Tuesday, October 28, 2014

What Went Wrong with my Soybeans Planted after Soybeans?

         Some growers who have not had good luck growing corn, have an expectation of lower input costs, want lower machinery requirements at harvest, or are looking at the grain market outlook when deciding to grow soybeans after soybeans.  Almost all sources say that if a grower chooses to go with a continuous soybean after soybean rotation, he most likely should expect potential yield loss.  Often times, this yield loss gradually increases, the longer that a field is planted with only soybeans.  Most sources say that there can be up to 5 to 15% yield loss potential in soybeans planted after soybeans, when compared to a corn after soybean rotation.  One source said that in stress years, there could be up to 20% yield loss in soybeans planted after soybeans.
However, Dr. Emerson Nafziger, from the University of Illinois says that yield loss was not as high as some expected after analyzing 15 years of data from continuous soybean plots planted at University of Illinois Research stations in western Illinois.  He found that in some locations, a field planted continuously to soybeans only yielded a few bushels less, when compared to a similar field planted to soybeans following corn.  In a nutshell, if you choose to grow soybeans after soybeans, the days of “planting soybeans and forgetting them” are over.  A soybean after soybean crop will need to be highly managed.  Field placement, fertility, production selection, disease scouting, and weed management will all be factors for determining potential yield loss.

The biggest disadvantage of a continuous soybean crop is the potential for an increase in disease as well as soybean cyst nematode populations.  Many are unaware of just how many disease management benefits are gained with crop rotation.  Soilborne diseases such as pythium and phytophthora don’t necessarily increase when soybeans are planted after soybeans, but they are something you will need to contend with if fields are stressed, low-lying, or consist of poor drainage.  Diseases that overwinter in soybean residue, such as bacterial blight, stem canker, pod and stem blight, Septoria brown spot, or frogeye leaf spot will increase.  Frogeye leaf spot is one example of a disease that can cause significant yield losses in soybeans if found at high levels during flowering (R1-R3) on susceptible varieties.  If survival structures of Sudden Death Syndrome or white mold are found in the soil, a continuous soybean crop will be truly challenging.  The greatest threat to soybeans is soybean cyst nematodes (SCN).  It is strongly recommended to conduct SCN test before planting soybeans after soybeans.

Not just any field will do if planting soybeans after soybeans.  This is because the yield penalty will only become greater in environments less suited for soybean growth.  Avoid fields that are low lying or have poor drainage as these are the fields that will be more vulnerable to root rot diseases.  The addition of seed treatments such as PowerShield® for early season disease and insect control is strongly recommended to encourage early season vigor.  In addition, fields that have a history of heavy SDS, SCN, or white mold infection or infestation are not suited for a continuous soybean operation.  Product selection of soybean varieties with disease resistance is critical, especially if there is a field history of soybean disease.  Most soybean varieties have disease ratings for diseases such as SCN, Phytophthora, brown stem rot, SDS, frogeye leaf spot, and white mold.  It is also recommended that you rotate between soybean varieties so that you are not depending on the same source of resistance every year, which can encourage the onset of disease resistance.  Often times, planting shorter season or shorter relative maturities of soybeans will help avoid diseases that tend to make their debut later in the season.  Some sources encourage a slight increase in planting populations within continuous soybeans; however, lower planting populations will be needed to reduce the onset of white mold.  In addition, increase row width to 30 inches will also help to reduce disease.  The planting date of soybeans may also need to be later for some diseases, such as SDS, that are more likely to infect soybeans in cooler or wetter soil conditions.

The next yield threat in a continuous soybean operation will be weeds.  This is because weeds are more apt to be kept in balance within a crop rotation.  Growing soybeans after soybeans can cause a shift in weed species or increase one particular weed species due to the likelihood of herbicide tolerance.  If weeds are a problem, later planting into warmer soils can aid crop emergence, so that the soybeans can better compete with weed infestation.  If the disease white mold is not an issue, then narrow rows may be a way to encourage a faster canopy to face weed competition.  Weeds will always try to fill bare soil.   On the other hand, wider rows leave the option for tillage of weeds or incorporation of residue, which could harbor disease; however, tillage of any kind might aid erosion.  When it comes to herbicides, a weed resistance problem is much more likely in soybeans following soybeans because it is much more difficult to rotate herbicides with different modes of action.  It is highly advised to always start with a clean field before planting soybeans.  A different herbicide program which consists of pre-emergence and post-emergence herbicides is required each year to keep weeds in check as well as curb the development of herbicide resistance within weed populations.  Consequently, rotation of herbicide traits, such as a LibertyLink®   followed by Roundup Ready®  is recommended.

