Category Archives: Disease

Now is a Good Time to Evaluate Your Varieties for Foliar Diseases

Posted on by

September is a great time to evaluate your crop and the performance of varieties that you chose.  In addition to general growth and health of the crop, take some time to determine if you have any of the below diseases.  If so, you could be losing some yield.  If you sprayed with a fungicide and still have disease, reconsider the product and rate used and the time that the fungicide was applied.  Keep in mind the weather conditions when the application was made and the conditions 2 to 3 weeks after or before the product was applied.  Cool temperatures (70’s) and high relative humidity (>95% for 12 hours or more) will usually increase disease incidence.

Another caution is to never diagnose a specific disease on the plant without verifying it with a person trained to identify plant pathogens.  Only when the reproductive structures are found on the leaf can a disease be confirmed.  Many things will cause look-alike symptoms.  Be sure before you cast the blame.  There are more diseases than just the ones shown below, but these are the most common.  Brown spot is normally found in the lower part of the crop canopy (the lower leaves), Cercospora blight and leaf spot will be found throughout the canopy, and the frogeye leaf spot and downy mildew tend to be found in the upper part of the canopy.

 

Foliar Fungicides for Soybeans

Posted on by

Ten years ago, few farmers considered applying foliar fungicides for soybeans.  These crop protection products had their home with higher-value crops such as vegetables and peanuts, where a return on investment was more likely.  But with soybean futures topping $16, more growers are using fungicides to protect their soybeans from common diseases and hopefully to increase yields.

The question that is still plaguing nearly everyone is will the fungicides pay for themselves this year?  The answer is really quite simple – We Don’t Know!  Why don’t we know?  First, the fungicides that effectively control our most common soybean diseases, the strobilurin fungicides (e.g., Headline, Quadris, etc.) are preventative.  In other words, they must be applied before the disease develops.  So, we’re applying a chemical to prevent a disease that may or may not progress to yield-reducing levels.  Unfortunately, our ability to predict soybean disease development is not very good.  Therefore, we rely on results from applied research, experience, and the probability of getting a response.

How likely are we to get a response to fungicides in Virginia?  We regularly test fungicides on soybean and have built up a fairly large database.  While disease incidence is reduced with fungicide application, a significant yield response occurs only about 1/3 of the time.  The average yield response is 3 to 4 bushels per acre, regardless of whether we average over only experiments with significant responses or over all experiments.  We’ve seen yield increases up to 12-14 bushels, but that type of response is rare.  So, if you can average 3 to 4 bushels per acre over all acres, then a fungicide will likely pay for itself – unless we experience one of those years where disease is nearly absent (remember how dry it was last year?).  If there’s no disease, we’ve wasted our money.

Although we do not yet have an effective weather model (such as is being used in peanut) that will guarantee success, we can make an informed, although not perfect, decision.  We must remember that three conditions must be present in order for a disease to develop: 1) a susceptible host; 2) the pathogen; and 3) a conducive environment.  This concept is commonly referred to as the plant disease triangle.  If any of these three is missing, then the disease will not develop.  We have the host – soybean.  And we usually have some yield-robbing pathogens present – especially the Cercospora species.  But the variety used must be susceptible to the pathogen.  This includes two things: genetics and stage of soybean development.  From the genetic standpoint, we have many varieties with resistance to frogeye leaf spot (Cercospora sojina), probably the most damaging disease that is annually present in Virginia.  So, if you’re using a frogeye leaf spot resistant variety, response to fungicides is less likely.  Certain varieties are more susceptible to cercospora blight (Cercospora kikuchii); some are less susceptible.  But few have true resistance.  Cercospora blight is likely most responsible for yield loss to to foliar disease and the reason that fungicides prevent soybean yield losses in Virginia.  Regarding stage of development, soybean disease usually appears after full flower (R2) and the soybean are just beginning to pod (R3).  This can be attributed to a larger canopy at this stage (which is related to the environment side of the triangle – see below discussion), but may also be related to physiological changes taking place in soybeans as plants transition from vegetative to reproductive growth.  So if the pathogen is present on a susceptible host, there is a greater likelihood that a disease will develop.  Still, the environment has to be right.

