corn growth and yield formation in light of fungicide ......july 2, 2007—integrated crop...

July 2, 2007—Integrated Crop Management—199

IC-498 (17) 199 July 2, 2007

In this issueCorn growth and yield formation

in light of fungicide applications at tasseling _____ 199–200

Before applying fungicides to corn: Stop! Look! Consider! _______________________ 201–203

Safety, restrictions, and precautions for spraying fungicides on corn _______________ 204–205

Monitor soybean aphid populations on PIPE __________ 206

Soybean aphid numbers increase . . . and decrease ____ 207

Economic thresholds for western bean cutworms _____ 208

Soybean rust update and outlook __________________ 209

Bean leaf beetle: Predicted peak first-generation dates __________ 210–212

Iowa Learning Farm participates in Midwest Strip-Tillage Expo _____________________ 213

Blooming time in Iowa ___________________________ 214

Crop Production

Corn growth and yield formation in light of fungicide applications at tasselingby Roger Elmore and Lori Abendroth, Department of Agronomy

Tassels top many of our corn fields with more to appear every day. The critical corn pollination time

is near indeed. Soon the roar of spray planes will add a sense of urgency as will high-clearance sprayers gliding over an ocean of tall corn. Fungicide applica-tions made to attain improved “plant health” while controlling limited disease pressure are prime topics in extension offices, coffee shops, elevators, and pickup trucks around the Corn Belt.

Through what mechanisms would fungicide application improve yields when applied at tasseling (VT)? In order to know this, first understanding how yield accumulates in corn is crucial in determining why stress at different times in a plant’s growth is especially negative. Yield is the accumulation of five key elements expressed as:

A 3 B 3 C 3 D 3 E = YIELD

Plants per acre 3 Ears per plant 3 Rows per ear 3 Kernels per row 3 Weight per kernel = Yield per acre

In an ideal environment with no stress agents, the order in the equation is the sequence in which the various elements are determined. Several forces can alter the progression of this. For example, plant removal at various times due to crusting, cultivator damage, insect feeding, hail, greensnap, stalk lodging, etc. will affect the entire process.

Tasseling has begun around Iowa.

200—Integrated Crop Management—July 2, 2007

To understand how fungicide protection at tasseling and shortly thereafter might affect final yield, it is necessary to see which of the yield components will be impacted:

(A) Plants per acre are normally determined by a combination of seed germination (seed quality) and soil environment at planting and germination, although other factors can alter this as noted above.

(B) Ears per plant as well as tassel size are both determined by the 5th or 6th leaf stage (V5 to V6). All leaves and ear shoots are visible with dissection by V10. Primary ears will form at the 12th to 14th leaf node depending on hybrid. Ear shoots are formed on all aboveground nodes except the upper six to eight nodes. Usually only one ear develops kernels.

(C) Rows per ear are determined by V6. Hybrid genetics is instrumental in determining the potential number of rows per ear, yet environmental factors have an influence, especially if severe.

(D) Kernels per row are determined between V12 and tasseling (VT). The maximum number of ovules (potential kernels) per row is set a week or so before silks emerge. Stress during this time will reduce kernel numbers per row. Some researchers estimate there are up to 1,000 potential ovules per ear (that could develop into kernels), yet only 400 to 600 will develop into harvestable kernels. An ovule develops into a kernel when its silk receives pollen, is fertilized, and then develops without aborting. Potential kernels per row are highly dependent on growing conditions prior to silking while actual kernels per ear are determined by condi-tions during and after silking.

At tasseling, VT, all the plants’ stover (leaves, stalks, husks, cobs) are in place and are at or near final weight. The amount of pollen produced within hybrid fields is typically sufficient for pollination as 2,000 to 5,000 pollen grains are produced for every one silk. One grain of pollen pollinates one silk. Pollen usually sheds slightly before silking. Silks are receptive for approxi-mately 10 days. If stress occurs during this time, the synchronization between pollen shed and silk receptiv-ity may not line up, resulting in reduced pollination and fertilization. At silking, R1, corn plants require up to 0.35 inches of water per day. This is the greatest per day usage of any time in the growing season; therefore, moisture stress at the R1 stage reduces kernel numbers. Moisture stress causes silk emergence to slow while pollen shed accelerates. Some kernels will simply not develop due to a failure in pollination or fertilization. Kernels can be aborted in response to stress from R1 through the milk stage (R3). Abortion typically starts at the tip of the ear; hence, it is referred to as tipping back.

(E) Kernel weights are determined until black layer (physiological maturity, R6). Stress that occurs before R6 reduces starch accumulation in the kernel, resulting in lighter seed (lower kernel weight). The converse is true as well: anything that reduces stress between R1 and R6 may increase kernel weights.

SummaryAlthough stress inherently reduces yield, the plant

has great capabilities for responding to adverse growing conditions. This is obvious with the multiple factors used in the yield equation. Yield is not a result of one or two factors but instead a combination of several criteria over the entire length of the growing season. Applying a fungicide that has a 2- to 3-week window of viability is simply protecting the plant for a small window of time. If conditions exist during that time that significantly reduce yield, then the producer may see a yield response to a fungicide application. Maximum yield potential is determined by VT. After this, the focus is completely on preserving that yield potential. Late-season stress will reduce that yield potential, but nothing can increase the maximum level of what is possible.