Soil testing and keeping Phosphorus (P) and Potassium (K) levels in check within continuous soybeans becomes extremely important.  Soybeans can remove up to .90lb of P and 1.50lbs. of K per bu/grain.  Most of the K is taken up by the soybean plant 2 to 3 months after emergence and, if soybeans are deficient in K, the size and fill of grain will be reduced.  The soybean grain will remove significant levels of K at harvest.  In addition, when compared to corn, soybeans will remove equal or greater amount of P from the soil due to the greater amounts of P take up by the soybean grain.  Soil testing will reveal nutrient removal levels and offer a basis for P and K replacement for the following soybean crop. If deficient sulfur and zinc micronutrient levels might need to be supplemented in higher yielding environments. It has been shown that biologicals, like those in the PowerShield soybean treatment, can establish favorable relations between the soybean plant and soil microorganisms, to enhance the uptake of critical nutrients such as nitrogen and phosphorus.  While taking soil tests, also be sure to check the soil pH and make sure it is around 6.5 to 7.0 for adequate soybean growth.  Lastly, when comparing a soybean/soybean rotation to a corn/soybean rotation, there could be less soil residue left on the soil surface; therefore, there could be a greater likelihood of a lower amount of organic matter or biomass returned to the soil, reduced soil structure or less stable soil aggregates, as well as a greater likelihood for soil erosion to take place.

Wednesday, October 22, 2014

Did Mother Nature cause corn disease to be a problem in 2014?

The main foliar corn diseases that were observed within the Burrus footprint during the 2014 growing season were as follows:  gray leaf spot, northern corn leaf blight, and Goss’s wilt.  All of these foliar diseases can significantly decrease yield, especially if their lesions cover leaves near the ear at corn pollination or cover upper leaves during grainfill.  Every corn hybrid has a disease rating for gray leaf spot, northern corn leaf blight, or Goss’s wilt.  Check the disease rating for your hybrid and if it is a low number, you could more likely to benefit from use of a fungicide (for gray leaf spot or northern corn leaf blight) at corn pollination. 
When it comes to plant disease spread, we usually assume a rainy season, humidity, moisture, and even dew are favorable conditions for foliar corn diseases.  Some made the assumption that most areas were getting precipitation during the 2014 growing season, but in reality, some areas received minimal rainfall in July within the Burrus footprint during corn pollination.  In addition, most plant disease pathogens require a narrow range of specific temperatures, before they will sporulate or initiate infection.  Gray leaf spot, a fungal disease, started to appear on susceptible corn hybrids in southern, western, and in areas of central Illinois in mid-July during 2014, just a few weeks before pollination.  These were areas within the Burrus footprint with the earliest planting dates during 2014.  So, the question was, why were we not seeing gray leaf spot in other areas of the Burrus footprint?  One reason could be that some corn fields may have had less residue present, which could harbor disease.  Another reason could be that the last few growing seasons were dry and there might be less build-up of disease inoculum within residue.  But, the main reasons were that some areas might not have been humid or temperatures were not favorable.  Gray leaf spot requires warmer temperatures (75° -85°) for disease infection to occur and  if conditions stay conducive, symptoms will develop after several weeks.  The average temperature range in northern Illinois, Wisconsin, and Iowa was 65° - 70° during July. Another disease called northern corn leaf blight began to be visible on susceptible hybrids in the Northern part of the Burrus footprint in mid–July of 2014.   This is because northern corn leaf blight symptoms will develop at lower temperatures, when compared to gray leaf spot.   
Gray leaf spot

Northern corn leaf blight

Gray leaf spot symptoms typically appear first on lower leaves because it overwinters on residue.   A wound is not needed for the infection of gray leaf spot or northern corn leaf blight disease.  Northern corn leaf blight also overwinters within corn residue, however spores can blow into fields from very, long distances, then infect susceptible hybrids.  Lesions can develop every 7 - 12 days in a favorable environment.  Northern corn leaf blight causes larger “cigar shaped” lesions, when compared to gray leaf spot symptoms, which are small, tan, and rectangular in shape.  Northern corn leaf blight causes larger lesions and can spread a bit quicker when compared the gray leaf spot; therefore, we become a bit more concerned about this disease in growing seasons that are favorable for northern corn leaf blight spread and infection.  Based on disease scouting during pollination, disease pressure, and forecasted wet weather, the grower can make the decision if a fungicide application is needed.   Northern corn leaf blight seems to be advancing at the most rapid rate of the afore mentioned diseases.

