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4/21/2008 - 4/27/2008

Improved Disease Resistance on the Way – One Key to Soybean Improvement

By John H. Hill  and Alan L. Eggenberger, Department of Plant Pathology

As spring planting season begins, it is difficult to predict those diseases that may be most problematic during the growing season of 2008. So much depends upon rainfall, temperature and a myriad of other factors that can affect crops and pathogens. This includes potential for resistance to various pathogens. Despite all the management practices we talk about, disease resistance remains one of the most practical and economic ways to control plant disease.  But, sometimes the genes that confer that resistance are not so easy to find. Iowa State University scientists, in research that would not have been possible without support by soybean check-off dollars, have recently been working to develop a means to find and characterize resistance genes to pathogens when resistance has been difficult to find. 

Plants can have elaborate mechanisms to recognize and counterattack against invading pathogens. Resistance occurs when a plant resistance gene (R gene) recognizes a gene in the pathogen (avirulence determinant) and starts a complex biochemical pathway to stop the pathogen. This recognition can occur either directly or indirectly. In soybean, very little information exists on genes that are essential for these biochemical pathways that control defense against disease. Identification of the genes that are essential for resistance is key to development of disease resistant soybean varieties.

To establish a road map that will be effective for examination of resistance to a multiplicity of soybean pathogens, the interaction of the resistance gene Rsv1 with soybean mosaic virus has recently been examined. Through utilization of several different methods, two genes of the virus have been identified as the avirulence determinants. By means of a unique evolutionary approach, researchers identified specific mutations in the viral genes that result in overcoming resistance. This occurs as a result of selection pressure imposed by R genes in soybeans. 

Resistance to soybean mosaic virus was chosen for study because the genetic studies of this system are much further along than for other soybean pathogens. Knowledge from these studies is being used as a guide to unravel the disease resistance network in soybean. Resulting from research supported by check-off dollars, it is revealing clues that will allow identification of genetic resistance to numerous yield-robbing soybean pathogens. Stay tuned. Improved resistance against several different problematic pathogens is on the way!

John H. Hill is a professor in the Department of Plant Pathology working on virus diseases of Iowa crops.  Alan L. Eggenberger is an assistant scientist with research responsibilities on virus diseases of soybean.

New Directory of Sensitive Crops and Apiaries for Pesticide Applicators

By Kristine Schaefer, Department of Entomology

The Iowa Department of Agriculture and Land Stewardship is developing a new registry that will provide a list of locations of sensitive crops in addition to apiaries.

This list of locations, available to pesticide applicators in an online directory, will be provided so pesticide applicators can be aware of sensitive areas prior to making pesticide applications.This will, in turn, help to minimize the potential for damage from pesticide drift.

Persons who raise sensitive crops (vineyards, orchards, certified organic crops and fruit and vegetable crops) and beekeepers will need to register with the Iowa Department of Agriculture and Land Stewardship. Field markers will be available to registered producers so sensitive areas are clearly marked and visible to both ground and aerial applicators. 

Kristine Schaefer is an extension program specialist with the Pest Management and the Environment Program.

Check Out the ISU Plant and Insect Diagnostic Clinic

By Laura Jesse, Plant and Insect Diagnostic Clinic

 

The Iowa State University Plant and Insect Diagnostic Clinic helps growers and agribusiness professionals throughout Iowa diagnose and manage plant disease, weed and insect problems on all kinds of plants. Clients are educated on ecologically sound plant disease, weed and insect management practices.

 

Services offered include:

Diagnosis of plant problems       $10
Identification of insects, plants, weeds or mushrooms Free
Soybean cyst nematode egg count $15 in state

$30 out of state

Complete nematode count (for corn nematodes) $30 in state

$60 out of state

Soybean cyst nematode HG type test $150

 

Submitting a sample for diagnosis 

When submitting a plant sample please provide plenty of fresh material and when possible submit the entire plant. Wrap the plant in dry newspapers and place it in a box for shipping. Do not add any moisture. 

 

Insects should be submitted in a container to prevent crushing in the mail. Hard bodied insects (beetles, grasshoppers) need no special preservation, but soft bodied insects (caterpillars, aphids) should be placed in a vial with alcohol or hand sanitizer.

 

Any background information you can provide also is a big help.  Please tell us about the chemicals applied, when you first observed the problem, the pattern and how quickly it spread, the history of planting in the field and anything else you think might be relevant. 

 

For specific information on submitting a soybean sample please see our new pamphlet.