“Before applying fungicides to corn: Stop! Look! Consider!” on pages 201–203 notes things to consider when deciding whether to apply a fungicide. An article in The Bulletin by Emerson Nafziger, University of Illinois, provides third-party yield data on fungicide performance. See Nafziger’s article at www.ipm.uiuc.edu/bulletin (Issue 14, Article 10, 29 June 2007).

By controlling diseases for two to three weeks after tasseling and/or affecting the plants’ physiology (by improving “plant health”), fungicides could potentially aid in kernel retention and increase kernel weights. Therefore, by reducing stress at or after VT, yield could be increased through more kernels per row and greater kernel weights.

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.

CORRECTION: An error was made in the article titled “2007: Tri-modal planting dates for corn” on pages 182–184 of the June 18, 2007, issue (www.ipm.iastate.edu/ipm/icm/2007/6-18/planting.html). The data for Region 5 (NE IA) show three distinct planting windows. This area of the state only had two main planting windows as producers planted primarily from April 19 to April 21 and from April 29 to May 16. We apologize for any confusion and appreciate the clarification and correction brought to our attention by ISU Extension field agronomist, Brian Lang. The online article has been updated to reflect this change.


Plant Diseases

Before applying fungicides to corn: Stop! Look! Consider! by Alison Robertson and Daren Mueller, Department of Plant Pathology

There is considerable interest across Iowa and the whole Corn Belt in applying fungicides to field

corn. In the past when corn prices were down below $2 a bushel, the decision to apply a fungicide was easy—no. This growing season, the high price of corn and increased disease risk due to increased corn-after-corn acreage has many producers considering fungicide applications as a means to increase yields.

Stop!Before applying a fungicide to corn this season, it

would be prudent to note the resistance to foliar disease of the hybrid being grown. The most common foliar diseases in Iowa are common rust, gray leaf spot, northern leaf blight, and, to a lesser extent, eyespot (see photos). Back in the mid-1990s, research done in southeast Iowa by Gary Munkvold, ISU plant pathologist, and others demonstrated a single application of fungicide could be profitable; however, profitability was strongly influenced by gray leaf spot susceptibility. In other words, the chance of increased yield and making a profit only was likely on highly susceptible hybrids. On intermediate and moderately resistant hybrids, any yield benefit would not cover the costs of the fungicide application.

Look!Many of the foliar diseases in Iowa start on the

bottom leaves of the corn plant and gradually move up the plant depending on environmental conditions. Thus, scouting the field can give us a very good indication of the disease pressure in that field. The best time to start scouting is immediately prior to tasseling. Look for disease development on the lower leaves of the corn plant up to and including the ear leaf. Remember, it is the ear leaf and leaves above that contribute 75 to 90 percent of the carbohydrates to grain fill. Thus, these are the leaves we need to protect. If disease is not present on the leaves below the ear leaf, a fungicide application may not be warranted. Continue scouting on a weekly basis.

Gray leaf spot. (Alison Robertson)

Northern leaf blight. (Alison Robertson)

Eyespot. (Alison Robertson)


Consider! Other factors to consider when deciding whether to

spray a fungicide or not include:

b Anticipated environmental conditions. Environ-ment plays a significant role in disease development (see the disease triangle below). Hot, humid weather favors gray leaf spot. Wet, warm (64–81 °F) conditions favor infection and spore production by the fungus that causes northern leaf blight. Dry weather halts the development of most fungal diseases.

b Planting date. Infection at earlier growth stages will have a greater impact on yield. In central Iowa, we usually see gray leaf spot starting on the lower leaves around the end of July and beginning of August. This year, corn has been planted at essentially three plant-ing dates (www.ipm.iastate.edu/ipm/icm/2007/6-18/planting.html). It is possible that the later planted corn (planted mid-May) will be at a greater risk for yield loss from foliar disease.

b Corn-following-corn fields. Many of the foliar disease pathogens survive winters in infested crop residue. Infested residue on the soil surface significantly increases the risk and development of gray leaf spot, northern leaf blight, and eyespot.

b Fungicide activity. Most fungicides are effective against disease for 14 to 21 days. Corn takes approxi-mately 60 days from silking to physiological maturity, depending on the hybrid. Therefore, spraying a fungi-cide too early could negate any beneficial effect against disease development that occurs during grain fill.

b Plant health. In last week’s issue of The Bulletin— Pest Management and Crop Development information for Illinois, Emerson Nafziger eloquently addressed the use of fungicides on corn for plant health (www.ipm.uiuc.edu/bulletin/article.php?id=793). In the absence of disease averaged over 10 sites, fungicides increased yields by 6.2 bushels per acre, which would not cover the cost of the fungicide plus application.

b Stalk rots. Foliar fungicides will have no direct effect on stalk rots. The fungi that cause stalk rots systemi-cally infect the corn plant and currently available fungicides are not systemic enough to be effective against stalk rot pathogens. However, foliar disease severity is highly correlated with stalk rot prevalence. Leaf spot diseases reduce the area of photosynthetic tissue and increase susceptibility to stalk rot. Therefore, fungicide applications may indirectly reduce stalk rots by managing foliar disease and reducing plant stress.

