Friday, August 22, 2014

The Bermuda Triangle

This simple diagram is a depiction of why diseases occur, why they are sometimes severe and why they sometimes are not.  Let me explain.
White mold in soybeans is a good example.  It can devastate a soybean crop with yield losses as high as 50%.
White mold, caused by sclerotinia spp, is a garden variety common disease pest.  It affects many different plant species including crops such as white beans, peas, canola, sunflowers and many vegetables.  I have also seen it on weeds like pigweed and lambsquarters.  In other words the pathogen is always present in big numbers because of the diverse host species. Because of this fact we can safely predict the pathogen part of the triangle is a permanent fixture.
The two environmental factors that have the biggest influence on white mold are moisture and temperature. As white mold growth matures, the white cotton moldy growth bonds together and forms sclerotia.  Sclerotia are hard black bodies, commonly referred to as "rat turds".  The sclerotia overwinter in the soil and can survive for up to 5 years when they are buried by tillage.  High soil moisture levels encourage the sclerotia to "germinate" and produce fruiting bodies shaped like small mushrooms.  The mushrooms produce spores and the cycle continues.
The temperature that favours white mold growth is 10-20 degrees C.   If you think about the temperatures that occur under a dense canopy of soybean leaves combined with lots of soil moisture, you can begin to appreciate how the disease can flourish in this "environment".
Given the fact that the pathogen and environment part of the triangle are often present, why do we not see it expressed in every soybean field?   They are all lush with dense canopies this year.  The answer to this question is found in the host part of the triangle.  In our immediate area it had become uncommon to find white mold in soybeans. In fact several times this year some intelligent growers have asked what is causing the dead plants to show up in soybean fields.  They have not seen the disease for many years and have just forgot what it looks like.  Going back 30 years to when soybeans were first becoming popular it was very common to have white mold infection in the soybean crop.  That was due to the fact that a couple of the popular soybean lines were very susceptible to white mold.  Any one remember the variety Evans?  There were not many varieties to choose from so growers were forced to take chances with susceptible varieties.  It takes a lot of work, but smart soybean breeders have been able to screen for white mold susceptibility within the genetic lines they work with.  There are many soybean varieties that are just not very susceptible to the white mold infection. Growers are quick to toss out varieties that they discover to be white mold susceptible in their fields because there are other options.  Despite this, extremely high pressure of white mold can on occasion overwhelm genetic defenses.
So the disease is predictable to a point.  We understand the disease, the environment and we know a lot about the genetic component.  We can identify fields and soil types that are more prone to infection.  We have some fungicide tools that reduce the impact of white mold, but the fungicide has to be applied at high rates, which is expensive if there is no white mold present and need to be applied at first flower.  We should be able to scout a field and identify the potential risk.  Just look for the mushroom shaped structures on the soil surface that I referred to earlier and you would know where and when to spray, the same way we do with weeds.  Not so simple.  Many pathogens in nature most of them harmless to the crop, use similar looking mushroom bodies as part of their life cycle.  Deb Campbell, a sharp eyed agronomist, has spent years trying to identify the specific white mold "mushrooms" as a service to her clients.  She admits to still not being able to positively identify the pathogen at that stage.  If Deb can't find them the rest of us don't have a ghost of a chance.
Farm management has a significant impact on white mold.  If we go back 30 years, conventional tillage was the norm for every crop.  No-till was a "radical fringe" element.  In the last 20 years no-till has become a normal farm practice.  The adoption of no-till had an impact on white mold because the turds laying on the soil surface are subject to predation by insects and mice.  And they need to be buried to germinate well. In the last 5 years, tillage has become much more fashionable again.  I believe, at least in this immediate area that the return of tillage is having a positive influence on white mold infection.
Thanks to the skill and effort of soybean breeders the disease is not as devastating as it once was.

In the corn crop leaf diseases that enjoy cool weather are now easy to identify.
The one disease that is on everyone's radar after last year is northern leaf blight.
You can find it in many fields by the cigar shaped lesions, but at the moment it is at low levels.
Common eyespot is a also present.  It is easy to identify because it looks like an eye.
This picture is not great, but I believe it is the early stages of common rust.
How much of a problem the combined effect of these leaf diseases have on the corn crop will depend on the weather in September.  It is the goal of fungicides to reduce the effect of these diseases and they do a good job in that respect.  It is well known that fungicides keep the plant alive longer by reducing the level of infection.  It is also why some of you resist fungicides because the plant is slower to die and the resulting drying bill causes heart burn.  It is important to remember fungicides they are not 100 % lethal to every fungal disease and you may still find leaf disease in a treated field.

While you are out looking, please remember that not every symptom found is disease.
This ugly spot is not something you do anything about.  It is pollen that was caught in the leaf collar and is now rotting.  I guess you could say the pollen is composting inside the collar and consuming some green tissue in the process.


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