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9/23/2013 - 9/29/2013

Tobacco Streak Virus on Soybean Confirmed in Iowa

By Daren Mueller and Erika Saalau, Department of Plant Pathology and Microbiology

Over the past several weeks, we have received several samples in the Plant and Insect Diagnostic Clinic with irregular blotches and necrotic lesions on the pods.

After eliminating all of the known causes, a sample was screened for several common bean viruses*. The sample came back strongly positive for tobacco streak virus (TSV).

This virus was first identified in Iowa and the United States in 1967. The soybean plants with “pods showing necrotic spots” were noticed in late-planted soybeans. From this earlier report, TSV reduced the number of pods per plant and delayed seed maturation. Other symptoms include stunting, bud blight, leaf mosaic (mottling), dwarfed leaves and stem discoloration.  This virus is mainly seed transmitted and several thrips species have been reported as TSV vectors.

If you have soybean pods with similar symptoms, you can send them to the ISU Plant and Insect Diagnostic Clinic. Confirmation of TSV costs $50.

Figure 1. Tobacco streak virus symptoms on soybean pods.

 

*Testing service provided by Agdia, Inc.

 

Daren Mueller is an assistant professor in the Department of Plant Pathology and Microbiology. He can be reached at 515-460-8000 or e-mail dsmuelle@iastate.edu. Erika Salaau-Rojas is a diagnostician in the Plant and Insect Diagnostic Clinic. You can reach her at 515-294-0581 or e-mail pidc@iastate.edu.

Avoid Tillage This Fall

By Mahdi Al-Kaisi, Department of Agronomy

As the harvest season is getting close, there are a few things we need to keep in mind this fall with regard to soil conservation concerns. Soil conditions are dry across the state. If they continue, it can be advantageous during harvest by reducing potential soil compaction. So, if the rationale for tillage is to reduce soil compaction, it is not valid. Therefore, pay attention to tillage intensity after harvest; tillage affects soil conditions and destroys soil structure, which can create significant problems. Tillage accelerates organic matter loss, which results in more problems of accelerating soil erosion and surface runoff. Those changes in soil condition with tillage during rain events after harvest can also reduce soil profile recharge due to increased surface run off.

Leaving crop residue on the soil surface has many benefits not only in minimizing future negative effects of soil erosion and sediment and nutrient losses, but also works as an effective method of trapping soil moisture, which later easily penetrates into the soil and recharges the soil profile. Tillage of any kind damages the soil by reducing the residue cover and its effect in protecting the soil surface.

A common misconception is that shredding or incorporating residue with tillage will enhance soil organic matter or improve other physical and biological properties, which are essential to a well-functioning soil. However, research documents that crop residue can be most effective when left intact on the soil surface protecting soil quality, such as soil structure, water infiltration, soil moisture holding capacity, and soil bulk density to name few. During dry conditions, removing residue or incorporating it can affect those soil qualities, especially at the soil surface, causing surface sealing during rain events and subsequent soil crusting.

 

Soil management considerations for this fall

  1. Avoid any unnecessary tillage this fall. Conventional tillage to incorporate residue, such as deep ripping, chisel plow and even vertical tillage, etc., can have negative effects, especially after persistent drought conditions when soil structure is weakened. 
  2. Managing residue - whether removing or shredding - needs to be done with care, especially on high slope areas where potential soil erosion can be significant when fields are exposed to high-intensity rain. Shredding residue after grain harvest will reduce its effectiveness in protecting the soil surface.
  3. Generally, standing residue is highly effective in trapping soil moisture and reducing water movement or surface flow over the field and also increases soil water infiltration and subsoil moisture recharge for the following season. Keeping crop residue intact on the soil surface with roots anchored in the soil can help protect soil and reduce soil erosion.
  4. Consider planting cover crops this fall. However, soil moisture conditions are critical for establishing cover crops. The use of cover crops will be a good option on fields where corn was cut for silage, especially on high slope areas. Cover crops help reduce soil erosion and increase soil water storage. Also, cover crops can help extract excess nitrogen in the soil profile after cutting corn for silage or grain harvest. This can be especially important in low-yield areas this season.
  5. If early harvest occurs, soils will be exposed to weather conditions for a longer period of time this year than normal; therefore, leaving crop residue intact will provide protection from potential late-season rain events. As mentioned in number 4, one management decision farmers should consider is the use of cover crops because if the weather holds, there will be a good window of time to establish them this year.