There are numerous fungicides listed for use on corn in Iowa. Research has shown that those containing active ingredients belonging to the strobilurin and/or triazole groups are the most effective against foliar pathogens of corn (Table 1).

In conclusion, producers who are considering making fungicide applications for “plant health” benefits should be aware that a significant economic response is not assured. The jury is still out. The profitability of fungicide applications on corn is determined by numer-ous factors, so stop, look, and consider the above factors before you spray.

SLIGHT DISEASE

Host

Pathogen Environment

NO DISEASE SEVERE DISEASE

Degrees of severity in the disease triangle.


Table 1. Fungicides listed for use on corn in Iowa.

Application Rate Application Adjuvants Coverage (gpa)Fungicide Target Diseases (fl oz/acre) Timing Ground Air Ground Air

Headline® Anthracnose 6–12 Prior to disease NIS @ 1 pt/100 gal COC @ 1 pt/acre 20 2–5 Common rust development Gray leaf spot Northern leaf blight Northern leaf spot Physoderma brown spot Southern leaf blight Southern rust Yellow leaf blight Quadris® Common rust 6–15.5 Prior to disease NIS @ 1 pt/100 gal Sufficient 5 Eyespot development water volume Gray leaf spot for adequate Northern leaf blight coverage Northern leaf spot Southern leaf blight Southern rust Quilt® Common rust 7–14 At onset of NIS @ 1 pt/100 gal NIS @ 1 qt/ Sufficient 2–5 Eyespot disease to 100 gal spray; water volume Gray leaf spot brown silk COC @ 1 pt/acre for adequate Northern leaf blight coverage Northern leaf spot Southern leaf blight Southern rust Stratego® Common rust 7–12 At onset of NIS @ 1 pt/100 gal COC @ 1 pt/acre 10 2–5 Eyespot disease through Gray leaf spot to end of Northern leaf blight silking (R2) Northern leaf spot Southern leaf blight Southern rust

Tilt®, Propimax® Common rust 2–4 Apply when NIS @ 1 pt/100 gal 10 5 Eyespot disease first Gray leaf spot appears Northern leaf blight Northern leaf spot Southern leaf blight Southern rust

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Daren Mueller is an extension plant pathologist with the Iowa State University Corn and Soybean Initiative

and the Pest Management and the Environment Program.


Table 1. Required personal protective equipment, according to the label.

Long-sleeved Shirts Shoes Chemical-resistant Protective Chemical-resistant and Pants plus Socks Gloves Eyewear Coveralls Clothing Headline® x x x x x xQuadris® x x x Quilt® x x x Stratego® x x x Tilt® x x x x

Plant Diseases

Safety, restrictions, and precautions for spraying fungicides on corn by Daren Mueller and Alison Robertson, Department of Plant Pathology

In the next month, there may be an unprecedented amount of fungicides being applied to field corn in

Iowa. While herbicides and insecticides are commonly used by Iowa farmers, fungicides are not. Like herbicide and insecticide labels, reading through a fungicide label will give you most of the needed information concern-ing safety for both yourself and others while spraying field crops.

SafetyPersonal protective equipment: With a few excep-

tions, fungicides labeled for use on field crops have minimal requirements for personal protective equip-ment, namely wearing the following:

b long-sleeved shirt and pants,

b shoes plus socks, and

b chemical-resistant gloves.

The Worker Protection Standard (WPS)The WPS is a federal regulation designed to

protect agricultural workers and handlers. If the pesticide that you are using has an “Agricultural Use Requirement” statement in the “Directions for Use” section of the label, you must comply with the WPS. The most recent information about the Standard may be obtained by checking the September 2005 updated WPS How to Comply manual. A helpful Web site that has information about the WPS is www.epa.gov/agriculture/twor.html.

The following Iowa State University Extension publications may be purchased or viewed by contacting Iowa State University Extension Distribution or by visit-ing their Web site, www.extension.iastate.edu/store:

PM 1663a Understand label precautions

PM 1663b What to do when clothes are soiled with pesticide

PM 1663c Wear the right gloves

PM 1663d Wear coveralls and aprons

PM 1663e Use eye and lung protection

PM 1878 For Pesticide Work Guard Your Hands with Gloves!

RestrictionsRestricted-Entry Interval (REI): All agricultural

pesticides labeled after April 1994 are required to have an REI stated on the label. REIs for fungicides, like other pesticides, are established to reduce pesticide exposure and are based on the product toxicity. In general, work-ers may not enter a treated area during a Restricted- Entry Interval unless they are wearing all the personal protective equipment required on the product label. Early entry that will result in contact with surfaces treated with pesticides is permitted in only three work situations:

b Short-term tasks that last less than one hour and do not involve hand labor.

b Emergency tasks that take place because of an agricul-tural emergency.

b Specific tasks approved by EPA through a formal exception process, which includes additional pesticide training for the worker.