 

For more information, contact your local ISU Extension County Office, or contact the Clinic directly at (515) 294-0581, sickplant@iastate.edu.

 

Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic.

Good News—Bean Leaf Beetles Hit Hard by Winter Cold

By Marlin E. Rice and Rich Pope, Department of Entomology

 

During the last 20 years, the bean leaf beetle has undergone tremendous population changes in Iowa. From 1989 to 1996, the populations (both first generation and second generation) were relatively insignificant and the insect was not considered to be a serious pest. But in 1997, the population in central Iowa began a yearly escalation until it reached a historical high in 2002. Populations that year were nearly 400 times larger than those of the mid-1990s and we believe that this was due in part to milder winters followed by earlier planting of soybeans. Since that time, the population has returned to more normal levels and is similar to what we witnessed at the beginning of the beetle explosion in the late 1990s.

 

Adult Bean Leaf Beetle - Photo by Marlin E. Rice

Adult bean leaf beetle. (Photo by Marlin E. Rice)

 

Since 2002, the bean leaf beetle has hit on "hard times." Insecticide spraying for soybean aphids during July and August may have greatly reduced the second generation. Each year that we spray aphids, fewer beetles survive the summer to go into hibernation. A resulting benefit from the soybean aphid applications is that the bean leaf beetle is now relegated to secondary pest status in many areas.

 

So what should we expect for 2008? It was a long, cold winter, and based on temperatures, we predict that the mortality of overwintering bean leaf beetles was very high—especially in the northern and central tiers of Iowa counties (see map). To place this predicted mortality into a historical context, for central Iowa the predicted mortality is 94 percent, which is the second highest prediction during the last 20 years (see chart).

 

Bean Leaf Beetle Overwintering Rates in Iowa

 

 

 

Bean Leaf Beetle Winter Mortailty in Iowa

Data Source: Midwest Climate Information Center

 

The beetles will be back again this spring but their numbers should be the lowest of the past seven years. However, the earliest emerging fields in an area (such as several sections) should be closely scouted and managed if necessary. Beetles are highly attracted to these early emerging fields. Fields that should be of greatest concern for damage from this insect, and the virus it spreads (bean pod mottle virus), are food-grade soybeans and seed beans where reductions in yield and seed quality can be significant.

 

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

Alfalfa Weevil Predictions for 2008

By Marlin E. Rice and Rich Pope, Department of Entomology

 

Cold! That is the best way to describe the first three and a half months of 2008. A cool spring has delayed alfalfa weevil hatch this year, however Iowa fields should have accumulated sufficient temperatures for larvae to hatch starting April 23-25 in southern Iowa. Naturally, weevils with hatch at slightly later dates in the central and northern counties of the state.

2008 Alfalfa Weevil Hatch Dates in Iowa

The top number is accumulated base-48 degree days between Jan. 1 and April 20, 2008. The bottom number is the projected date of alfalfa weevil hatch for each Iowa crop reporting district.

 

Effective management of alfalfa weevil depends on timely scouting, correct identification and effective determination of insect populations. From that information, better economic decisions can be made that may entail spraying with an insecticide or cutting the hay crop early. Alfalfa weevil larvae can be very destructive to first-cutting alfalfa, so fields should be scouted. Larvae remove leaf tissue, beginning with the new leaves at the top of the plant, then work down the stem to other leaves. This feeding reduces forage quality and quantity.

Alfalfa Weevil larva. Photo by Marlin E. Rice
A young alfalfa weevil larva. (Photo by Marlin E. Rice)

Scouting for alfalfa weevil should begin at approximately 200 degree days in fields south of I-80 (the three southern crop reporting districts), and 250 degree days in fields north of I-80. The map indicates the accumulated degree days across the nine crop reporting districts. Begin scouting in southern Iowa based on the projected hatching dates, but remember that scouting should start on south-facing hillsides. Larvae typically will hatch here first because these areas warm up more quickly than north-facing hillsides.

Using a sweep net can save some time when first scouting a field for alfalfa weevil larvae. With the sweep net, a field can be quickly and easily checked to determine whether larvae have hatched. If no larvae are found in the net, then move on to the next field. However, if larvae are found in the net, then collect 30 stems and count the larvae in the upper leaves.

Economic thresholds for alfalfa weevils and related management options will be provided in an article during the next week on Integrated Crop Management News.

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

Delayed Burndown Applications in No-Till

By Bob Hartzler, Department of Agronomy

As the end of April nears with little field work accomplished, getting the crop in the ground becomes the priority for many farmers. No-till farmers may plant fields prior to killing weeds with a burndown herbicide, with the intent of returning later to control the weeds with an early postemergence application. While this strategy can be effective, it is important to realize that planting into established weeds greatly shortens the time required for weeds to impact crop yields (critical period).