In summary, tillage can be very destructive to soil in terms of reduction of the residue cover in compromising soil quality. We must maintain our soil quality to sustain yield and reduce nutrient loss during the off season. Corn residue left on the field after harvest is a critical source of soil organic matter. It provides protection for the soil against water and wind erosion, and it contributes to the improvement of soil water storage and water quality.  All this will depend on the intensity of tillage this fall.

 

Mahdi Al-Kaisi is a professor of agronomy with research and extension responsibilities in soil management and environmental soil science. He can be reached at malkaisi@iastate.edu or 515-294-8304.

Be on the Lookout for Green Stem Syndrome in Soybeans

By Daren Mueller, Department of Plant Pathology and Microbiology, and Clarke McGrath, ISU Extension and Outreach

Reports are trickling in about green stem syndrome (GSS) in Iowa soybean, which is like rubbing salt in the 2013 soybean crop wound. Before we get into details of the disorder, we wanted to let you know that the IPM program is doing an analysis to determine the most common causes of GSS. This article discusses the confusion about what actually causes this disorder. If you have a field with any level of GSS, we would really like to include it in our analysis. Please contact Nate Bestor (bestor@iastate.edu), Iowa State University pest management specialist,  to see how you can participate.

 

Symptoms

Green stem syndrome (Fig. 1) is a soybean disorder in which stems remain green after pods and seeds are fully mature and ready to harvest. Cutting affected plants during harvest is difficult and seed quality can be reduced.

There is a classic definition of how GSS appears, but in reality, the symptoms vary quite a bit. Sometimes, plants with GSS can have just a few pods on the upper nodes, or there can be leaves still on at upper nodes in some spots. Also, immature green pods may be clustered on the upper part of the plant, and even empty pods have been associated with GSS.

Bottom line: if there are any mature pods in a field ready to harvest, and green, chewy stems are also present, this is considered GSS by our definition.

Figure 1. Green Stem Syndrome. Used with permission, Laboratory for Soybean Disease Research, University of Illinois.

 

Causes

Depending on the location, many different factors have been associated with GSS, including viruses (especially bean pod mottle virus), soybean aphids, thrips, stink bugs, leafhoppers, bean leaf beetles and Cercospora leaf blight. However, USDA research in Illinois showed no relationship between green stem syndrome and many of these factors. This doesn’t necessarily mean these factors do not cause green stem syndrome, but what is evident is that you can get GSS without these factors. Also, some of the symptoms described in scientific literature are not what we see in Iowa fields.

Some plant pathologists think that there is a separate viral or fungal pathogen that causes GSS, and it is yet to be identified. Some think that it is physiological. Evidence for a physiological effect comes from a research project from Kentucky where they dramatically increased GSS by severe pod removal. What conditions can cause a reduction in pods and an increase in GSS? Well, over the past few years, season long soil moisture stress seems to be a trigger, especially post flowering. When a soybean plant is growing normally at the beginning of the season, it produces high levels of carbohydrates to feed itself; when drought or other stress hits, pods can fall off or not fill in relation to the carbohydrate supply in the plant. If the plant has a pool of carbohydrates in reserve, it may stay green longer. That is one theory.
 
Other factors that have been associated with GSS, at least anecdotally, include lower fall humidity and higher fall temperatures that lead to faster drydown for the grain and not enough time for stems to dry. Also, foliar fungicides have sometimes been associated with GSS. This year the drought limited the number of acres receiving fungicide applications so this may not be a large factor this fall. Plant populations also have been associated with GSS, but oddly, both lower and higher populations have been accused of being the problem.
  
There have been many observational or anecdotal reports about varietal tendencies, but this is really hard to accurately assess. For now, the best thing to do is to note the varieties that have more GSS and try to avoid these varieties in future years.

 

What can we do about green stem syndrome and harvest?

