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10/5/2009 - 10/11/2009

Crop and Weather Update – First Week of October

ISU Extension climatologist Elwynn Taylor, economist Dan Otto, and soybean agronomist Palle Pedersen are guests this week for the crop and weather update.

Pedersen expresses disappointment in central and northern Iowa yields, feeling the crop has been influenced by yield robbing diseases and cool July temperatures, but he is impressed by southern Iowa yields. Current weather will help soybean plants have more uniform maturity when harvested.

Otto visits about the significance of agriculture to Iowa’s economy where one out of every six jobs are related to agriculture and says 20 Iowa counties have over 50 percent of their jobs related to agriculture.

Taylor says current rainfall isn’t all bad. At the beginning of this week all regions of the state needed at least four inches of precipitation before Nov. 1 to have near normal subsoil moisture levels going into winter.

Fall Is Great Time to Sample For SCN, But Not Corn Nematodes

By Greg Tylka, Department of Plant Pathology

There is a chill in the air and thoughts are focused on harvesting corn and soybeans.  Another annual fall ritual is to collect soil samples. it is important to keep a few things in mind when considering collecting soil samples for nematode testing in the fall.

Fall is NOT a good time to collect soil samples to check for corn nematodes. Corn nematode population densities (numbers) typically decrease in the latter part of the growing season. And it is not possible to calculate back in time to determine what the numbers were in the earlier part of the growing season. Therefore, low corn nematode numbers obtained from fall soil samples are not very informative. Of course, if population densities of corn nematodes are high in soil samples collected in the fall, it is reasonable to assume that the numbers were high earlier in the season as well.

The exception in this situation is with the needle and sting nematodes. These two nematode species occur only in soils that are at least 70 percent sand and they migrate down into the soil profile during the heat of summer. So needle and sting nematodes are best detected in soil samples collected in the spring or in the fall.


Fall is ideal time to collect soil samples to test for SCN, in contrast to the situation with corn nematode soil sampling described immediately above. Fields in which corn or soybeans were grown in 2009 can be sampled for SCN. Sample results will indicate if fields are infested with SCN or if SCN population densities are being kept in check in SCN-infested fields that have had SCN-resistant varieties grown in the past.


Guidelines for collecting soil samples to test for SCN in the fall

• Samples should be collected using a soil probe.
• Soil cores should be collected to a total depth of 6 to 8 inches.
• Collect soil cores from 15 to 20 places in a zigzag pattern in a sampling area.
• Collect a separate set of soil cores for each 20 acres or so.
• Combine and mix soil cores, and fill a sample bag with one cup or more of soil.
• Label the outside of each sample bag with a permanent marker.
• A soil sample can be used to test for SCN and for nutrient analysis.

Numerous private soil testing laboratories in Iowa can test soil samples for SCN.  The Iowa State University Plant and Insect Diagnostic Clinic also analyzes soil samples for SCN. The mailing address of the Diagnostic Clinic is 327 Bessey Hall, Department of Plant Pathology, Iowa State University, Ames, IA 50011-1020. The Diagnostic Clinic’s current fee for SCN analysis is $15 per sample and the Plant Nematode Sample Submission form should be submitted with the sample. 

 

soil sampling in corn stubble

tylka sampling

Fields in which corn or soybeans were grown in 2009 can be sampled for SCN after harvest.

 

 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.Tylka can be contacted at gltylka@iastate.edu or by calling (515) 294-3021.

Flurry of Forage Questions Come with First Fall Frost and Freeze

By Stephen Barnhart, Department of Agronomy

The first frost of the autumn generally brings a flurry of forage related questions.These questions usually center on three general topics:
• Toxic prussic acid potential and management of frosted sudangrass and sorghum sudangrass hybrids
• Suspected toxicity of frosted alfalfa to grazing animals
• Post-frost harvest of last alfalfa cutting
 

Managing frosted sorghum sudangrass and sudangrass
The potential for prussic acid poisoning and management suggestions are related both to the size of the plant when frosted and the extent of frost damage. Producers should be aware that the risk of damaging levels of prussic acid is very unlikely.
 