When crops are planted into a weed-free seedbed, weeds typically compete with the crop for three to four weeks after crop emergence without impacting yield, but this isn’t the case when planting into established weeds. Factors that will influence the critical period include:

  • density of weeds
  • size of weeds
  • cultural practices (row spacing, seeding rate, etc.), and
  • environmental conditions.

Because of the many variables involved, it is impossible to accurately predict when weeds begin to affect yields; however, weeds present at planting may affect yields as early as the V1 stage of crop development. Thus, appropriate herbicides should be applied as soon after planting as possible to remove these weeds.

Due to the increased cost of glyphosate, an increase in the use of preemergence herbicides is anticipated in order to reduce the number of postemergence glyphosate applications. Certain preemergence herbicides cannot be applied if the crop has emerged, so be sure to determine label restrictions and crop stage of development prior to application.

Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities.

Inoculating CRP Ground

By Palle Pedersen, Department of Agronomy

 

The Prospective Plantings Report by the National Agricultural Statistics Service (NASS), Agricultural Statistics Board, U.S. Department of Agriculture was released on March 31, 2008. Iowa corn acreage in 2008 is expected to be 1 million acres less than in 2007 and it appears that this, along with other ground (CRP, hay and small grains), is being directly switched over to soybean (increase of 1.25 million). With the increase in soybean acres do we need to inoculate our soybean seed when planted on some of the “new” soybean acres? It depends. 

 

Nitrogen fixation is the process of converting atmospheric nitrogen into a usable form for the plant and is critical for producing higher yields in soybean. For nitrogen-fixation to occur, the nitrogen-fixing bacteria known as Bradyrhizobia japonicum need to be readily available in the soil or must be applied to the seed to form nodules on the soybean root. 

 

When the seed germinates, the bacteria invade the root hairs of the seedling and begin to multiply. Nodules, which house the bacteria, form on the roots. Under field conditions, nodule formation can be seen shortly after emergence but active nitrogen fixation does not begin until about the V2 to V3 stage. After this, the number of nodules formed and the amount of nitrogen fixed increase with time until about R5.5 (midway between R5 and R6), when they decrease sharply. 

 

There is a mutual benefit in the relationship between the Bradyrhizobium bacteria and the soybean plant. For nitrogen fixation to occur, Bradyrhizobium bacteria need to be present in the soil. The plant, in turn, provides the bacteria's carbohydrate supply. A relationship such as this, where both bacteria and plant profit from the other, is called a symbiotic relationship. 

 

Nitrogen (N) fertilization of soybean is not a common practice because it generally does not increase grain yields. The total number of root nodules that form decreases proportionately with increasing amounts of applied N. In addition, N fertilizer applied to a soybean plant with active nodules will render the nodules inactive or inefficient, proportionately to the amount of N applied. Thus, although the soybean plant can use both fixed N from bacteria and soil N (both mineralized and fertilizer N), but soil N is used in preference to fixed N if available in large amounts. 

 

Although soybean does not respond with increased yield to the addition of N, plants remove a significant amount of it from the soil. In Iowa soils, which have appreciable residual N levels, up to 50 percent of the total plant N has been attributed to N2 fixation. Increasing nitrogen supply by adding fertilizer, animal manure, sludge, or a green manure crop simply substitutes nitrogen from these sources for nitrogen that would otherwise be fixed by the bacteria in nodules on the roots.

 

I have conducted multiple inoculant experiments across Iowa every year since 2004. Our recommendation in Iowa, based on our data, is very similar to other states in the Midwest. It is recommended to inoculate the seed if nodulated soybean has not been grown in a field in the past 3 to 5 years (like CRP), if soil pH has not been maintained above 6.0, if fields have sandy soils and are irrigated, and then if fields get flooded frequently. 

 

For the common soybean grower in Iowa with a corn-soybean rotation or a corn-corn soybean rotation we do often not see an advantage (or disadvantage) of using an inoculant. The major reason is the high frequency of growing soybean in rotations and the widespread soybean production both of which keep the inoculant level adequate in most fields due to soybean growth and dust movement. In addition, Iowa soils are fertile and have a great soil supply of plant available N that decreases the chance for severe N shortage in Iowa. However, it is recommended that if you put CRP ground into production in 2008, it would be a wise investment to inoculate your seed just to be sure that you do not have a shortage of N. More information about inoculants can be found on www.soybeanmanagement.info with other information about inoculant and the data from our inoculant evaluation trials in Iowa in 2007.   

