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7/7/2008 - 7/13/2008

Reseeding Flood and Weather-Damaged Forage Crops in Late Summer

By Stephen K. Barnhart, Department of Agronomy
New forage seedings are susceptible to a wide array of problems and stresses -flood, hail, drought, weed competition, insects, etc. Late summer can be a good time to ‘patch in’ bare or thin spots in forage stands or reseed entire fields. However there are some risks. The following steps will improve the chances for successful forage stand establishment in late summer.

Step One – Plan ahead.
Planning is best done a year or more ahead of your new forage seeding. Planning should include these steps:
• Test soils and apply needed, corrective lime and/or fertilizer in the cropping seasons before the forage seeding. Small amounts of corrective fertilizer can be incorporated during forage seedbed preparation.
• Begin to control problem perennial weeds a year or more ahead of seeding.
• Be careful with herbicide selection on crops grown in the field before the forage seeding because some may have residual soil activity and will harm new forage seedlings if proper waiting periods are not observed. Read the labels for details.

Step Two - This is late-summer seeding, not fall seeding! 
Seed as early as possible. Perennial forage seedlings require 6 to 8 weeks of growth after emergence to have adequate vigor to survive the winter. Seed by August 10 in the northern third of Iowa, by August 20 in central parts of the state, and by September 1 in southern Iowa. 

Forage legumes such as red clover and alfalfa can be seeded up to the dates listed above if moisture is present. Slow establishing species like birdsfoot trefoil or reed canarygrass should be planted in early August. Most forage grasses such as Kentucky bluegrass, smooth bromegrass, orchardgrass, tall fescue, and timothy can be seeded five days than the dates listed above. 

Keep in mind that the above dates assume sufficient moisture to establish the crop. Planting later than the dates mentioned above is sometimes successful depending on fall and winter weather patterns, but there is increased risk of failure and reduced yield in later years if late emerging seedlings are damaged by frost.

Step Three - Prepare a firm seedbed if using tillage.
Loose seedbeds dry out very quickly. Deep tillage should be completed several weeks ahead of seeding so rains can settle the soil before final seedbed preparation. A cultipacker or roller is an excellent last-pass tillage tool. The soil should be firm enough for a footprint to sink no deeper than 3/8 to one-half inch.

Step Four – Utilize interseeding and no-till forage seeding.
Late-summer pasture interseeding and no-till forage seeding is an excellent way to conserve moisture, provided weeds are controlled prior to seeding. Remove all straw after small grain harvest. Any remaining stubble should either be left standing, or clipped and removed. Do not leave clipped stubble in fields as it forms a dense mat that prevents good emergence. 

Step Five - Don't plant alfalfa immediately after older established alfalfa. Autotoxic compounds are released by old alfalfa plants that inhibit growth and productivity of new alfalfa seedlings. It is best to rotate to another crop for a year or more before going back to alfalfa; however, thickening up seedings within 12 to 15 months of the original planting date is considered to be a low-risk practice  because autotoxicity concerns are greatest with older alfalfa fields.

Step Six – Seed when there is moisture.
Seed when soil moisture is adequate or a good rain system is in the forecast. There is a higher risk of seedling failure when planting seeds into dry soil, as there may be just enough moisture to germinate the seed but not enough for seedling establishment.

Step Seven - Plant seed shallow and in firm contact with the soil.
Carefully check seeding depth, especially when no-tilling. Most small-seeded forage grass and legumes should have a final seed placement depth of one-fourth to one-half inch. Drills with press wheels usually provide the greatest success in the summer. Broadcasting seed on the surface without good soil coverage and without firm packing is usually a recipe for failure in the summer.

Step Eight - Use high quality seed of known varieties.
Cheap seed often results in big disappointments and shorter stand life. Make sure legume seed has fresh inoculum of the proper rhizobium.

Step Nine - Do not harvest new summer seedings this fall.

 


Stephen K. Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management.