What is the right environment?  Temperatures ranging from 60 to 85 F in periods with moisture provided by rainfall, dew, or high relative humidity is usually required.  The greater number of days that these conditions are met, the more disease there will be. The weather service is pretty good at predicting temperatures, but not so great with rainfall.  Additionally, even if rainfall is predictable, the environment within the soybean canopy (micro-environment) is usually what matters most.  A tall soybean crop that has completely closed its canopy and is growing in good soil moisture will result in a micro-environment more conducive to disease development than a short crop that has not closed the canopy and is growing in a relatively dry soil.

In summary, all three conditions shown on the disease triangle must be present for a disease to develop to yield-robbing levels.  We do not yet have a good model to predict common soybean diseases – but we are working on it.  Until such a model is developed, Dr. Pat Phipps and I suggest the following guidelines on whether or not to spray a foliar fungicide on soybeans:

  • Soybeans are growing well, have a full canopy, and are in the R3 (beginning pod) to R5 (beginning seed) stage.  Research indicates that applying fungicides at R3 is better than later stages.  Note that most fungicides cannot be legally applied after they have reached the R6 (full-seed) stage.
  • Daily air temperatures averaging between 60 and 77 F and accumulations of rainfall were ≥ 0.5 inches in the previous 5 days or ≥ 1.0 inches in the previous 10 days, or periods of relative humidity were ≥ 95% for ≥ 12 hours per day.  The greater number of days that these conditions are met, the greater likelihood for a response to fungicide application.

When soybeans were $5 to 6 per acre, very little foliar fungicide was being applied to soybean.  However with today’s prices, yield is king; therefore, a small yield increases resulting from an external input such as foliar fungicides can be cost effective.

Fungal Seedling Disease in Soybean

Posted on by

David Holshouser, Extension Agronomist

Pat Phipps, Extension Plant Pathologist

Rhizoctonia Damping Off and Root Rot.  Rhizoctonia root rot is probably the most common soilborne disease in Virginia soybeans.  Even if other diseases pre-dominate in a diseased plant, rhizoctonia could easily be a component of the problem.

Preemergence symptoms are typical of common seed rots, but are not usually recognized just because these plants never emerge.  More recognizable is the damping off that occurs in the seedling stage.  This will usually occur before the first trifoliate leaf develops.  Infected plants will have a reddish brown lesion on the emerged shoot at the soil line.  This lesion is most visible after the seedling is removed from the soil.

 Resistance to rhizoctonia is not available; variations in variety tolerance have been reported though.  Stresses such as herbicide injury, poor soils, insect damage, and feeding by soybean cyst nematode will increase damage.  Several fungicide seed treatments are effective for this disease.

Fusarium Root Rot.  Fusarium is another common disease in Virginia.  It is one of the diseases that has been implicated in “Essex Syndrome” that we continue to battle in some parts of Virginia.  There are several species of fusarium and each can cause a different plant reaction and/or disease.

 Two of the species, F. oxysporum and F. solani can cause root rot.  The root rot caused by F. oxysporum usually develops on seedlings and young plants during cool weather (<60O soil temperatures).  Older plants are generally less susceptible than younger ones.  Seedlings will emerge very slow and the resulting seedlings are stunted and generally unhealthy.  Symptoms are usually found confined to the roots and lower stems.

F. solani causes preemergence damping-off and root rot.  Damping off after the seedlings emerge is less of a problem, but can occur.  Lesions are generally on the roots and are dark brown to reddish brown to black.  Lesions can also occur on the young stem.

This disease is common in nematode-infested fields.  Soybean cyst, root knot, and sting nematodes will predispose seedlings.  Soybeans growing in soybean cyst nematode-infested fields will frequently develop fusarium symptoms.  This is less likely in root knot infested fields because the injury to the plant from root knot nematode is limited to the root tip.  In contrast, larvae of soybean cyst nematode migrate within the cells and cause more wounding.  In addition, F. oxysporum often interacts with rhizoctonia.

