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Dec 11

The Extension Connection Column by Jeff Stachler

 

Frogeye Leaf Spot - A New Soybean Disease in North Dakota

Happy Friday to all of you!  Thank you for the complements concerning my column.  I hope you are finding them useful.  We are getting quite dry and this is a concern to me.  Let’s pray for rain. 

Frogeye Leaf Spot, a foliar disease of soybean, was found for the first time in North Dakota in September of 2020.  It was found in Sargent County.  This disease can cause yield loss.

The pathogen causing Frogeye Leaf Spot is a fungus called Cercospora sojina.  The pathogen is very diverse genetically.  There are between 12 and 20 different races of the pathogen.  A race are genetically similar individuals of the pathogen having the same combination of genes making them able to cause the disease on a particular variety of soybean. 

Symptoms usually just appear on leaves, but can appear on pods and stems in years when the disease is severe.  Symptoms can appear shortly after flowering or later in the season.  The youngest leaves are the most susceptible to the pathogen.  Lesions (spots) appear as small gray spots with reddish-brown to purple borders.  On the underside of the leaf, the lesion appears brown to gray with tiny dark “hairs”.  The hairs are the long conidia or infective spores of the fungus.  Severe leaf drop can occur when the disease is severe.  Pods and stems can become infected if frequent rainfall and high humidity persists.  The lesions (spots) on pods are reddish brown, shrunken, and circular to elongate in shape.  Older lesions on pods become brown to dark gray, usually with a narrow, dark brown border.

The disease can spread by seed and soybean residue.  It is unsure at this time if the Frogeye Leaf Spot fungus can over-winter in North Dakota, but it likely will.  Spores are carried by wind and rain from other areas, so even if it does not survive in North Dakota it could still come up from the south.  Rain splashes spores from soybean residue up onto young leaves in the canopy.  The pathogen prefers warm (77 degrees F to 86 degrees F) and wet (rain, heavy dew or >90% relative humidity) conditions to cause infection and disease development.  Symptoms develop in 7 to 12 days after infection, depending upon temperature.  The disease will complete multiple life cycles during the season, so the disease will get worse on individual plants and throughout the field.

If Frogeye Leaf Spot shows up prior to or at flowering then substantial amounts of the disease can develop and will have a negative impact upon yield.  Frogeye leaf spot can reduce yields by 10 to 60%.  If the disease is first found at or after R4 (full pods on one of upper four nodes) to R5 (beginning seed (1/8 inch seed) in one of upper four nodes) then very little yield loss is expected.

One of the biggest problems with Frogeye Leaf Spot is that the pathogen can be resistant to strobilurin fungicides such as Headline, Quadris, Aftershock, Evito, and Aproach.  NDSU has determined that about 50% of the pathogen in Sargent County was resistant to strobilurin fungicides, making them ineffective for this disease.

The best way to manage Frogeye Leaf Spot is to plant varieties resistant to the pathogen!  Purchase varieties with the Race (RCS) 3 gene.  The next method to managing Frogeye Leaf Spot is to completely bury the soybean residue and/or rotate to corn, wheat, or alfalfa, which are non-host crops to the disease.  Planting continuous soybeans will only increase the problem.  The only other management strategy is to apply fungicides.  For fungicides to be effective, timely scouting is required.  Fungicides should be applied when one or two lesions (spots) can be found in every 25 foot of row when soybeans are at the R2 (full flower) growth stage.  Effective single active ingredient fungicides include Topguard, Proline, Domark, and Topsin-M.  Remember that strobilurin fungicides only are no longer recommended for managing Frogeye Leaf Spot due to the likelihood of resistance.

Keep an eye out for this disease next season, but I doubt it will be a yield limiting problem!

 

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