Renovating Pastures or Hay Meadows Damaged by Flooding

By Stephen K. Barnhart, Department of Agronomy
Flooding has damaged pasture and hay meadows across the state of Iowa. While most forage will likely recover, other areas that were eroded or silt- or sand-covered will require some reseeding. The following considerations may guide you in your planning and management.
 
Fertilization. Fertilizer, particularly nitrogen, is the quickest way to increase grass production in pasture areas adjacent to your reseeding. Research over the years has shown that, with good rainfall, a modest application of nitrogen (30-50 lb. N/A) on grass pastures in late spring or late summer can provide an economical increase in dry matter production and associated animal gain per acre. 

To get the full benefit from nitrogen there must also be suitable soil pH, phosphorus (P) and potassium (K) levels, and soil moisture. Partial benefit is achieved where other fertility levels are deficient. Benefits from summer-applied nitrogen will be less without adequate and timely rainfall. Areas to be reseeded should be soil sampled, and needed lime, P and K incorporated during any seedbed preparation.

Reseeding.  When deciding what to plant, consider what is compatible with the forage surrounding it. Ideally, this is something not very much different. For hay, plant something that will mature similarly, and if pasture, try to plant species that have similar palatability as the residual sod. The more difference, the less uniformly it will be grazed in the future. See ISU Extension bulletin Pm- 1792, Selecting Forage Species for traits of various forage species and some suggested mixtures.

If you will be designing your own mixture, consider using only two or three compatible and well adapted grasses and or legumes.

There are several methods for planting forage crops this time of year. The most costly is complete renovation, using a drill or broadcast seeder in a tilled seedbed. Refer to ISU Extension bulletin Pm- 1008 Steps to Establish & Maintain Legume-Grass Pastures. Accumulated sand or silt may have to be spread to level the area before final seedbed preparation. 

A slightly less expensive approach is using a no-till forage drill and planting into a killed sod or undisturbed surface, if conditions are suitable. For more on this method refer to ISU Extension bulletin Pm-1097, Sod-seeding and no-till pasture renovation.

As with most of the pasture improvement alternatives, there are some additional issues to consider when contemplating a change in pasture species.  Among these questions:

• Is seed of your selected forage species and varieties available?

• Are the weather patterns and soil moisture conditions suitable to provide for establishment and stand development? Mid-summer can be a challenging time for establishing shallowly-placed, small-seeded forage grasses and legumes. If soil moisture is not adequate and if timely rainfall for the remainder of the summer is uncertain, it would be best to delay planting until mid- to late-August or the first week of September for your planting. Weeds may grow on the flood-killed areas before you can accomplish the new seeding. Mowing closely before no-till seeding may be adequate; mowing and tilling may be necessary for seedbed preparation in some cases. There is a risk of delaying too late into the autumn. Late emerging new seedlings may not have enough time to establish sufficiently to survive the winter.

• Will weed competition be sufficiently controlled to provide for the successful establishment and persistence of the newly introduced species?

• Will grazing animals be kept off the newly seeded areas until the new seedlings are established?

• If the seeding activity leads to a temporary decrease in forage production, are there sufficient forage resources to support the existing animals until the forage improvements are realized?

 


Stephen K. Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management.

Twisted Whorls

By Roger Elmore, Department of Agronomy and Alison Robertson, Department of Plant Pathology

Corn is seemingly racing toward tassel and silking thanks to a spell of relatively good weather.  Field appearances improve daily. As we scan the tops of canopies a few plants stand out, not because of height but because of color; yellow, bright yellow leaves seem to wave to us in the breeze.  These trapped, sun-starved leaves emerge as splotches of bright yellow in a dark green sea of plants.

Interesting symptoms, descriptive terms, and strange timing
These symptoms are of what some describe as buggy whipping, rapid growth syndrome, accelerated growth syndrome, roping, wrapped whorls, onion leafing, and twisted whorls.  Take your pick!  We’ll use the last term in this report. 

Whatever term you chose, it describes a situation where the uppermost plant leaves are tightly rolled and do not unfurl normally. When entrapped leaves break free, they are bright yellow. After some exposure to the sun, they will turn green.