There is some variety resistance to the disease, but this information is not always published in the company literature.  Warm soils that are well-drained are helpful in managing the disease.  Good soil fertility should be maintained and soil compaction avoided.  Fungicide seed treatments provide some, but limited control.

Pythium Damping-Off and Root Rot.  There are many different species of pythium and the dominant species that is present will vary from geographical region to region, usually depending on temperature.  Pythium will cause pre- and postemergence damping-off during the young seedling stages.  It can also cause a root rot in later vegetative stages.  Seedlings may fail to emerge and will have a short, discolored root.  After emergence, symptoms can resemble those of other seedling diseases, especially fusarium and phytophthora.  The disease begins as water-soaked lesions on the young stem or on the cotyledons (seed leaves), and then followed by brown soft rot.

Variety resistance to pythium is not available, but fungicide seed treatments containing metalaxyl or mefenoxam will control the disease.  The best way to avoid the disease is to avoid planting into cool soils (<60oF).

Phytophthora Root Rot.  Of all the seedling disease that you may have heard about, phytophthora is probably the one that you hear and read about most.  It is a serious problem in the Midwest and affects young seedlings and older plants.  Many of our varieties that we grow in Virginia have varying levels of resistance to multiple races of phytophthora.  Yet, most of you have probably never had the disease.  Why is that?

Phytophthora rot is most severe in poorly drained clay soils that are readily flooded.  Most of our soils are sandy in nature, or if a clay, are well-drained.  This doesn’t mean you can’t have the problem just that it is less likely.  Plant loss can occur in lighter soils or on well-drained soils if they are saturated for an extended period of time when the plants are young.

Symptoms are the typical root rot and pre- and postemergence damping off.  The disease is often not diagnosed because it is confused with flooding damage.  Root and stem rots occurring later in the season will occur under similar, saturated conditions.  Tolerant cultivars may escape damage.  Damage does increase with reduced tillage, especially no-till, mainly because those fields absorb more rainfall and can be more easily saturated if the field is poorly drained.  Like most diseases, continuous soybean will increase likelihood of infection and damage.

Is it Too Late to Spray Fungicides?

Posted on by

Over the past week, it seems that we’ve been living in a rainforest.  Today is the first day  in several that it has only rained once.  Pluse, we’ve had several very hard rains each day.  In many areas, we’ve had more rainfall after Hurricane Irene that we received during the that storm.

These wet and humid conditions have set up our double-crop soybean for some late-season foliar disease.  If you haven’t sprayed your soybean and they have not yet reached the R6 (full-seed stage), you may want to consider another fungicide application.  We have seen yield responses, when conditions were right, as late as the R5 stage. The soybean pod below is still in the R5 stage (the seed are not yet meeting), but will be at R6 in just a few days.  R5 is as late as you can legally spray soybeans with fungicides.

If you decide to spray, stick with the strobilurin fungicides such as Headline, Quadris, or Stratego.  Using a triazole fungicide (Folicur, Domark, Tilt, etc.) are not effective on most of our common foliar soybean disease.

Soybean Rust Update

Posted on by

Below is the latest risk assessment on soybean rust.  Basically, conditions are less than ideal for disease development.  Rust has been identified in Louisiana and Florida, but only on kudzu.  Although a tropical storm or other major weather system could change things, the risk of soybean rust moving into Virginia this year is low.

Foliar Fungicides for Soybean

Posted on by

Ten years ago, few farmers considered applying foliar fungicides for soybeans.  These crop protection products had their home with higher-value crops such as vegetables and peanuts, where a return on investment was more likely.  But with soybean futures topping $13, more growers are using fungicides to protect their soybeans from common diseases and hopefully to increase yields.

The question that is still plaguing nearly everyone is will the fungicides pay for themselves this year?  The answer is really quite simple – We Don’t Know!  Why don’t we know?  First, the fungicides that effectively control our most common soybean diseases, the strobilurin fungicides (e.g., Headline, Quadris, etc.) are preventative.  In other words, they must be applied before the disease develops.  So, we’re applying a chemical to prevent a disease that may or may not progress to yield-reducing levels.  Unfortunately, our ability to predict soybean disease development is not very good.  Therefore, we rely on results from applied research, experience, and the probability of getting a response.