Not much. Harvesting green, stemmy beans is frustrating. Grinding through them with an expensive combine when they are yielding average to below average is even more frustrating. One option is to wait until a hard frost or wait until stems dry down. However this is risky as pods can split or shatter, beans can become too dry or bad weather can wreck fields. Some people have considered using Gramoxone to desiccate the plants and speed the process up, but we have not heard of this being done with any tangible success.
 
Unfortunately, grinding slowly through the fields prior to a hard frost is likely the best option. No management changes for future crops at this time will help with reducing GSS. We will continue to investigate this disorder to hopefully provide some more useful management information for future years. Again, please contact us if you have any amount of GSS in your fields. The information we get may help us to understand and manage this disorder.

 

Daren Mueller is an assistant professor. He can be reached at 515-460-8000 or e-mail dsmuelle@iastate.edu. Clarke McGrath is an extension program specialist. He can be contacted at 712-215-2146 or by email at cmcgrath@iastate.edu.

Corn Belt Corn Crop Conditions and September Yield Forecasts – 2013

By Roger Elmore, Department of Agronomy

Crop conditions across the U.S. Corn Belt varied dramatically in 2013.  Iowa was no exception and this year sits in a rare position in terms of yield potential: lower than several other Corn Belt states.

 

Iowa – 2013 growing conditions

Iowa conditions were dry in January 2013 - drier, in fact, than in January 2012. Several ICM articles addressed our concerns and management suggestions dealing with dry soils: for example, plant population changes; hybrid changes; soil management; and  planter adjustments. March and April rains and snow relieved our concerns about drought. In late April, a few days of good planting conditions encouraged Iowa farmers to plant 2 percent of their acres in late April. Early May snows hampered planting for some time and conditions continued to be wet and cool. Only half of Iowa’s corn was planted by mid-May in dramatic contrast to 2010 when half of our corn was planted by April 18th.

Half of our state’s 2013 corn was silked by July 28th (Figure 1), in contrast to 89 percent for the five-year average and 99 percent in 2012. Although conditions around silking were reasonable and likely resulted in good kernel set, hot, dry conditions occurred after that and continued into mid-September.  Heat unit accumulations ranged above normal during a several week period, hastening crop development and likely reducing yields.

Figure 1. Percent of corn rated good and excellent across the Corn Belt during the 2013 growing season. Columns starting from the left for each state are for reports issued weekly from June 2 through Sept. 15, 2013, respectively. Crop condition data were compiled and adapted from USDA-NASS Quick Stats. The single yellow column among the blue columns for each state represents the first week at which 50 percent or more of the crop was silked based on USDA Crop Progress data. The first number in the rectangles overlaid over the columns for each state indicates that state’s final 2012 USDA yield. The number to the right in each box is the September 2013 yield forecast for that state. Yield data are derived from the USDA-NASS September 2013 Crop Production report. Full-size image

 

USDA-NASS crop condition reports

The USDA-NASS publishes crop condition reports for major corn-producing states weekly in Crops & Weather. The crop condition segment of these reports derives from subjective data. Subjective data are based on opinions of volunteers who survey crop conditions weekly and report their findings. Subjective data are more open to interpretation and may possibly be influenced by emotions. On the other hand, objective data, like that which the USDA-NASS yield forecasts are based upon, are in large part fact-based, measureable, quantifiable and repeatable. With this understanding, USDA-NASS uses the following criteria to rate crop conditions subjectively during the growing season.

  • Very Poor - Extreme degree of loss to yield potential, complete or near crop failure. Pastures provide very little or no feed considering the time of year. Supplemental feeding is required to maintain livestock condition.
  • Poor - Heavy degree of loss to yield potential that can be caused by excess soil moisture, drought, disease, etc. Pastures are providing only marginal feed for the current time of year. Some supplemental feeding is required to maintain livestock condition.
  • Fair - Less than normal crop condition. Yield loss is a possibility but the extent is unknown. Pastures are providing generally adequate feed but still less than normal for the time of year.
  • Good - Yield prospects are normal. Moisture levels are adequate and disease, insect damage and weed pressures are minor. Pastures are providing adequate feed supplies for the current time of year.
  • Excellent - Yield prospects are above normal. Crops are experiencing little or no stress. Disease, insect damage and weed pressures are insignificant. Pastures are supplying feed in excess of what is normally expected at the current time of year.  http://www.nass.usda.gov/Publications/National_Crop_Progress/Terms_and_Definitions/index.asp#condition