The difference this year from previous occurrences is in timing.  Corn plants expressing these symptoms this year range from V10 to V12 (tenth to twelfth leaf stage). In contrast, of twisted whorls have been on much younger corn plants (V5-V6 leaf stage).  An interesting point here is that the leaf crinkling (often associated with leaves that are or were previously twisted or trapped) begins on about the tenth leaf, relatively high in the plant canopy.  This suggests that the plants were ‘normal’ until sometime shortly before the tenth leaf stage.

In some west-central Iowa fields the incidence of plants with twisted whorls is as high as 16 percent, with affected plants scattered uniformly over a field according to Mark Licht, Extension Field Agronomist (July 9, 2008, CropWatch Blog ). Unfortunately, these symptoms are not confined to a single field or management system.  Plants in scattered central Iowa fields are expressing the symptoms as well. Several hybrids, with different herbicides, and other management practices are affected. Mark estimates 30 percent of fields in some areas of west-central Iowa have these symptoms.

twisted whorl in corn 2

twisted whorl in corn 3

Plants exhibiting twisted whorls, 9 July 2008, Story Co. IA, Roger Elmore.

 

Possible causes
Since symptoms are wide-spread in central west-central Iowa, an environmental stress is the likely cause.   All season long corn has grown under far less than ideal conditions in Iowa.  An additional stress factor may result in otherwise normal plants expressing themselves as ones with twisted whorls.  Stress factors could simply be a wide range of temperatures sometime prior to V10, hail, strong winds, or an over-the top application of a herbicide, insecticide, or fungicide, etc.  Earlier-season (V5-V6) 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).  Herbicides may play a role is some of the situations we are seeing now.

The symptoms may also be caused by the recent change in weather which has encouraged rapid growth, in sharp contrast to weather experienced earlier this year. We also know that some exotic germplasm incorporated into adapted hybrids can exhibit twisted whorls this late in the growing season. Exotic germplasm is genetic material originating from outside of the Corn Belt.

Twisted whorls are not a symptom of a biotic disease. However, injuries caused as a result of twisted whorls can increase smut infections (see Photo). The yellow leaves resulting from twisted whorls are also not the same as those related to genetic stripe (see Photo).

twisted whorl with leaf smut in corn

Leaf smut symptoms on a corn plant affected by twisted whorl, 9 July 2008.  Alison Robertson

 

twisted whorl with green stripe

Genetic stripe. White or yellowish stripes usually run the entire leaf length.  30 June 2008. Story Co. IA. Roger Elmore.

 

Yield impacts?
If these, late-season twisted whorls follow the same pattern as their early-season counter parts, we would expect these plants to unfurl within a week or two.  If the malformation causes a delay in either growth or development, it could result in yield reductions.

 

 

Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases.

Yellow corn, Wet soils, and Nitrogen loss (Part 3)

by John Sawyer, Department of Agronomy

Earlier I provided observations in two articles (June 19 and June 26, 2008) on corn growth and response to nitrogen (N) applied in an anhydrous ammonia study being conducted at the ISU research farm between Ames and Boone. Following are some final observations (July 7, 2008).

Response to Anhydrous Ammonia Timing – more observations
In this study, anhydrous ammonia was applied in late fall (October 31, 2007), spring preplant (April 30, 2008) and sidedressed (June 18, 2008) at different depths of injection and application rates with corn following soybean. Corn was planted May 15. The study is “low” on the landscape, and like many fields in Iowa this year part of the study area was wetter than the rest, with a part where the corn is dead due to standing water.

Plant growth has progressed well and corn is into its dark green phase. As of Monday, July 7 the corn on the “higher” ground continues to grow better (is waist high) than plants on the “lower” ground and where water had ponded (is at best knee high). Plants on the lower ground are responding to the fall and spring applied N with increased green coloration, but the growth differential between where N was applied and where no N was applied is not as large as where the plants were not as impacted by wet soils.