Prussic acid, more correctly called hydrocyannic acid (a cyanide based compound) is formed in sudangrass or sorghum sudangrass hybrids which are severely stressed or frost damaged. The hydrocyannic acid develops within a few hours after the frost and usually dissipates within a few days. The safest management is to remove cattle and sheep from frosted fields for several days. Livestock can be returned to frost injured sudangrass that is 18 inches or taller and sorghum sudangrass 30 inches or taller after about three or four days. If the grass was shorter than these heights when frost injured, withhold cattle and sheep for 10 days to 2 weeks following the frost to avoid problems. Then watch for new shoot regrowth, (tillers or “suckers”) on partially frost killed plants! Direct grazing of these fresh new shoots can be toxic too. Where new shoots appear following frost, avoid grazing until two weeks after the killing frost that kills the new shoots.
 
Prussic acid poisoning is not a common occurrence. Very few verified cases are reported by veterinarians - maybe Iowa producers are just using good management. Consider the recommendations above to be at the low risk or conservative level.
 
If in doubt, move the livestock to another type of forage. Livestock can be returned to the sudangrass or sorghum sudangrass fields following a killing frost and appropriate post-frost delay period.
 
Frost damaged sudangrass or sorghum sudangrass hybrids can be cut and stored as silage.  Hydrocyannic acid is dissipated during wilting and partially during the ensiling process. Observe proper ensiling technique, particularly moisture content, when ensiling these
crops.
 
Sudangrass and sorghum-sudangrass hybrids are difficult to dry thoroughly enough for safe storage as dry hay. As with wilting and ensiling, most if not all of the hydrocyannic acid is dissipated in the drying process.
 
Producers who want to get frosted sudangrass or sorghums tested for hydrocyannic acid content should first contact a forage or plant tissue analysis laboratory near you and ask first whether they can do the test for you and what they recommend as the proper procedure for collecting, handling and shipping of the sample to the lab. Forage Testing Laboratories PM 1098A can help you locate a laboratory. Sudangrass or Sorghum-Sudangrass should never be used for horse pasture.
 
Is frosted alfalfa toxic?
Frost injured alfalfa, clovers, and the commonly used perennial cool-season forage grasses Do NOT have the potential to form hydrodynamic acid, are NOT considered toxic and can be safely grazed or harvested for hay or silage following a frost.  There is probably a slightly higher bloat risk for grazed alfalfa and white clover the first few days after a frost. Follow normal bloat preventing grazing management when grazing alfalfa and clover.


Now that we've had frost, should I harvest the last alfalfa cutting?
There is not a simple answer. In general, it will depend whether the frost was a killing frost or not.  A killing frost is not the first light frost of the season; rather, it is a 23 or 24 degree F freeze that lasts for four to six hours or so.
 
If the producer does not need the forage, it is best for the alfalfa plants to leave them uncut and standing through the winter.
 
If it was the hard, killing freeze and the producer needs the forage, harvest as soon as possible after the freeze to salvage as much of the nutritive value as possible. The longer the delay, the greater the weathering damage and leaf loss from the standing frosted plants. To improve plant crown insulation over the winter, consider leaving a 5 to 6 inch stubble at this late-season harvest.  
 
If the frost were a light, non-killing freeze, the tops of the alfalfa plants will be visibly damaged but will not likely stop the plants' growth for the season. The damaged tops will deteriorate in nutritive quality for the remainder of the autumn, but the plant will still be attempting to regrow from crown buds and will be using stored sugars. The best management for the plant is to allow it to continue to grow using whatever green leaf area it still has until the hard, killing freeze.  Then if the producer needs the forage, it can be cut and harvested for hay or silage; or grazed.
 
Alfalfa plants cut immediately after a partial freeze (non-killing frost) and which experience further normal growing temperatures will start new regrowth from crown buds, using accumulated proteins and carbohydrates that would otherwise be used for over wintering and regrowth the following spring. When these late-recovering plants experience a killing freeze a few days or weeks later, they will be physiologically weaker and more susceptible to winter injury.