Gray leaf spot
Northern corn leaf spot

(Cercospora zea-maydis)
(Setosphaeria turcica)
Fungal spore survival
overwinters in leaf debris
overwinters in leaf debris
75° - 85°
64° - 80°
high humidity/wet 
prolonged moisture/dew
Plant part affected
silking to maturity
silking or after silking
lesion development
1  1/2 - 2 weeks
7 - 12 days
resistance/tillage/rotation fungicides

Another corn disease that made an appearance during early August within northern Illinois, Wisconsin, and Iowa was Goss’s wilt.  It is a leaf blight caused by a bacteria that infects after corn silking; however, on rare occasion, this bacterial pathogen can also cause a systemic wilt.  This bacterial pathogen can survive in corn residue from 10 months - 2 years.  Many think that it is becoming more widespread because of susceptible hybrids, corn after corn rotations, conservation tillage, and favorable weather.  Most of Burrus hybrids have fairly good Goss’s wilt ratings, but keep in mind, no hybrid is immune from this disease.  Since Goss’s wilt is a bacterial disease, fungicides are useless.  The main way to manage this disease is to plant resistant corn hybrids.
Suspect Goss's wilt on competitor's corn
Goss’s wilt favors temperatures from 70° -  80° for infection and the average tempertures in parts of northern Illinois and southern Iowa during mid-July to August were within the favorable range for Goss’s wilt infection to develop.  But, in order for bacterial infection to occur, there must be wounds made by hail, blowing debris, or other mechanical injury.  One example of a storm that brought hail occured on June 30th  and this is only one of many storms that occurred during 2014 that could have caused wounding for entry of the Goss’s wilt bacteria, which was needed for disease infection to take place on susceptible corn hybrids. 
Lastly, at the end of the 2014 growing season, there were many concerned that there could be some issues with corn root lodging or stalk lodging at harvest.  Often times, if early corn growing conditions consist of no stress before pollination (high moisture or nitrogen), which most likely increases the potential of grain production, but later in the growing season (post pollination), factors can take place that hinder corn photosynthesis, which can hinder the flow of sugars to grain.  Sugars can then be taken from the stalk, which could predispose the corn plant to be infected by stalk rot pathogens.  
In fact, research has shown that high incidences of foliar diseases such as gray leaf spot, northern corn leaf blight, and Goss’s can indirectly predispose corn to stalk rot.  In addition, insects such as corn rootworm larvae can feed on roots and cause root lodging.  European corn borer (ECB) (usually the 2nd generation) can tunnel into the corn ear shank and cause ear dropping or tunnel into the stalk and cause stalk lodging.  Of course, ECB injury is more often found on corn hybrids without GM traits to help suppress or control the ECB.  Often times, if there is ECB injury above the ear, stalk rot can ensue.
Depending on soil type, amount of soil moisture, and root development, corn hybrids can be blown over during a wind event.  However, some strong winds can overcome the best root development.  Corn damaged due to weather can be more susceptible to disease and infection of stalk rot pathogens at points of injury on the stalk. For example, if hail occurs after pollination, stalk rot is almost always inevitable.  On June 30th, massive storms swept through areas of the Burrus footprint.  The winds and hail that this storm brought were a true evaluation of the strength of the corn hybrids, which were nearing growth stages during pollination. 
Other stresses such as high plant populations, extremes in soil moisture, nutrient deficiencies or imbalances, cloudy weather, drought, corn-on-corn rotations, high ear placement, or hybrid stalk strength can cause corn to be prone to lodging or stalk rot.  High plant populations can increase plant competition for light and reduce stalk diameter.  Extremes in soil moisture can cause root rot or compromised roots due to lack of oxygen.  We were especially concerned with nitrogen loss this year.  If corn plants are without adequate levels of nitrogen, they could be less vigorous and put all their energy into grain development; therefore, stalks could be compromised and more likely to be infected by stalk rot pathogens and lodging.  On the flip side, corn growing within higher nitrogen levels (promotes lush foliage growth) along with low levels of potassium (encourages premature stalk death) can also cause corn to be more prone to stalk rot or lodging.  Late season cloudy weather, which occurred mostly in the northern part of the Burrus footprint in 2014, may hinder photosynthesis in order to keep ears alive.  If the drought threshold of corn is reached, this could also cause a drastic reduction in photosynthesis, so irrigation should not be reduced during grain fill. 
Anthracnose stalk rot