Looking at the plant coloration and growth, I still generally see a better response to the spring applied ammonia compared to the fall applied ammonia at the lower N rates. It is difficult to tell what amount of N may have been lost this spring, but it appears there might be some difference between the fall and spring applied ammonia. The corn is also showing response to the sidedress applied ammonia. Overall the corn coloration and height is showing a classic response to N rate (with more height differentiation between rates where corn is growing the best). In the control plots where no N was applied, you can see the impact of soil derived N – even in this very wet spring. Of course plant growth and color is less than where N was applied, but it is remarkable the soil resource for supplying crop available N.

At this time, where the corn is growing better (higher ground), a visible N response difference appears to be up to the 120 lb/acre rate. As the plants grow and take up N during this rapid growth phase, differentiation between N rates will become more clear. Growing conditions during the rest of the season, and of course grain yield, will be the final determinate of maximum N need.

yellow corn
The middle stake is in the border between two four row plots, spring applied ammonia. The plot on the left (stake is in the middle of the plot) had 120 lb N/acre and the plot on the right (stake is in the middle of the plot) had 80 lb N/acre. (J.E. Sawyer, July 7, 2008)

 

yellow corn 1
The middle stake is in the border between two four row plots, fall applied ammonia. The plot on the left had 160 lb N/acre and the plot on the right had no N applied. These plots are in the area more impacted by ponded water and with slower corn growth. (J.E. Sawyer, July 7, 2008)

 

yellow corn 2

yellow corn 3
The middle stake is in the border between two four row plots, sidedress applied ammonia. In the top photo, the plot on the left had 80 lb N/acre and the plot on the right had no ammonia applied. In the bottom photo, the plot on the left had 160 lb N/acre and the plot on the right had 80 lb N/acre. (J.E. Sawyer, July 7, 2008)

 

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

Managing CRP for Hay or Pasture

By Stephen K. Barnhart, Department of Agronomy
The  extensive flooding in the Midwest U.S. and its effect on forage and livestock enterprises have prompted our government officials to permit haying and grazing on land enrolled in the Conservation Reserve Program (CRP) through several ‘emergency use’ programs. One permits haying or grazing after August 1 with a nominal ‘administration fee’. The other permits only grazing - sooner, but at a cost of 25 percent of the annual CRP rental payment. If you are considering the use of CRP land for haying or grazing there are some considerations and important first steps that you need to follow.

Both USDA-Farm Service Agency (FSA) and the USDA-Natural Resources Conservation Service (NRCS) will be involved in the administration of the emergency use of CRP contract land. This means you will have to visit both local offices for details and final guidelines.  Rules regulating use varies between the program options, so consult your local USDA-FSA and NRCS representatives to compare program costs and rules.

There is not much uniformity in CRP vegetation. Much of it is smooth bromegrass- or tall fescue-dominant; some is warm-season grass-dominant. Most CRP has some compliment of weeds and often some volunteer trees or brush. There may be considerable surface roughness due to erosion, rodent or ant mounds, etc. Fences are likely in disrepair.

The quantity and nutritive quality of the forage will vary with forage species, stand density, soil fertility, and degree of old, decomposing vegetation in the lower canopy. You are encouraged to do a thorough inspection of your fields to determine current conditions and any limitations that need to be addressed for each particular use, either hay or grazing.

How much forage is in those CRP fields? What is its feeding value? Data on this type of forage is limited, but my guesses are:

Estimated standing crop yields on July 1 - One and one-half to three tons per acre, depending a lot on stand density, soil type, and how much 'residual, or old dead stems are present.

Estimated yields on August 1 (as first use) - About the same, only a small amount of 'new’ regrowth would be accumulating in the bottom of the canopy

Estimated additional yields of regrowth during summer and autumn  - One-third to one-half  ton per acre more yield -  with 'normal' Iowa summer weather, having warm, intermittent rain. With higher than normal summer/fall rainfall and cooler than normal temperatures, there would only be a slight increase in summer/fall regrowth, and little or no additional regrowth would be expected with higher than normal summer temperatures and no appreciable rain through late summer and fall.

