Tuesday, September 30, 2014

My Soybeans had White Mold, Now What?

Many farmers in Northern Illinois, Iowa, Wisconsin, Michigan, and Minnesota are all too familiar with the significant yield loss that white mold can cause to soybeans.  However, many found white mold in their fields further south for the first time in 2014, due to the cooler temperatures.  Actually, a soybean plant showing white mold symptoms was found as far south as Arenzville, IL. 

According to the Illinois State Climatic Data Center, this summer was the 29th coolest on record.  " Daytime highs were much cooler than average while the nighttime lows were near-average." - more on this article can be found at:  Illinois and Global Temperatures for June-August

White mold or Sclerotinia stem rot is caused by Sclerotina sclertiorum and actually has a fairly wide host range.  If your soybeans appeared to affected this year, you now are aware that this fungus has somehow made its way to your field and has survived in the soil as fungal survival structures called sclerotia.  If environmental conditions are right (soils 40 to 60 degrees), these sclerotia will send up mushrooms called apothecia, which can release millions of spores that can infect senescenced flowers at bloom. 

Soybean infected with white mold.  If the canopy is humid, white mycelium will form on the outside of the stem.  Black sclerotia will form within the white mycelium or inside the stem.  (Picture provided by Jim Hughes)
Field infected with white mold (Picture provided by Jim Hughes)

START BECOMING CONCERNED ABOUT WHITE MOLD IF TEMPERATURES ARE BELOW 85 DEGREES WHEN SOYBEANS START TO BLOOM (R1 GROWTH STAGE)!

1.) If you have NOT ever had white mold in a field:  scout for the apothecia (mushrooms of white mold) before soybeans bloom on the soil surface.

2.) If you have had white mold in a field:  spray fungicide at soybean growth stages R1-R3 to SUPPRESS or CONTROL THIS DISEASE before infection takes place (if canopy conditions are cool).  Fungicides will only be effective if the are applied at the correct time, with good coverage within the canopy!  Fungicides will not offer total protection (0 to 60% disease reduction). 
Fungicides for soybean foliar diseases:
https://www.extension.purdue.edu/extmedia/BP/BP-161-W.pdf

Some herbicides (Cobra or Phoenix) do not have an effect on this fungal pathogen, but may help to reduce the incidence of white mold by reducing crop canopy or delaying/reducing soybean flowering time.

3.) If you find white mold in a soybean field:  IT IS TOO LATE TO APPLY A FUNGICIDE.  KEEP RECORDS OF FIELDS THAT HAVE WHITE MOLD FOR FUTURE YEARS.

What puts a field at risk to have white mold?

A.  The white mold disease pathogen has to be present in order for soybeans to become infected.  HARVEST INFECTED FIELDS LAST, SO THAT THE COMBINE DOES NOT SPREAD THE (SURVIVAL FUNGAL STRUCTURES) TO OTHER FIELDS.  CLEAN COMBINES!

B.   What is the rating of your soybean for white mold?  Check to see if your soybeans have a good rating (higher number) for white mold.  No soybean variety is completely resistant to white mold!

C.  The white mold disease pathogen will only infect under cool conditions.  If temperatures are above 85 degrees when soybeans bloom, infection most likely will not take place.  

D.  Factors that increase canopy closure, will cause fields to be more at risk for infection because this will allow for cooler, humid conditions for disease developmentThe factors that increase canopy closure are early planting, narrow row spacing, higher plant populations and higher soil fertility.  There are arguments as to whether row spacing causes a higher risk for disease development.  From experience and what I have read, I don't think that row spacing is a factor, because spore dispersal is still possible within various row spacings.
White mold infection that was intensified by a higher planting population (200,000 plants per acre) (Picture provided by Dennis Mueller)
E. A minimum rotation of 2 to 3 years to a non-host crop (corn or small grains) is required to help reduce the fungal survival structures within the soil.  But, keep in mind, that I visited a field this year with a high incidence of white mold that had previously been in corn for 5 years!

F.  There are also mixed feelings on the effect on tillage, when it comes to the prevention of white mold.  Deep tillage (8 to 10 inches) can help to bury fungal survival structures in the field; however another tillage pass could also bring fungal survival structures to the soil surface and increase disease incidence. Some say that fungal survival structures will degrade faster within no-till soybeans.

G.  Some weeds and cover crops can make conditions more favorable for white mold infection.

H.  There has been limited research done on biological controls that help to reduce or biodegrade the survival structures (sclerotia) of white mold within the soil.  The most popular biological control is Coniothyrium minitans (Contans or KONI).  Most of these biologicals should be incorporated (within 2 inches) of the soil at least 3 months before white mold infection.  Studies have shown up to 95% reduction of sclerotia or reduction of disease incidence up to 10 to 70%.  But, keep in mind, if you have a heavy amount of disease inoculum in your field, you will never be able to completely free the soil of the fungal pathogen that causes white mold. 

Control of white mold within soybeans requires an integrated approach or the implementation of many different management strategies (listed above).










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