Preharvest Interval (PHI): These intervals state the minimum amount of time that must pass between the last pesticide application and the harvesting of the crop, or the grazing or cutting of the crop for livestock feed. Some fungicides have restrictions based on growth stages instead of a specific number of days— e.g., fungicides cannot be applied “after silking.”

Table 2. Restrictions for fungicides labeled for corn.

Product Rate (fl oz/acre) REI (hours) PHI (days)

Headline® 6–12 fl oz 12a 7Quadris® 6–15.5 fl oz 4 7Quilt® 7–14 fl oz 24 After brown silk Stratego® 7–12 fl oz 24 After silkingTilt® 2–4 fl oz 24 After silkingaREI is 7 days for bare-hand detasseling activity.

PrecautionsFungicide resistance: Indiscriminate use of

fungicide this season may have consequences that are not immediately apparent. Fungicides like Headline® and Quadris® are strobilurin fungicides. The strobilurin fungicides have been characterized as being at “high risk” for fungicide resistance occur-ring; fungicide resistance has been documented in several fungal pathogens that cause diseases of different crops.

After 30 years of glyphosate use, the devel-opment of glyphosate-resistant weeds has been documented in the Midwest. Selecting for fungicide resistance in a fungal pathogen population is similar. If fungal pathogens are repeatedly exposed to fungi-cides, the chances of selecting for a “resistant” strain within a fungal population increases. Also, many fungal pathogens of corn produce very mobile spores

that can travel relatively long distances, and the numbers of spores that can be produced by fungal pathogens under suitable conditions are far greater than the number of seeds produced by any weeds.

In the short term, “high risk” fungicides may result in yield increases, even at times in the supposed absence of any disease pressure. However, these fungicides used indiscriminately will only increase the selection pressure for selecting fungicide-resistant strains of fungal pathogens. Hypothetically, there could be a time when fungicides are NEEDED to manage a foliar disease on corn, only to find out that these fungicides will no longer be effective against these pathogens. To ensure “high risk” fungicides continue to be useful tools in the future, these fungicides should be used responsibly now—i.e., only for the management of fungal diseases causing economical losses.

To reduce the risk of fungicide resistance development, follow these practices: 1. Apply a fungicide only when it is warranted. Use IPM practices, and base fungicide applications on good scouting observations.

2. Observe recommended fungicide rates. Applying a fungicide at a sub-lethal rate can increase the risk of fungicide resistance development.

3. Mix or alternate fungicides with different modes of action.

4. Do not make back-to-back applications of fungi-cides with the same mode of action.

For more information about fungicide resistance, go to the Fungicide Resistance Action Committee (FRAC) Web site (www.frac.info).

Daren Mueller is an extension plant pathologist with the Iowa State University Corn and Soybean Initiative and the Pest Management and the Environment Program. Alison Robertson is an assistant professor of plant pathology

with research and extension responsibilities in field crop diseases.


Insects and Mites

Monitor soybean aphid populations on PIPEby Carol Pilcher, Department of Entomology

The Pest Information Platform for Extension and Education (PIPE) was developed in 2004 to provide

electronic access to data for soybean rust. In 2006, soybean aphid was added to the system, and it is antici-pated that other pests will continue to be added to this national monitoring network. You can view the current soybean aphid monitoring results at www.sbrusa.net (to view the aphid map, select the second dropdown box under the date on the left-hand side of the screen).

Currently, extension field agronomists are moni-toring fields throughout the state of Iowa for soybean aphids. In each location, they examine 20 plants and determine the presence or absence of aphids. If aphids are present, they count the number of aphids per plant. This information is then recorded and entered in the national database on a weekly basis. PIPE provides growers with a tool to tell them when to start scouting for soybean aphid. It also gives them a general idea of the potential populations in their county. It does not, however, indicate the number of aphids in their field. Aphids are a sporadic pest and fields that are only separated by a road can have dramatically different populations. It is essential that a grower scout each field on an individual basis for this particular pest.

To date, aphid numbers have been relatively low (see “Soybean aphid numbers increase . . . and decrease” on page 207). Extension field agronomists report that they are finding aphids, but again, the numbers are low. What will happen to aphid popula-tions? That depends on two conditions: weather and the natural enemies of the aphids. Warm and dry weather

conditions favor aphid development and reproduction. The recent weather conditions have been ideal for aphids.

Continue to keep track of aphid populations through the ICM newsletter and the PIPE Web site, and keep track of what is occurring in your individual fields.

Carol Pilcher is the interim director of Iowa State Univer-sity’s Pest Management and the Environment Program and coordinator of the Integrated Pest Management Program.


Insects and Mites

Soybean aphid numbers increase . . . and decreaseby Marlin E. Rice and Matt O’Neal, Department of Entomology

Not only can aphid populations go up, or down, but they are also expected to be variable and uneven within the same field. We scouted a field near Nashua on June 28 and found eight consecutive plants in a row where each plant had 300 or more aphids, but on the plants in the immediate adjacent rows, there were 0–1 aphid per plant. This example should serve as a reminder that scouting across a field as opposed to a single spot in a field is imperative. The critical concept to remember coming into July is that soybean aphid populations are going to change—sometimes dramatically—during the next four weeks and that field scouting is absolutely critical for making informed decisions regarding pest management. Additional information on soybean aphids and the “speed scouting technique” can be found at www.soybeanaphid.info.

Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. Matt O’Neal is an assistant professor of entomology with extension and research responsibilities in field crops.

As might be expected, soybean aphid populations have increased slightly in some locations around

Iowa. The most notable increases, although they have been small, were reported from northeastern Iowa by Brian Lang, extension field agronomist in Decorah. The average number of aphids per plant increased from 6.6 to 30 with the percent infested plants also increas-ing from 59 to 95 percent (Table 1). The population in northeastern Iowa most closely parallels the same trends from 2005 with respect to aphid numbers and percent infested plants. Fortunately, we are not yet observing a population increase that would mirror the major out-break year of 2003, and that is good news.

Table 1. Soybean aphid population trends, Decorah, 2002–2007.

2002 2003 2004 2005 2006 2007 Date - - - - - - - - - - - - Infestation (%) - - - - - - - - - - - - - -

June 1 — — 0 6 0 2 June 7 — — 0 15 2 23 June 14 — 40 0 33 6 56 June 21 7 90 0 31 12 59 June 28 15 100 0 85 13 95 July 5 70 100 4 99 24 July 12 93 100 8 100 71

2002 2003 2004 2005 2006 2007 Date - - - - - - - - - - - - - Aphids/Plant - - - - - - - - - - - - - -

June 1 — — 0 0.1 0 0.1 June 7 — — 0 1.4 0.4 3.4 June 14 — 10 0 2.5 0.3 5.8 June 21 1 115 0 4.0 0.8 6.6 June 28 2 341 0 48 0.7 30 July 5 14 745 0.5 179 1.8 July 12 25 2,803 1 713 6.6

The aphid population in some areas also appears to have decreased in some areas. Jim Fawcett, extension field agronomist in Iowa City, reports that soybean aphids couldn’t be found during the last week of June whereas they were present in eastern Iowa the third week of June. He attributes rainfall as one possible reason for this population decline.

Soybean aphid colonies start small, usually on the underside of the youngest soybean leaflets, and they are often tended by ants collecting the aphid honeydew. (Marlin E. Rice)


Insects and Mites

Economic thresholds for western bean cutwormsby Marlin E. Rice, Department of Entomology

In his book, Entomology and Pest Management, Larry Pedigo discusses economic thresholds and the

foundation for their development and use. He states that one type of very common economic threshold is what entomologists know as the nominal threshold. A nominal threshold is one that is based upon a person’s understanding of the pest’s biology tied together with field experience, and it is rarely based on rigorous research. Nominal thresholds tend to be static—that is, unchanging with changes in crop value, control costs, or plant development stage.

The western bean cutworm is a pest of field corn in which we have used a nominal threshold developed at the University of Nebraska. Their threshold was devel-oped during the years when field corn was worth about $2/bushel, but with corn in the $3.50/bushel range now, it would make sense to cut the threshold in half to 4 percent of the plants infested with an egg mass.

It sometimes doesn’t hurt to get a second opinion, so last winter I spoke to Earle Raun, a private crop consultant from Nebraska whom I highly respect, and he shared his nominal threshold with me. You will notice that Raun’s threshold (shown in the right-hand column) is about half of the University of Nebraska threshold, or right in the ballpark if we halved the Ne-braska threshold based upon a current higher cash value for the crop. But the perspective I also appreciate with this threshold is that it incorporates a “field experience” component and increases as the crop matures, suggest-ing that it would require more insects to cause economic damage in the later plant stages.

I present both economic thresholds to you for your consideration. To my knowledge, neither has been tested side by side in the field, so the rigorous field validation aspect is missing here between these two thresholds. Also, remember that timing an insecticide application can be critical and applications should be made before larvae enter the silks.

Western bean cutworm eggs are “cantaloupe”-shaped and are white in color when newly laid. (Marlin E. Rice)

Western bean cutworm eggs that are ready to hatch are dark purple in color. (Marlin E. Rice)

Nominal threshold #1 (University of Nebraska)1. 8 percent of plants with egg masses or small larvae

2. If eggs have hatched, spray at 95 percent tassel emergence or

3. If tassels already emerged, when most of eggs are expected to hatch

Nominal threshold #2 (Nebraska crop consultant)1. 5 percent of plants with egg masses or small larvae on silking/blister/early milk-stage corn (R1–early R3)

2. 20 percent of plants with egg masses on mid- milk-stage corn (R3)

3. No threshold on late milk/dough/early dent-stage corn (late R3–early R5) unless seed production field

Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops.


Plant Diseases

Soybean rust update and outlookby X. B. Yang, Department of Plant Pathology, Iowa State University;

and Zaitao Pan, St. Louis University

Since May, Louisiana and southern Texas are consid-ered important source areas of soybean rust spores to

northern soybean production. In Louisiana, the disease was found in kudzu plants near New Orleans earlier in the spring. Latest reports indicate that the disease is developing locally with increased severity. Because the wind in June was not suitable to northward movement, limited northward spore movement occurred as indi-cated by modeling results. On June 20, the disease was found in central Louisiana in sentinel soybean plots and a production field. The finding in central Louisiana is about 10 days earlier than the detection last year. So far, the reported infested areas in central Louisiana are rela-tively small with a limited amount of spore production.