Modest amounts of nitrogen (25-40 lbs N/A) would not be expected to add much added yield if applied in July before harvest or grazing.  A modest nitrogen application in mid- to-late August , may provide economical yield returns if temperatures and rainfall support good fall growth.

The nutritive value of the standing crop is relatively low; the 'damage has already been done.’ This damage comes from allowing the forage plants to mature fully through June,  thus accumulating fiber and diluting protein. Old, residual stems in the lower canopy add yield, but are lower yet in nutritive value. I don’t think that there would be much change in nutritive value whether first cutting or grazing comes in July or early August. Crude protein (CP) of the standing crop on a percent of dry matter basis will likely range between 6 and 10 percent and dry matter digestibility, 45-50 percent, depending on forage species.

There would only be a slight increase in crude protein and digestibility if harvest or grazing is delayed and some new, leafy growth accumulates in the lower canopy. The greatest nutritive value benefits would be in the regrowth following initial cutting or grazing. Leafy, new growth would have a relatively higher protein and dry matter digestibility. Modest applications of nitrogen following initial grazing or harvest would add to CP and digestibility slightly too.

Some thoughts specific to grazing
The two current USDA ‘grazing programs’ vary. The ‘after August 1’ program restricts the proportion of the area or amount of forage that can be used. Both programs will likely include the basic conservation management guidelines for CRP that restricts grazing along riparian or waterways.

Fencing and water sources should be evaluated. Grazing livestock will be selective in their grazing and trample and waste considerable forage. Any effort to ‘concentrate more animals in smaller areas (increased stock density), using temporary fencing, will improve the utilization of the mature forage.

If harvesting hay
Debris, stumps, rodent mounds or ant hills, ditches, etc. can damage harvest equipment. Thoroughly inspect fields that will be harvested.

Even though the nutritive value of CRP hay is relatively low, it still has an economic value. Minimize waste by using good storage and feeding management. To most efficiently use CRP hay in livestock rations, you are encouraged to sample and have it tested for forage quality. Some producers will be buying or selling CRP hay. Its price will vary by nutritive quality, bale type, and condition. Summer and autumn auction prices for CRP hay in 2007 ranged from $24 to $55 per large round bale and $45 to $70 per ton. Winter auction prices for 2007-2008 ranged from $57 to $59 per large round bale and $50 to $120 per ton.

 

Stephen K. Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management.

Estimated Corn Silk Dates and Yield Potential, Iowa 2008

Roger Elmore, Department of Agronomy
Silk dates mark a milestone in growth and development of corn. This year in particular estimates of silk date are of interest since they provide an idea on timing of fungicide application as well as providing a look forward for conditions that might occur during this time critical for corn development. Crop simulation models are useful tools to project silk dates based on season-to-date weather conditions. Models also provide estimates of yield potential.

With this in mind, I used Hybrid-Maize to predict silk dates for different areas of Iowa.  In each area, the approximate date when half of the corn was planted in 2008 (based on USDA reports) was included in the model. Planting dates for the western third of the state were thus set at May 10;  May 15 was used for the remainder of the state.  Different hybrid maturities represented the different locations as well.  Hybrid maturities were 105, 110, and 115 days in the Northern, Central, and Southern thirds of the state, respectively. Given those inputs and weather data up through July 7, 2008, the model estimated the silk dates. Silk date ranges are shown in Figure 1.

iowa silking dates map

Estimated corn silk date ranges based on Hybrid-Maize simulations. The simulations used weather data from automated stations through July 7, 2008. 

To compare these estimated silk dates with those of previous years, I again used Hybrid-Maize estimates for Ames with May 15 as the planting date with a 110 day hybrid.  These comparisons are shown in Table 1.  Silk dates for the different yield possibilities range from 1 to 12 days behind those of previous years.  The least difference occurs in the years with the best yields. The largest difference in silk dates occurs in the worst-yielding years. Days to maturity (R6) are also shown.  Best yields are associated with the latest maturity date.

