases of soybean, check for green stem tissue at the soil level. If the lesion extends from below the soil surface, the disease is likely Phytophthora root rot, if the lesion begins above soil level, stem canker is likely. 

Phytophthora- resistant varieties are available although the pathogen continues to diversify within fields so that one resistant gene will not usually be effective. Try to select varieties that also have some partial resistance or field tolerance. Northern stem canker is reported from time-to-time in Iowa. There is no known resistance to the disease. Good reviews of both diseases are available at the Plant Health Initiative website.


Figure 1. Wilted plants with a brown lesion extending from the tap root up the plant stem are characteristic symptoms of Phytophthora root rot.


Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at or phone 515-294-6708.

Corn or Soybeans Looking Stressed? Nematode Damage is Becoming Apparent

By Greg Tylka, Department of Plant Pathology

Plant-parasitic nematodes are microscopic worms that primarily live in the soil and feed on the roots of plants, including corn and soybean. More than 70 percent of Iowa’s fields are thought to be infested with a particularly damaging nematode, the soybean cyst nematode (SCN). And most every field in the state harbors one or more nematode species that can feed on corn. Most nematodes are not thought to be damaging to corn until their numbers increase to high levels. Now is the time of the season when obvious damage from nematode feeding on corn and soybeans is likely to become apparent.


Two very damaging nematodes that feed on corn are the needle and sting nematodes, and they severely stunt the growth of plants and cause obvious leaf yellowing very early in the season (Figure 1). But needle and sting nematodes only occur in soils with at least 50 percent sand content, so they are not of great concern in most Iowa fields.

Most other nematodes that feed on corn are present at low population densities in the beginning of the season, then their numbers can increase to damaging levels, but this buildup takes many weeks. Symptoms of damage from these nematodes include stunting and foliar yellowing (Figure 2), and the symptoms typically begin to appear in July and August. Guidelines for checking stunted and/or yellowed corn for possible nematode damage was reviewed in the June 2011 ICM News article, Sampling for Nematodes that Feed on Corn. There is no reason to check for damaging population densities of plant-parasitic nematodes on corn if symptoms are not occurring.



SCN has been recognized as a serious yield-limiting pest of soybean in Iowa and throughout the Midwest since the late 1980s. This nematode was a high-profile pest in the 1990s and 2000s, but interest and concern about SCN have waned in the past several years. Soybean yields can be reduced by 30 percent or more due to SCN without obvious above ground symptoms appearing, and significant yield loss (5 to 10 bushels per acre) can occur even in years of adequate to excess rainfall. Also, dormant SCN eggs can survive in dead females, called cysts, in the soil for a decade or more. Consequently, fields are vulnerable to yield loss from SCN every year.

When above ground symptoms of damage from SCN occur, the soybeans are stunted and yellow (Figure 3) and rows are slow to close over. At times, these symptoms can be very severe. The symptoms typically appear mid to late July or later and occur more frequently when soil moisture declines in the second half of the growing season. Fields can be checked for the presence of SCN by looking for white SCN females on soybean roots at this time of the year; guidelines for doing this were discussed in the June 2011 ICM News article, Females Now apparent on Soybean Roots.

Figure 1. Early season stunting and yellowing of corn caused by needle nematode.
Photo credit Tom Hillyer.


Figure 2. Mid-season stunting caused by nematode feeding on corn.

Figure 3. Mid-season stunting and yellowing of soybean from SCN. Photo credit Jim Fawcett.


Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

North American Manure Expo July 20 in Nebraska

By Angie Rieck-Hinz, Department of Agronomy

The 2011 North American Manure Expo will be July 20 in Norfolk, Neb. at the Northeast Community College Agricultural Complex.

The expo is hosted by the University of Nebraska-Lincoln (UNL) and other land-grant universities in partnership with Nebraska custom manure applicators and businesses and the Iowa Commercial Nutrient Applicators Association.

"Professionalism in Manure Management" is the theme of this year's expo. Educational topics will include: manure stockpiling, manure pit foaming and safety, winter application of manure on frozen soils and more. These are just a few of the educational programs — 23 different sessions in total, repeated twice during the day — that will be offered at the expo. Many of the sessions will be approved for continuing education units for certified crop advisors and others.

Fifty-three vendors from across North America will demonstrate the latest technology in manure management. Equipment demonstrations with manure are planned by vendors, attendees will be able to compare 15 solid spreaders, 13 liquid equipment (towed hose applicators, liquid tankers and sprinkler systems) and six GPS enabled equipment demonstrations. Attendees also will be able to compare sprinkler and solid spread patterns and injector disturbance between different kinds of equipment.

This will be a great opportunity for commercial manure applicators and livestock producers from Iowa to see the latest in manure application equipment and technology and to attend a wide variety of educational session all in one place. The expo is open from 7 a.m. - 6 p.m. There is no cost to attend. For more information about the North American Manure Expo visit


Angela Rieck-Hinz is an extension program specialist for Iowa State University Extension and is the coordinator of the Iowa Manure Management Action Group (IMMAG). Rieck-Hinz can be reached at (515) 294-9590 or by emailing

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