 

Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production.

Will 2008 be the “Perfect Storm” for Soybean Seedling Disease?

by Palle Pedersen, Department of Agronomy and Alison Robertson, Department of Plant Pathology

 

There have been several reports in the press about poor soybean seed quality for this growing season. Seed quality is a problem for many companies this year but all are doing everything they can do to ensure we have the best quality seed possible. In Iowa, however, seed quality issues are minor compared to the south.

 

Seed quality

Seed companies usually sell high quality seed but this year the standard will be lower for some varieties for a number of reasons;

  • demand for soybean seed is high this season due to low supply because seed from some production fields was rejected because of low quality and an increased in soybean acres this year;
  • a high prevalence of seedborne fungal infection; and
  • lower germination as a result of mechanical damage due to thinner and cracked seed coats.

 

In Iowa, the lower germination rates have mostly been attributed to mostly to fungal infection. Seedborne fungal infection can result in seedling disease under the right environmental conditions and some seed treatments are effective against these seedborne fungi.

 

Seedling disease

Several fungi are able to infect germinating seed and cause damping off on soybean. Most of these fungi are soilborne, e.g., Phytophthora sojae, Pythium, Rhizoctonia and Fusarium spp, however there are seedborne fungi, e.g., Phomopsis, which also can affect germination and cause damping off. Each pathogen is favored by slightly different environmental conditions at and soon after planting. Very, wet conditions are required for infection by Phytophthora and Pythium, where as infections by Rhizoctonia and Fusarium are favored by drier soil conditions. Soil temperatures also are important: Pythium prefers cool soils (<60 degrees F), while Phytophthora prefers warmer soils (65 – 75 degrees F) and Rhizoctonia prefers even warmer soils (>75 degrees F). 

 

Seedling diseases of corn result in reduced plant stand, poor plant vigor and consequently lower yields. In contrast, however, a reduced stand of soybean does not always result in a lower yield. If the germination is lower than the “typical” 90 percent we will need to be sure to adjust and increase our seeding rate to account for this difference. The problem is however, that even though we adjust the seeding rate to take into account lower germination (as confirmed in a seed testing laboratory), we never really know how the seeds will perform under field conditions.

 

Fungicide seed treatments

Fungicide seed treatments can be effective in preventing or reducing damage from pathogens that may be carried in/on the seed or pathogens present in the soil that cause seed decay, seedling blights and root rots of soybeans. However, not all seed treatment fungicides are equally effective against all fungal pathogens. Products that contain the active ingredients metalaxyl or mefenoxam, e.g. Allegiance, and Apron XL are effective against Pythium and Phytophthora. Other active ingredients, e.g., azoxystrobin, captan, carboxin, fludioxonil, PCNB, thiram and thiabendazaole are effective against Fusarium, Phomopsis and Rhizoctonia. Usually choice of a fungicide seed treatment will depend on knowledge of what disease problems are prevalent in a particular field. Combination seed treatments are available, and can be used when this information is not available.

 

With the wet and cool soil conditions that many growers are facing this year the risk is high for seedling blight caused by Pythium in addition to damping off caused by seedborne Phomopsis. Remember however, fungicide seed treatments will not improve germination of seed that has poor quality and lower germination as a result of mechanical damage because of a thin seed coat, cracked seed coat and other physiological factors.

 

There have been rumors that germination rates are further lowered if low quality seed is treated in advance. Some rumors have even suggested we should not be treating seed because potential mechanical damage from the treating process also may lower germination. We do not know of any independent data to support these claims. If you plan to treat your seed, do it when time allows and handle all seed carefully, particularly if they have thin seed coats and are fragile.

 

Be patient when it comes to planting

Since the germination levels in some seed lots is below the “normal” 90 percent, it is imperative that we get a good stand the first time around, and avoid replanting since that will lower yield potential due to delayed planting, and also, we may have to replant with seed that may be of even worse quality. The key thing this year is to communicate fully with your seed company and your agronomist and know the condition of your seed. This year, everybody should read the seed tag and know the percent germination of their seed. Since there are large differences among seedlots, each bag or unit should be checked.

 

Based on the condition of available seed and the cold wet start to the growing season, it appears that we may have the “perfect storm” regarding soybean seedling disease. The bottom line: Be patient. Do not get stressed out with the current weather pattern and soil conditions. Plant only when soil conditions are suitable. A seed treatment will likely be a good investment this year. 