Delayed silk dates will be the norm across Iowa in 2008. If fungicide applications are necessary, schedule them based on crop growth stage rather than on calendar date. Although silking will be delayed this year, weather forecasts for the next month are for normal precipitation and below normal to normal temperatures. These are both promising forecasts for corn development.

2008 silking dates on iowa corn chart

Table 1.  Estimated silk dates and estimated yield potential for an array of years from Hybrid-Maize for Ames, Iowa.  Compare these with days to silk estimated for all other years in the weather data base. Weather data included: January 1986 to 7 July 2008.

The model also estimates maximum potential yields.  Included in Table 1 is an estimate of yield as a percent of the best possible yield estimated from May 15 plantings from 1986-2007.  In the best of years, yield is only 4 percent less than maximum potential yield. However, in the worst yielding year, yields are only 45 percent of maximum. We might expect an average yield of 77 percent of normal.

Crop model simulations provide an idea of crop development rates and yield potential. But, please remember, as the Hybrid-Maize developers state, “As with all simulation models, Hybrid-Maize still represents a simplification of the ‘real-world’ system and, as such, model predictions may differ from actual outcomes. Therefore, the results of model simulations should be considered approximations and not taken as fact."

Summary:
• 2008 estimated silk dates could range up to 12 days later than in previous years.
• A longer growing season and a delayed frost may help maximize yield.
• Excellent yields are still possible.

 


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production.

With the Increased Value of Corn, What are the Economic Thresholds for First Generation European Corn Borer?

By Jon Tollefson, Department of Entomology

The 2008 crop season has been unusual; not only has the weather been extreme, but the value of crops is higher than it has been historically. In the European Corn Borer Ecology and Management bulletin published in 1996 the economic injury levels for first generation European corn borer attacking corn were calculated for corn that sold for $2.50 per bushel. Now field corn can be sold for nearly three times that amount. How will this affect the number of larvae that should be present when it is decided to apply a control?

The number of pest insects that are present to justify paying to control them should increase with an increase in the cost of control (if it will cost more you don’t spend the money as soon), and decrease when the crop value increases (if corn is worth more you will pay for the treatment sooner). I have recalculated the economic injury level (when the loss of corn equals the cost of control) for first generation European corn borer for corn that can be sold at $6 and $7 per bushel and control that costs $15 and $18 per acre. 

The data represented in Table 1 has been compiled in an Excel spread sheet. The Iowa State University Extension field agronomists have this spreadsheet and can provide it to you to make calculations. The spreadsheet uses the same assumptions as the corn borer management bulletin: that 40 percent of the larvae will survive to tunnel into the stocks, yield loss will be 5.9 percent per larvae in the stock, and that the field corn is in the tenth leaf stage. Setting the returns near zero (the value of yield saved is equal to the cost of control) and by calculating backward the Economic Injury Levels can be derived. The values that result are:

Control costs are $15 if corn is figured at $6 per bushel and there is an average of 0.88 Second stage larvae per plant. Control costs are also $15 if corn is figured at $7 per bushel and there is an average of 0.76 larvae per plant.

Control costs are $18 if corn is figured at $6 per bushel with an average of 1.06 larvae per plant. Control costs are also $18 if corn is figured at $7 per bushel corn with an average of 0.91 Second stage larvae per plant

These numbers will change if you change your assumptions. For example, if you assume that the larval mortality will be lower, then you can change 40 percent for the surviving larvae to a higher value. The value of corn and control costs can also be changed. To make the changes you will need the Excel spreadsheet with the mathematical functions imbedded in it so that it will react to your changes in the variables. (Access the Microsoft Office Excel spreadsheet from the link on the right.)

Table 1. European Corn Borer 1st Generation Economic Threshold at $6 and $7 Corn.

Table 1: Economic Injury Level
Jon Tollefson is a professor of entomology with extension and research responsibilities.

Questions and Answers on Soybean Fungicide Applications

XB Yang, Department of Plant Pathology
 As the difficult planting season passes, we continue to receive not so positive reports on soybean and corn with the development of foliar diseases due to weather conditions.  In Iowa there have been many observations and questions on soybean foliar diseases, especially brown spots, by field crop specialists and producers. Producers have questions about how and when to use fungicides to control foliar diseases. This article addresses some of those questions.
 