 

Iowa Corn Conditions: Correlated with reduced yield forecasts - 2013

For most of us in Iowa, it was no surprise to see our corn crop conditions slide weekly for 10 weeks from early July through mid-September (Figure 1). By mid-September, only 35 percent of our crop rated in either the ‘Good’ or ‘Excellent’ categories. That suggests that only 35 percent of our crop was expected to yield normally or greater than normal. On the other end of the scale, 28 percent of the crop rated ‘Poor’ or ‘Very Poor;’ expectations for these crops are for “heavy” or “extreme” losses in yield potential. 

September’s USDA-NASS corn yield forecast of 162 bushels per acre for Iowa is nearly 17 bushels below the 30-year trend line – a 9.4 percent reduction from my 2013 trend line yield estimate of 178.9 bushels per acre. That still sounds better than last year’s yield of 137 bushels per acre for Iowa!  Be aware though that these crop condition data are summarized as state averages. Obviously, conditions range widely in Iowa as reflected by September’s USDA-NASS yield forecast

 

Corn Belt corn conditions and yield forecasts  - 2013

Corn growing conditions also varied widely across the Corn Belt during the 2013 growing season (Figure 1).  While Iowa’s corn conditions fell for 10 straight weeks, Missouri’s fell for five weeks and Wisconsin’s nine weeks. These three states all have less than half of their acreage in good and excellent condition and thus should expect below trend line yields based on subjective data from the crop condition reports. Iowa’s September forecast yield as mentioned is 9.4 percent below trend line. September yield forecasts for Missouri, 125 bushels/acre, and Wisconsin, 143 bushels/acre, are the lowest among the state’s shown in Figure 1.

On the other hand, Illinois and Minnesota ratings currently lie between 50 and 60 percent good and excellent. They might expect normal yields. Although ratings fell for several weeks in Indiana, Nebraska and South Dakota, their good and excellent ratings remain at or above 60 percent. Corn conditions in Ohio hovered above 70 percent good and excellent all season. Expect to see very good yields from Ohio! September 2013 yield forecasts across the Corn Belt seem to correlate with crop condition reports after silking (Figure 1).
Considering the kind of year Iowa corn experienced, we are fortunate to have a forecast yield as high as it is at this time.  Modern hybrids, improved management systems and Iowa’s excellent soils certainly pay off in a stressful year like 2013.

 

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

Harvest 2013: Another Set of Extremes

By Charles Hurburgh, Department of Ag and Biosystems Engineering

This sounds like a broken record, but we are looking at yet another harvest driven by weather extremes and combinations that are hard to predict. The major events were the extremely late planting in very wet soils, followed by an almost complete turnaround in many places to a steadily growing drought condition. In July, the market believed that our major harvest risk would be very wet and late crops vulnerable to even an average frost. By Sept. 1, protracted heat changed the picture completely to the point where corn harvesting has begun in several markets. The quality and management forecast now differs sharply between corn and soybeans.

 

Corn

The recent extension of 90+ temperatures with inadequate rainfall has rapidly increased maturity of corn regardless of planting date. This demonstrates the principle that timing of events is at least as important as the average conditions. On average this year was “average” temperature and rainfall. However, corn quality is driven by conditions during grain fill. Kernels are small and shallow; the extent of kernel fill will be variable depending on timing of rains. Last year drought-stressed plants put unexpectedly large amounts of dry matter into kernels, resulting in the highest test weights and protein contents in many years. Areas that had enough rainfall to continue root development in June and July may experience the same result, but shallower rooted plants likely will have reduced fill and, therefore, lower test weights. 