Comparing this year with the last two years, Texas has had conditions favorable for the occurrence for soy-bean rust. There was more rainfall in June, and disease was detected north of Houston. The Great Plains had a lot of rainfall in June, which is good for soybean rust occurrence. Fortunately, June is too early for soybean rust to establish itself since soybeans were just emerging or in the seedling stages. Spores were likely dead after been transported there by storms because unavailability of advanced soybean plants.

July outlook. Computer models with the latest climate data predict that there is a good chance for soybean rust spores to move as far north as Oklahoma from Texas. From Louisiana, the model predicts spore movement as far north as central Missouri and Kentucky. Rainfall in these areas is moderately low in favor of the establishment of these northward soybean rust spores. Analysis of computer calculations and current soybean rust monitoring results suggest that the risk of having a damaging outbreak in Iowa or neighbor-ing states is not high, but this season’s weather condi-tions (recorded and predicted) are much more favorable than the previous two seasons.

During the last two seasons, which were dry, we already declared the “risk is over” after June. In this season, July rainfall is predicted to be normal. Because the disease was found to be active in the Gulf Coast (Louisiana and Texas) and because there will be normal precipitation for the rest of season in main soybean pro-duction regions, we still need to watch the development of disease in the South. It is good to be cautious. After soybean rust was first found in Louisiana in fall 2004, this season is the first one to have normal precipitation (if the prediction is correct) in the Soybean Belt. The previous two seasons were dry.

Other foliar diseases. While we are watching soybean rust in the South, we should not forget the endemic soybean diseases in Iowa. Cool summer temperatures and good moisture are ideal for foliar diseases, and this year’s weather in Iowa so far has been good for the development of foliar diseases. White mold can be a concern to some growers in eastern Iowa, especially northeastern Iowa. In a season with high risk, soybean flowering time is time to control white mold with fungicides or Cobra®. In Iowa, the disease has been a problem in even years. In an odd year like this with later planting, most soybean fields in Iowa will not have a dense and closed canopy during the flowering time, a condition needed for white mold infection. Therefore, these fields are likely to escape the disease. However, for soybean fields that had problems in odd years and where the soybean canopy is already closed, the risk could be high if the fields are wet or saturated with moisture.

X. B. Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases at Iowa State University. Zaitao Pan is a climatologist at St. Louis University.


Insects and Mites

Bean leaf beetle: Predicted peak first-generation datesby Marlin E. Rice, Department of Entomology, and Rich Pope, Department of Plant Pathology

Bean leaf beetle feeding on soybean pods can lead to significant reductions in seed quality and yield.

Management during the pod setting and filling stages can be frustrating because beetles may feed on pods for a couple of weeks before the population reaches the economic threshold. In this situation, some loss in seed quality and quantity occurs before an insecticide application can be economically justified. Several years ago, Larry Pedigo and his students in the Department of Entomology developed research-based information to help make a management decision for second- generation bean leaf beetles based upon the population size of the first-generation bean leaf beetles.

The beetle has two generations a year in Iowa. The overwintered population (actually the second generation from last year) feeds on soybean during May through June. Females from this overwintered population lay eggs that develop into first-generation beetles that emerge in late June or early July. First-generation adult populations usually peak in the early reproductive soybean stages, whereas the second- generation adults peak during the pod-fill stages later in the summer. Feeding by first-generation beetles on soybean leaves very rarely, if ever, results in economic yield losses, but second-generation feeding on pods in late summer can cause significant yield reduction.

A degree-day model was developed by the Pedigo researchers to estimate the occurrence of first-generation adults. The degree days for the first-generation adults were estimated to be 1,212 degree days with a develop-mental base threshold at 46 °F. The overwintered female beetles usually begin to lay their eggs after colonizing the bean fields. The degree-day estimation for the first-generation adults is calculated by accumulating the temperature at the week of soybean emergence. Table 1 shows the accumulated degree days and predicted dates for the peak emergence of first- generation adults for 2007.

The value of this information is that the first genera-tion can be sampled to predict the size of the second generation. Tables 2 and 3 refer to the size of the first-generation bean leaf beetle population that may occur in the field during mid-July. Again, these are first- generation beetles, and if the population exceeds the thresholds, the field should not be sprayed at this time but instead sprayed later during the beginning of the

Bean leaf beetle. (Marlin E. Rice)

second generation of beetles. When the soft, gray teneral beetles start to appear in August, then this is the begin-ning of the second generation and if the first generation thresholds were exceeded, then the second generation should be sprayed at their first appearance. The value of field scouting now is that it will help you predict the possibility of economic pod damage from bean leaf beetles at the beginning of soybean pod development a month from now.

This pest management concept is to sample the first-generation beetles and then to use this information to manage the second-generation beetles. Here is how it works:

1. Determine what week your soybean plants emerged from the soil.

2. Consult the left-hand column of Table 1 and find the dates that match your soybean emergence date.

3. Determine which of the five Iowa locations is closest to your field.

4. Where the date (row) and location (column) intersect represents the predicted date for peak first-generation beetle emergence.

