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8/17/2009 - 8/23/2009

Sudden Death Syndrome in Soybean Widespread this Summer

By XB Yang, Department of Plant Pathology

This summer has been very cool with record cool temperatures in July. This cool and wet summer has led to widespread occurrences of sudden death syndrome (SDS) in soybean fields. The SDS reports this year are unlike other years. In 2006, the disease was reported by producers and agronomists to be widespread in eastern and central Iowa. This year, wide occurrences of the disease were observed by Mark Licht, ISU Extension field agronomist, in west central Iowa with incidence as high as 75 percent. Mark Wuebker, ISU Extension field agronomist, reported that "SDS is spreading by the day in Polk and Story counties. Fields that aren’t showing some are the exception."

Delayed planting has been a measure to reduce SDS risk in Iowa. This year’s occurrence is a reason for consideration of future changes in recommendations for delayed planting. In recent years, we have observed that even fields where planting was delayed have had severe occurrence of this disease because some springs were cool and long. With cool temperatures occurring in Iowa several springs in a row, use of delayed planting would not be an effective management practice for SDS risk. In a paper published several years ago, ISU climatologists reported that in a warming climate, the Midwest will have a warming hole in summer, which means that summer temperature in Iowa will be cooler instead of being warmer. If this prediction is correct, we may see more SDS and other cool temperature diseases in future.

Sudden Death Syndrome in Soybean

Symptoms of Sudden Death Syndrome in Soybean

SDS can cause premature defoliation of soybean plants in the later summer. Premature dying has been found in soybeans infected by the SDS pathogen. Plants with premature dying lack the typical drastic defoliation symptom of SDS, but diseased plants yellow and die gradually. By closely examining diseased plants, you can find symptoms typical of SDS plants.

Cool temperatures also are favorable to brown stem rot (BSR), a disease that causes foliar symptoms similar to those of SDS. It is important when scouting this fall to correctly distinguish BSR from SDS because management measures for the two diseases are different. One simple way to identify the two diseases is that SDS causes root rot and the pith of the infected soybean stem remains white, while with BSR, the pith is brown and there is no root rot.

 

XB Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or xbyang@iastate.edu.

Degree Days – Steady as She Goes

By Rich Pope, Department of Plant Pathology

Another August week down and it was again a favorable week with nearly normal temperatures. What we need most now is to avoid extremes in temperatures throughout the next month, with periodic rainfall. Last week was actually the first in nearly two months that was, albeit slightly, above normal in heat accumulation.


degree days

Crop conditions statewide are quite good relative to the rest of the Corn Belt. Fallout from hail and high wind events remain major issues, especially in bands from Winneshiek through Fayette counties and also from Ida through Grundy counties.

Soybean aphids are the big story among pests this week. Aphids are now easily found in fields in nearly all parts of Iowa, and populations at or near the treatment threshold of 250 aphids per plant and increasing are not uncommon. Some fields are getting close to the R5.5 stage when soybeans have progressed past the stage where soybean aphid damage is significant. Good treatment decisions this coming week will be a test. With that, there are many aerial applicators active.

 

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted at ropope@iastate.edu or by calling (515) 294-5899.

Risk of Mycotoxins Associated with Hail Damaged Corn

By Alison Robertson and Gary Munkvold, Department of Plant Pathology

Hail storms that occurred recently across the state have caused considerable damage to corn crops that will likely result in reduced yields.  Bruises on stalks and ear husks may allow pathogen entrance that could result in stalk and ear rots, and consequently stalk and grain quality issues. In particular, there may be increased risk of mycotoxin contamination on grain.

Unfortunately there is little information available to quantify the increased risk of mycotoxin contamination of corn as a result of hail injury. However we do know that injury to the ear does favor certain rots, namely Fusarium ear rot (Figure 1) and Aspergillus ear rot (Figure 2).  The fungi associated with these ear rot diseases can produce mycotoxins that are harmful to livestock. Other fungi that do not produce mycotoxins may also colonize damaged kernels and reduce their feed value.  

fusarium ear rot

Figure 1.  White mold characteristic of Fusarium ear rot associated with wounds caused by hail.  Photo G. Cummins.


 
aspergillus ear rot

Figure 2.  Powdery olive green mold characteristic of Aspergillus ear rot associated with insect damage.  Photo A. Robertson

 

It is important to be able to recognize the toxin-producing ear rot diseases because their potential impact is very dependent on the particular fungus involved. Once the corn is harvested, it can be more difficult to recognize the symptoms, but if there is a major problem, it will be evident in the grain.