Test weight is one of two reliable indicators of storability, the other being the variation in moisture at harvest. Moisture variation will be an issue this year; even within the same planting date, there are large differences in maturity within fields or even the same rows. If there are large areas of  replants, harvesting around them is a good option, but within fields there is little choice but to harvest straight through, which creates challenges for drying and handling. Recognize that early harvest will happen in warm weather. Rapid drying and cooling will be critical to preserving the storage life of 2013 corn. Actions in the first few days after harvest can either preserve or waste the future storage life of grain.

 

Soybeans

Soybeans are small but will probably be dry, except those planted quite late (June and after). Late-planted soybeans may still have some frost risk, especially in areas that received enough rain in August and early September to slow down the maturing process. Growth in the late season will mean harvesting with green stems and mixed quality.  We will have more information on frost impacts and handling of frost damage if this problem occurs.

 

Harvest planning and preparation

Scout fields for fungal infections.  Until the very recent hot weather, fungus and related mycotoxin problems seemed unlikely. However, deteriorating conditions and repeated small rains may encourage field fungi. It is important to understand which fungus can produce which toxin. There is a video tutorial on mycotoxins on the Iowa Grain Quality Initiative website.

 

Scouting for ear rots

Fields should be scouted for ear rots from black layer development onward. (See Figure 1.) At several locations in a field, peel back the husks of several ears and examine the ear for signs and symptoms of ear rot. Take note of what ear rot is present.  If more than 10 percent of the ears in the field have ear rot, the field should be scheduled for harvest as soon as possible. This is particularly important in the case of ear rots that are associated with mycotoxin production (Aspergillus, Fusarium and Gibberella). Wet weather after maturity while the grain is drying down may increase the risk of toxin production by favoring growth of the fungus.

Figure 1.

 

Grain management

Following basic principles of grain management will be important; the high variability in quality will not leave much room for error.

  • Immediate cooling after harvest (and drying) - shelf life begins right away.
  • Have adequate aeration (0.1 cfm/bu or more) -  all bins with all grains should be aerated.
  • Run a cooling cycle every 10-15 degree average change in outside temperature, starting at harvest. With 0.1 cfm/bu, a cooling cycle will take about 150 hours; proportionately less for higher airflows.
  • Get grain below 40F as quickly as possible.
  • Take out the center core of fines. Variable quality and lower test weight will mean more fines.
  • Inspect grain and monitor temperature weekly until December; every two weeks thereafter. Automated temperature cable systems are very useful; the larger the bin the less likely a manual check will be adequate.
  • Responding to temperature change is as important as the actual temperature. A 3 to 5 degree change between readings, even if from 40 to 45 F, is indicative of spoilage if the fan had not been run in the interim.
  • Stay within temperature-moisture guidelines.  These are listed below. (See Figure 2.)

Figure 2.

 

Operating suggestions

  • Recalibrate your yield monitor.
    – Seed size and density are factors in yield monitor response.
  • Take more than one moisture sample any time moisture is being measured.
    – Average of at least three separate samples
  • Check dryer moisture often (out and in)
    – This applies especially to continuous flow and automated batch dryers.
  • Check accuracy on freshly dried grain.
    – Freshly dried grain normally reads low; test 5 to 10 sealed samples 4 to 6 hours later to establish a rough correction factor.
  • Keep good records on what grain where.
    – Tracking of wet grain can indicate where problems could occur later.
    – Insurance records often need traceability by unit.
  • Separate by test weight.
    – Check each new field as you open it.
    – Sell light grain first and grain that clearly had large moisture variations.
    – The yield monitor moisture will give a good indication of high variability.
  • Expect hot spots in storages.
    – No dryer can completely even out wide moisture variances.
    – February and March will be the most likely time that problems will arise.
    – Good temperature systems are needed to detect hot spots deep in large bins.
  • If you have late planted corn in spots, drive around them and come back later.
    – Moisture will be higher and test weight will be lower.
  • Take out the center core of fines in all bins. 
    – Variable quality and lower test weight will mean more fines.
    – Fines disrupt and restrict airflow.

 

Charles Hurburgh is a professor in the Department of Ag and Biosystems Engineering. He can be reached at 515-294-8629 or e-mail tatry@iastate.edu.



This article was published originally on 9/30/2013 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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