5. Sample your soybean fields one week after the predicted peak emergence. If the number of beetles reaches or exceeds the threshold (Table 2 or 3), stop sampling. If the sample is below the threshold, sample the following week. If the sample remains below the threshold, sample a third and final week. If the thresh-old is not reached, an economic infestation of bean leaf beetles should not occur in your pod-stage soybeans.


6. If the first-generation population is above the thresh-old, do not spray now, but scout the fields again in August to monitor for the first-emerging beetles of the second generation. When the first soft, gray beetles start to appear, spray the field with an insecticide (45-day

Drop cloth techniqueb Walk 100 feet in from the field edge and scout each

field and each variety separately.

b Place a 3-foot-wide strip of cloth on ground between the rows.

b Bend the plants on one row over the cloth, and shake them vigorously.

b Count the number of beetles on the cloth.

b Repeat the procedure four times for each 20 acres of the field.

b Determine the average number of beetles per 3 foot of row.

b See Table 3 for the number of beetles per 3 foot of row necessary to justify insecticide treatment for the second-generation adults in August or September.

b If the number of beetles is below the economic threshold, sample your fields again the following week, or a third week if necessary.

Sweep net techniqueb Walk 100 feet in from the field edge and scout each

field and each variety separately.

b Take 20 sweeps down the row.

b Repeat the procedure four times for each 20 acres of the field.

b Determine the average number of beetles per 20 sweeps.

b Table 4 shows the number of beetles per 20 sweeps that justifies insecticide treatment for the second- generation adults.

b If the number of beetles is below the economic threshold, sample your fields again on the following week, or a third week if necessary.

preharvest interval or less). Based upon the population size of the first generation, it is expected that the second generation will exceed the economic threshold. Fields can be sampled for first-generation beetles by using either a drop cloth or a sweep net.

Table 1. Degree-day accumulations and predicted dates for peak emergence of first-generation bean leaf beetle adults (1,212 degree days with developmental threshold of 46 °F) from the date of soybean emergence through June 27, 2007.

Degree-Day Accumulations and Predicted Peak Emergence

Storm Lake Oelwein Ames Atlantic OttumwaDate of Soybean Emergence (Northwest) (Northeast) (Central) (Southwest) (Southeast)

May 1–7 1,184 1,089 1,185 1,237 1,232 June 29 June 28 June 29 June 26 June 26

May 8–14 1,068 987 1,072 1,113 1,104 July 3 July 7 July 3 July 1 July 1

May 15–21 937 879 942 978 985 July 9 July 11 July 9 July 6 July 6

May 22–28 778 723 779 811 816 July 15 July 17 July 15 July 12 July 12


Table 2. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 3 foot of row).

Management Cost ($/acre)

$/bu 7 8 9 10 11 12 13 14 15

13 2.0 2.2 2.5 2.8 3.0 3.3 3.5 3.8 4.1

14 2.1 2.4 2.7 2.9 3.2 3.5 3.8 4.1 4.3

13 2.3 2.6 2.9 3.2 3.5 3.8 4.1 4.3 4.6

12 2.4 2.8 3.1 3.4 3.7 4.1 4.4 4.7 5.0

11 2.6 3.0 3.3 3.7 4.1 4.4 4.8 5.1 5.5

10 2.9 3.3 3.7 4.1 4.4 4.8 5.2 5.6 6.0

9 3.2 3.6 4.1 4.5 4.9 5.3 5.8 6.2 6.6

8 3.6 4.1 4.5 5.0 5.5 6.0 6.5 7.0 7.5

7 4.1 4.6 5.2 5.7 6.3 6.8 7.4 7.9 8.5

6 4.7 5.3 6.0 6.6 7.3 7.9 8.6 9.2 9.9

5 5.6 6.4 7.2 7.9 8.7 9.5 10.3 11.1 11.8

4 7.0 7.9 8.9 9.9 10.9 11.8 12.8 13.8 14.8

Table 3. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 20 sweeps).

Management Cost ($/acre)

$/bu 7 8 9 10 11 12 13 14 15

15 8.1 9.2 10.2 11.3 12.4 13.4 14.5 15.6 16.6

14 8.6 9.8 10.9 12.1 13.2 14.3 15.5 16.6 17.8

13 9.2 10.5 11.7 12.9 14.2 15.4 16.6 17.9 19.1

12 10.0 11.3 12.6 14.0 15.3 16.6 18.0 19.3 20.6

11 10.8 12.3 13.7 15.2 16.6 18.1 19.5 21.0 22.4

10 11.8 13.4 15.0 16.6 18.2 19.8 21.4 23.0 24.6

9 13.1 14.8 16.6 18.4 20.2 22.0 23.7 25.5 27.3

8 14.6 16.6 18.6 20.6 22.6 24.6 26.6 28.6 30.6

7 16.6 18.9 21.2 23.5 25.8 28.1 30.3 32.6 34.9

6 19.3 22.0 24.6 27.3 30.0 32.6 35.3 38.0 40.6

5 23.0 26.2 29.4 32.6 35.8 39.0 42.2 45.4 48.6

4 28.6 32.6 36.6 40.6 44.6 48.6 52.6 56.6 60.6

Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. Rich Pope is an extension program specialist working with the Iowa State University Corn and Soybean Initiative.