 

Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production. Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases.

Surface Waters: Ammonium is Not Ammonia

By John Sawyer, Department of Agronomy

 

A recent article in The Des Moines Register newspaper has caused considerable controversy regarding nitrogen in Iowa streams and rivers. The article (High ammonia levels threaten D.M.’s water, April 6, 2008) featured information about “ammonia” levels in certain Iowa surface water systems during the recent winter time period.

 

The implications were that manure and fertilizer application to cropland, and subsequent snowmelt and runoff, had resulted in higher than normal “ammonia” levels in surface waters. In the article there was a comparison of the reported levels to an ammonia reading of 0.10 parts per million considered harmful to aquatic life. Unfortunately, measured surface water concentrations (and as reported in the article) are not ammonia-N. Instead they are ammonium-N plus ammonia-N. Therefore, a comparison of the reported values to a concentration of ammonia toxic to aquatic life is inaccurate.

 

Ammonia is un-ionized, and has the formula NH3. Ammonium is ionized, and has the formula NH4+. The major factor that determines the proportion of ammonia or ammonium in water is water pH. The activity of ammonia also is influenced by temperature and ionic strength. This is important as the unionized NH3 is the form that can be toxic to aquatic organisms. The ionized NH4 is basically harmless to aquatic organisms.

 

The chemical equation that drives the relationship between ammonia and ammonium is:

 

NH3 + H2O ↔ NH4+ + OH-

 

When the pH is low, the reaction is driven to the right, and when the pH is high, the reaction is driven to the left. In general, at a temperature of around room temperature, at a pH less than 6.0, the proportion of ammonium-N plus ammonia-N as NH3 is very-very low and as NH4+ is very-very high. At a pH around 8.0, the proportion as NH3 is 10 percent or less, and at a pH slightly above 9.0, the proportion is about 50 percent. The activity of aqueous ammonia also is much lower at low temperatures and higher at warm temperatures. This means that at low temperatures and low pH the activity as NH3 is even lower, and as NH4+ is even higher. Therefore, sensitive aquatic organisms can tolerate a higher total “ammonium-N plus ammonia-N” at low temperatures than at high temperatures due to much less aqueous NH3 being present in the water.

 

The laboratory method used for analysis of water measures ammonium-N plus ammonia-N. It is very difficult to directly determine the activity of aqueous ammonia, so instead the surrogate of ammonium-N plus ammonia-N is used, and then tabled values of ammonium-N plus ammonia-N are used to determine if a measured concentration will provide ammonia at a level that is detrimental to aquatic organisms, for acute and chronic conditions. These tabled values are a surrogate since the measured concentration is a total of the ammonium-N plus ammonia-N, and the concentrations in the tables for chronic or acute levels are set to reflect back to likely concentrations of ammonia-N for specific water pH and temperature.

 

The acute and chronic criteria for “ammonia” have been established for Iowa streams designated for aquatic life uses (Chapter 61, Iowa Administrative Code; tables 3a, 3b and 3c). One has to carefully use the tables as the listed concentrations are for ammonium-N plus ammonia-N, not ammonia-N (the header to the tables says “ammonia”). As expected, chronic criteria (ammonium-N plus ammonia-N concentration) are higher for low pH and low temperature water (ex. pH 6.5 at 0 degrees C is 6.67 mg N/l, early life stages present) and lower for high pH and high temperature water (ex. pH 8.0 and 26 degrees C is 1.16 mg N/l).  Similarly, acute criteria are higher for low pH water (ex. at a pH of 6.5 the criteria for class B (WW1-3) and B(LW) is 48.8 mg N/l) and lower for high pH water (ex. at a pH of 8.0 is 8.4 mg N/l).

 

The early February 2008 ambient monitoring levels from the Iowa Department of Natural Resources Storet database for the rivers identified in The Des Moines Register article ranged from 0.13 to 1.00 mg N/l (ammonium-N plus ammonia-N). At the water pH and temperature during that time (7.6 to 8.1 pH and 0 to 1.0 degrees C), the monitored values are well below both acute and chronic criteria for those conditions (acute criteria 17.0 to 6.95 mg N/l and chronic 3.98 to 2.10 mg/l). Measured ammonium-N plus ammonia-N tends to be variable during the winter months, but the variation and levels this year are not higher than recent history.

 

Bottom line, aqueous nitrogen is complex, as is the effect on aquatic life. For evaluation of water quality monitoring data, it is important to know what is being measured and to be careful and don’t compare apples and oranges.

 

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



This article was published originally on 4/28/2008 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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