 

Steve Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management. Barnhart can be contacted at (515) 294-7835 or by email sbarnhar@iastate.edu.

Stalk and Ear Rots Prevalent in Iowa

By Alison Robertson, Department of Plant Pathology

The cool, wet growing season has favored infection and the development of certain corn ear and stalk rots in Iowa.  Foliar diseases that occurred earlier in the growing season may have increased the risk of stalk rots in fields. In corn trials across the state, anthracnose and Gibberella stalk rots are common. Ear rots that are being reported across the state include Diplodia, Gibberella and Fusarium. 

Since stalk rots lead to standability problems, fields should be scouted before or at black layer. At least 100 plants, scattered through out the field, should be assessed. Look for visible symptoms and test stalk firmness at the lower internodes with thumb and forefinger. If more than 15 percent of the stalks are rotted, schedule for the earliest possible harvest because significant lodging is possible.  

Similarly, fields should be scouted for ear rots before harvest for a number of reasons:  (1) ear rot diseases can reduce yield and quality of the corn harvest; (2) some ear rot fungi may produce mycotoxins which are harmful, and can be fatal, to livestock; and (3) ear rots can continue to be a problem in storage if the grain is not stored under optimum conditions.

Recognizing which stalk and ear rots are prevalent in a field is important for future management decisions. Susceptibility to these diseases differs amongst hybrids, so choosing a hybrid with a better disease tolerance score can help reduce disease risk in subsequent years. Rotation to soybean can also decrease disease since some pathogens survive in corn residue.

Diplodia Ear Rot
Diplodia ear rot is characterized by a dense white to grayish mold that starts at the base of the ear and is evident growing between the kernels (Figure 1).  Oftentimes the husks of the ear are difficult to remove and appear “glued” to the ear by the mold. Very small, black fruiting bodies can be found scattered on husks or embedded in cob tissues and kernels.

Infected kernels are lightweight and have reduced nutritional value. Mycotoxin contamination of the grain under field conditions has not been reported in Iowa. Damage caused by Diplodia ear rot is usually limited to the field, but the pathogen can be a problem in storage if grain moisture is 20 percent or above.

Gibberella Ear Rot
A pink to red mold that usually starts at the tip of the ear is characteristic of Gibberella ear rot. This ear rot also has been detected on hail damaged ears (Figure 2) that have been received as part of the Hail Damage-Grain Quality Survey. The fungus that causes this ear rot produces several mycotoxins including DON (vomitoxin), zearalenone and T-2 toxin.

Fusarium Ear Rot
Symptoms of Fusarium ear rot are a white to pink- or salmon-colored mold, which occurs anywhere on the ear or on scattered kernels. Often the mold is associated with insect-damaged kernels. We have also found Fusarium ear rot associated with hail damaged kernels (Figure 2). Infected kernels are often tan or brown, or have white streaks. The fungi that cause Fusarium ear rot produce mycotoxins known as fumonisins.

Gibberella Stalk Rot
A pink to reddish discoloration of the pith of corn stalks is symptomatic of Gibberella stalk rot (Figure 3). No distinct lesions occur on the outside of the stalk but small, round, bluish-black bodies may occur scattered around the nodes of the stalk. These are the fruiting structures of the fungus and they can be easily scraped off the stalk with a thumbnail. 

Anthracnose Stalk Rot
Black shiny lesions on the rind of the stalk are typical symptoms of anthracnose stalk rot (Figure 4). Internally, the pith of plants is discolored and shredded.


diplodia

Figure 1.  Diplodia Ear Rot

 

 

hail damaged ears

Figure 2.  Gibberella and Fusarium ear rot on hail-damaged corn

 

 

Gibberella stalk rot

Figure 3.  Gibberella stalk rot

 

 

anthracnose stalk rot

Figure 4.  Anthracnose stalk rot

 

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu.



This article was published originally on 10/12/2009 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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