Fusarium ear rot is caused by several species of Fusarium.  Symptoms of Fusarium ear rots are a white to pink- or salmon-colored mold, beginning anywhere on the ear or scattered throughout. Often the decay begins where kernels have been damaged. Infected kernels are often tan or brown, or have white streaks. These fungi can produce mycotoxins known as fumonisins. 

Aspergillus ear rot is caused by Aspergillus flavus.  This olive-green, powdery mold is usually associated with damaged kernels. In Iowa, Aspergillus is much more common in hot, dry years, but it can grow extensively in damaged kernels under a wider range of weather conditions.  Aflatoxins can be produced when A. flavus colonizes corn.

Another ear rot to look out for, since mycotoxins are also associated with it, is Gibberella ear rot (Figure 3), which is caused by the fungus Gibberella zeae, also known as Fusarium graminearum. This ear rot is not typically associated with damaged kernels. Instead, it usually infects through the silks, so it begins at the tip of the ear and appears red or pink, or occasionally white. Gibberella sometimes rots the entire ear. Infections occur more commonly in cool, wet weather after silking and through the late summer. Gibberella can produce vomitoxin and zearalenone.

gibberella ear rot

Figure 3.  A pink mold that starts at the ear tip is characteristic of Gibberella ear rot.  Photo A. Robertson

 

When evaluating an ear rot problem, remember that certain ear rots are a warning sign to suspect toxins, but ear rots do not always lead to toxin problems. If hail-damaged fields are still intended for grain harvest, ears should be inspected before harvesting. If more than 10 percent of ears in a field have a significant amount of mold (25 percent of the ear or more), these fields should be harvested and the corn dried as soon as possible. The combine will remove some of the moldiest kernels.

Options for using corn at risk of mycotoxins
The best option for moldy grain is to feed it or sell it instead of storing it. However, it should be tested for toxins before feeding. Mycotoxin testing is available through the ISU Veterinary Diagnostic Laboratory. Testing for mycotoxins can be done before putting the grain in storage. The best sampling method is to take a composite sample of at least 10 pounds from a moving grain stream, or to take multiple probes in a grain cart or truck for a composite 10-pound sample. If toxins are present, it is possible that it can be fed to a less sensitive livestock species, such as beef cattle (depending on the specific toxin and its concentration). A veterinarian or extension specialist can help with these decisions. If the grain is sold, there may be a reduced price due to mold damage.

Hail-damaged fields that are to be salvaged for silage may have mycotoxin risks originating from both the grain and the stalks. Stalks can be colonized by mycotoxin-producing fungi, especially following hail damage. If silage is taken at standard moisture content, and properly ensiled, usually these fungi can no longer develop, but any mycotoxins produced in the field will still be there. Also if the silage is not well-packed, there can be too much air exposure and some additional mycotoxin development. There are a few other fungi, such as some species of Penicillium, that produce less well-known toxins such as PR toxin in silage. There are good sources of information on molds and mycotoxins in silage from UW Madison, and Pennsylvania State University.

Once fungal colonization begins, it can continue as long as temperatures are favorable and the plant moisture content is high enough. As long as grain moisture remains above about 21 percent, any of the mycotoxin-producing fungi can continue to grow and produce mycotoxins. Aspergillus flavus can continue to do so down to about 16 percent. Silage remains wet enough to sustain fungal growth, but the anaerobic conditions and low pH in fermenting silage will arrest the development of aerobic, mycotoxin-producing fungi. So reducing mycotoxin risk in hail-damaged corn can be achieved by chopping early and ensiling, if the moisture content is low enough for proper ensiling (see articles on the Hail Damage page of the ISU Extension Disaster Recovery website). Fields that can be held for grain should be considered for early harvest and artificial drying. Mold inhibitors can be used in grain or silage, but they will not repair decay that has already occurred nor reduce existing mycotoxin levels.