Dates to Remember:Crop Diagnostic Clinic—July 18–19

Soybean Cyst Nematode Clinic—August 7

Soybean Rust First Detector Training—August 20–24 (5 statewide locations)

Manure Management Clinic—August 28–29

Alfalfa Management Clinic—September 5

Late-season Crop Disease Clinic—September 6

Soils Clinic—September 18

To register or for more information

about these summer events at FEEL:

www.aep.iastate.edu


Announcements

Iowa Learning Farm participates in Midwest Strip-Tillage Expoby Carol Brown, The Leopold Center for Sustainable Agriculture

Iowa Learning Farm team members, agronomist Mahdi Al-Kaisi and agricultural engineer Mark

Hanna, will be presenters at the Midwest Strip-Tillage Expo ’07 in Waterloo, Iowa. Tillage specialists from three states will join together on July 31 for the Expo held at Hawkeye Community College.

The program will include concurrent educational presentations and field demonstrations starting at 9 a.m. and repeated at 1 p.m. Al-Kaisi will be presenting information regarding strip tillage for continuous corn, and Hanna will discuss selecting the proper tools for strip tillage.

Other educational programs include presentations on fertility management for strip tillage and auto-steer and -guidance technology. At noon, a panel of veteran strip tillers will discuss their experiences and techniques.

Field demonstrations will give farmers a chance to compare features of a variety of strip-tillage equipment and related products, including auto-guidance systems and fertilizer injectors. Manufacturers’ representatives will be on hand to answer questions.

The Iowa Learning Farm’s rainfall simulator will be demonstrated during the lunch hour. The rainfall simulator illustrates the potential impacts of conserva-tion and land management practices by distributing rainfall on five different surfaces. The runoff from these scenarios is collected in glass jars so one can see the results. The rainfall simulator shows how keeping various amounts of residue on the land surface can reduce sediment loss.

The Midwest Strip-Tillage Expo is organized by research and extension programs of Iowa State University, the University of Minnesota, the University of Wisconsin–Madison, and Hawkeye Community College. There is no charge for the expo. Lunch will be available for a nominal cost.

For more information on the field day go to http://wrc.umn.edu/outreach/striptillageexpo/midwest.

The Iowa Learning Farm’s focus is on helping strong conservationist farmers teach other farmers about systems that will improve the quality of the soil and water on their farms, while remaining profitable and sustainable. The Iowa Learning Farm is a partnership between the Iowa Department of Land Stewardship, Iowa State University Extension, the Leopold Center for Sustainable Agriculture, Iowa National Resources Conservation Service, Iowa Department of Natural Resources; in cooperation with Conservation Districts of Iowa and Iowa Farm Bureau.

Carol Brown is a communications specialist for The Leopold Center for Sustainable Agriculture.

Matt Helmers demonstrates the Iowa Learning Farms’ rainfall simulator. (Jackie Comito)


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Integrated Crop Management is published by Iowa State University Extension, with funding support from the Integrated Pest Management program. Subscriptions are available for $55 a year from the Extension Distribution Center at ISU. To subscribe or change the address of a current subscription, write to 119 Printing and Publications Building, Iowa State University, Ames, Iowa 50011 or call 515-294-5247. Please indicate that you are inquiring about Integrated Crop Management. Marlin E. Rice, Department of Entomology, is executive editor of ICM newsletter; Keven Arrowsmith, Extension Communications and Marketing, is managing editor; and Donna Halloum, Information Technology Services, is production designer.

. . . and justice for allThe U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Jack M. Payne, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.

Degree Days

Blooming time in Iowaby Rich Pope, Department of Plant Pathology

July dawns with some corn starting to tassel. Corn is a dioecious plant; that is, the male and female flowers

are borne in separate structures with the male flowers in the tassel and female flowers in the ears. This process increases the chances that the plants will crossbreed. Pollen (male) is shed to the wind, landing on silks (female) to achieve pollination. The separation of the flowers, and also in timing—the tassels generally shed pollen one to a few days before the female silks are receptive—increases the chances of natural interbreed-ing. However, in many modern hybrids, male and female flowering is closer and tassels and ears emerge close to the same time.

Conversely, soybeans are mostly self-pollinated. In fact, the only cross pollination that occurs with most soybeans is if insect feeding (or a soybean breeder) physically rips open the anther (where the pollen is borne) and transfers pollen to a receptive flower on another plant.

1083(+114)

1049(+84)

1027(+84)

1120(+77)

1110(+112) 1138

(+111)

1153(+67)

1131(+81)

1189(+97)

Accumulated base: 50 °F degree days

and departure from normal—May 1 through July 1, 2007

We are still ahead of long-term averages for growing season degree-day accumulations throughout the state, and that has contributed to the rapid growth to flowering for both corn and soybean.

Rich Pope is an extension program specialist working with the Corn and Soybean Initiative.

corn growth and yield formation in light of fungicide ......july 2, 2007—integrated crop...

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