What diseases to look for?
For this season, two diseases especially deserve our attention, brown spot and frogeye leaf spot. 

The most common disease is brown spot, which is caused by the fungus Septoria glycines. Disease symptoms occur on the lower leaves of soybean plants. The fungus is spread by splashing rains, which has been frequent from the beginning of spring. 

brown spot 2

Soybean leaves infected with brown spot.

Symptoms include many irregular, dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches, and you will not miss it when scouting. Brown spot usually does not cause damage unless the disease progresses quickly with frequent rains later in the season. Currently the disease is prevalent in many areas with incidence over 90 percent according to surveys, a much higher incidence compared with other years. 

Another disease to look for is frogeye leaf spot caused by the fungus Cercospora sojina, which has become a frequent visitor to Iowa soybean fields in the past, especially in river bottom fields. The disease causes small, gray spots with reddish-brown borders to appear on the upper leaves. In severe cases the disease can cause premature leaf drop and form brown spots on stems and pods.

 

frogeye leaf spot

Soybean frogeye leaf spot.

An erroneous report of soybean rust damage in 2006 was actually a case of severe defoliation by frogeye leaf spot. It is quite early to observe frogeye leaf spot in the first week of July, as in most seasons it is found in later July or early August.

Is there a need to spray?
Field spray experiments or stripe spray trials by the Iowa Soybean Association consistently suggest that yields benefit when foliar diseases were present. With the weather so wet and soybean price so high, it is a no brainer to conclude that spray fungicides this season can control soybean foliar disease and increase yields. 

We had debates on the use of fungicide in other years, and this year would be the year to see the benefits unless we are fooled by Mother Nature and the rest of season becomes hot and dry. The diseases have already established in many soybean fields and could become worse in the late season, like what happened in 1993.  The chance to see yield benefits from a spray so far is much higher than most normal years.
 
When is it time to spray? 
In our data from the last few years’ experiments in Nashua and other locations, the best control with one spray in terms of yield increase for later season foliar diseases is at R3-R4 growth stage (last week of July and early August). However, this year the disease occurs earlier, and a spray at R3 growth stage or later may not reproduce the results that we have had in a normal weather year. The outcomes depend on the weather in July and August. 

If the rainfall is less intensive in the two months, a spraying at R3 may still control the disease well. If the weather in August is like last August’s, which had a record rainfall, one spray may not stop the disease, as most fungicides only provide two-three week protection period. But we do not recommend two sprays because of damage to soybeans caused by driving in the fields in August.
 
Bacterial blight
Also reported is bacterial blight, a foliar disease similar to brown spot. Bacterial blight also requires rainfalls for its spread, and its symptoms are similar to brown spot. However, this disease cannot be controlled by fungicide application because the casual agent is bacterium. 

Besides the three diseases mentioned, Cercospora leaf spot has also been found.  Another disease we should not forget is pod and stem blight. The disease was reportedly widespread last year and caused seed quality problems this spring.  Fungicide sprays should help reduce the risk of this disease, especially for seed productions.

 

XB Yang is a professor of Plant Pathology with responsibility of soybean diseases.

Knee High by When?

By Rich Pope, Department of Plant Pathology

Independence Day has come and gone, and scant acres of corn in most of Iowa have reached that old, outdated saw of knee high by the fourth of July. On my weekend trip to western Iowa, I saw corn ranging from V2 to VT! Admittedly, very little acreage was at either extreme, but it illustrates the continuing range of crop conditions that we will deal with through and after harvest this year.

Degree day accumulation summary as of July 6

Degree days accumulated during the first week of July were again slightly behind average. Because of crop plant’s limited root growth from the onslaught of the cool and wet conditions during the late spring, receiving timely rainfall will remain an ongoing issue through the remainder of the season.

Rich Pope is an extension specialist with responsibilities in the Corn and Soybean Initiative.



This article was published originally on 7/14/2008 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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