The only way to determine mycotoxin levels with certainty is to have samples tested for specific mycotoxins. The most accurate samples will be chopped silage, sampled just before ensiling, or harvested grain as already described. Silage samples are more time-consuming to process because of the lack of accurate quick tests for mycotoxins in silage. Grain can be sampled pre-harvest, by collecting ears at random from throughout the field; at least 25 ears should be sampled; larger samples tend to provide a better estimate of whole-field mycotoxin levels.


 
 

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. Gary Munkvold is an associate professor of plant pathology and seed science endowed chair in the Iowa State University Seed Science Center with research and teaching responsibilities in seed pathology. He can be reached at (515) 294-7560 or by email at munkvold@iastate.edu.

Harvesting High-Quality Corn Silage

By Steve Barnhart, Department of Agronomy

Proper harvest management is critical for high-quality corn silage, and it starts with harvest timing. This ensures that the harvested crop is at the optimum moisture for packing and fermentation. Silage that is too wet may not ferment properly and can lose nutrients through seepage. If silage is too dry when harvested, it has lower digestibility because of harder kernels and more lignified stem fiber. In addition, dry silage does not pack as well, thus increasing the potential for air pockets, excessive heating, and mold.

Optimum silage moisture at harvest ranges from 55-60 percent for upright oxygen-limiting silos; 60-65 percent for upright stave silos; 60-70 percent for bags; and 65-70 percent for bunkers. Due to variability among hybrids and growing conditions, best management is to collect a representative chopped sample and check whole-plant moisture content using a commercial forage testing laboratory, forage moisture tester, or microwave oven rather than simply estimating it from the kernel milkline. Instead, kernel milkline should be an indicator of when to collect the first silage samples for moisture testing.

A general guideline is to begin moisture testing when the milkline is about one-fourth of the way down the kernel from the dent end for horizontal silos, and about 40 percent of the way down the kernel for vertical silos. Then, under normal crop development conditions, assume a constant drydown rate of about 0.6 percent per day, and measure moisture again prior to harvest. 

Grain Processors
Length of cut and crop processing are also important for obtaining high-quality corn silage. A grain processor on the chopper breaks cobs and kernels, and increases surface area which improves digestibility, reduces cob sorting, and results in higher density silage that packs better.

Although grain processors on the chopper are expensive and require more energy, the higher-quality silage produced can increase milk production by 300 pounds per cow per year. The benefit of crop processors is greatest when there are drier, harder kernels resulting from delayed harvest or drought. When using a grain processor, chopper cut length can be increased to reduce horsepower requirements while maintaining optimum particle size. For unprocessed corn, ideal chop length is three-eighth inch theoretical length of cut. For processed corn, recommended settings are a three-fourth inch theoretical length of cut with 0.08 to 0.12 inch roll clearance.

A 4 to 6 inch cutting height is generally recommended for corn silage, as it maximizes silage yield and milk per acre. However, drought-stressed corn can accumulate nitrate in the lower part of the stalk, thus increasing the potential for nitrate poisoning, particularly in older livestock on lower-energy rations. The potential for high nitrate silage can be even worse if drought-stressed silage is harvested within 10 days after rainfall, since rainfall can lead to a sharp increases crop uptake of soil N.

Some producers increase the height of cutting to increase silage quality.  While this will produce silage with a higher ratio of grain to stover, and thus a higher silage quality, it will reduce harvested yield and reduce the contribution of fiber from silage in rations. 

Silage with high nitrate levels can be managed by dilution with other feeds or by increasing the cutting height to 12 inches, or 18 inches, thus leaving more of the higher nitrate lower stalk in the field. However silage cut at this greater height has been shown to have 8 to 15 percent less silage yield and 2 to 12 percent less milk per acre. So, the lower tonnage with high-chop silage is seldom justified in the absence of high nitrate levels.

When harvest begins, fill silos rapidly to reduce exposure of silage to oxygen and to reduce fungal growth. For bunker silos, pack silage as tightly as possible in progressive wedges in depths of 6 inches or less. Then cover and seal well.


Adapted by Stephen K. Barnhart, Iowa State Extension Agronomist, from an August 2009 newsletter article by Jeff Coulter, UMN Extension Corn Agronomist
 

 

Stephen K. 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.



This article was published originally on 8/24/2009 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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