How likely are we to get a response to fungicides in Virginia?  We regularly test fungicides on soybean and have built up a fairly large database.  While disease incidence is reduced with fungicide application, a significant yield response occurs only about 1/3 of the time.  The average yield response is 3 to 4 bushels per acre, regardless of whether we average over only experiments with significant responses or over all experiments.  We’ve seen yield increases up to 12-14 bushels, but that type of response is rare.  So, if you can average 3 to 4 bushels per acre over all acres, then a fungicide will likely pay for itself – unless we experience one of those years where disease is nearly absent (remember how dry it was last year?).  If there’s no disease, we’ve wasted our money.

Although we do not yet have an effective weather model (such as is being used in peanut) that will guarantee success, we can make an informed, although not perfect, decision.  We must remember that three conditions must be present in order for a disease to develop: 1) a susceptible host; 2) the pathogen; and 3) a conducive environment.  This concept is commonly referred to as the plant disease triangle.  If any of these three is missing, then the disease will not develop.  We have the host – soybean.  And we usually have some yield-robbing pathogens present – especially the Cercospora species.  But the variety used must be susceptible to the pathogen.  This includes two things: genetics and stage of soybean development.  From the genetic standpoint, we have many varieties with resistance to frogeye leaf spot (Cercospora sojina), probably the most damaging disease that is annually present in Virginia.  So, if you’re using a frogeye leaf spot resistant variety, response to fungicides is less likely.  Certain varieties are more susceptible to cercospora blight (Cercospora kikuchii); some are less susceptible.  But few have true resistance.  Cercospora blight is likely most responsible for yield loss to to foliar disease and the reason that fungicides prevent soybean yield losses in Virginia.  Regarding stage of development, soybean disease usually appears after full flower (R2) and the soybean are just beginning to pod (R3).  This can be attributed to a larger canopy at this stage (which is related to the environment side of the triangle – see below discussion), but may also be related to physiological changes taking place in soybeans as plants transition from vegetative to reproductive growth.  So if the pathogen is present on a susceptible host, there is a greater likelihood that a disease will develop.  Still, the environment has to be right.

What is the right environment?  Temperatures ranging from 60 to 85 F in periods with moisture provided by rainfall, dew, or high relative humidity is usually required.  The greater number of days that these conditions are met, the more disease there will be. The weather service is pretty good at predicting temperatures, but not so great with rainfall.  Additionally, even if rainfall is predictable, the environment within the soybean canopy (micro-environment) is usually what matters most.  A tall soybean crop that has completely closed its canopy and is growing in good soil moisture will result in a micro-environment more conducive to disease development than a short crop that has not closed the canopy and is growing in a relatively dry soil.

In summary, all three conditions shown on the disease triangle must be present for a disease to develop to yield-robbing levels.  We do not yet have a good model to predict common soybean diseases – but we are working on it.  Until such a model is developed, Dr. Pat Phipps and I suggest the following guidelines on whether or not to spray a foliar fungicide on soybeans:

  • Soybeans are growing well, have a full canopy, and are in the R3 (beginning pod) to R5 (beginning seed) stage.  Research indicates that applying fungicides at R3 is better than later stages.  Note that most fungicides cannot be legally applied after they have reached the R6 (full-seed) stage.
  • Daily air temperatures averaging between 60 and 77 F and accumulations of rainfall were ≥ 0.5 inches in the previous 5 days or ≥ 1.0 inches in the previous 10 days, or periods of relative humidity were ≥ 95% for ≥ 12 hours per day.  The greater number of days that these conditions are met, the greater likelihood for a response to fungicide application.

When soybeans were $5 to 6 per acre, very little foliar fungicide was being applied to soybean.  However with today’s prices, yield is king; therefore, a small yield increases resulting from an external input such as foliar fungicides can be cost effective.