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Watch for soybean diseases after big rains

I looked at the soybean prices on Sunday – all were still less than $10/Bushel.  This price combined with yield losses due to late planting, extra expenses for additional late weed control, and flood injury really put the kibosh on all but the most guaranteed return on investment for the remainder of 2015.  Here are a few guidelines, results from our studies in Ohio that point to the best return on investment.

Foliar pathogens have the most impact on soybeans at the later growth stages (R3 to R6) by reducing the photosynthetic area of the leaves that contribute to pod development and seed growth (http://www.oardc.osu.edu/soyrust/2007edition/10-SoybeanGrowthandDevelopment.pdf).  Soybeans also have an uncanny ability to compensate for missing neighbors.  The profitability measure for the 2015 season will be to scout for the occurrence of diseases after flowering R3 and choose the best fungicide if necessary.

  1. Septoria brown spot.  This is a lower canopy disease, which surprisingly, we have not been getting too many reports of this year.  Where we are, it is from fields that are planted into continuous soybean and have heavy residue.  Even in these situations, the yield loss for this is still on average 2 to 3 bu/A.
  2. Frogeye leaf spot.  This disease we are monitoring, not only because there are a few highly susceptible varieties but also because there are reports from Illinois, Indiana, and up and down the Mississippi of populations that are no longer managed by the strobilurin class of fungicides.  If you see it, please send this to the lab ASAP, so we can run some tests.  We have seen yield differences with low levels of disease (5 to 12% leaf area affected) of 5 to 10 bu/Acre.  This is the one to keep an eye out for but the timing for sprays is between R3 and early R4.
  3. Sclerotinia stem rot or white mold.  For those fields with a long history of this disease, this can cause problems when we have cool nights (a.k.a no air conditioning turned on in your house) and heavy dews.  We have started our scouting for this pathogen as fields begin to get closer to flowering.  However, for those historic areas where white mold is always present AND a susceptible to moderately susceptible variety was planted, a fungicide may be necessary this year.  The key is the timing, and coverage of the fungicide in the field.  The target area is the lower part of the stem.
  4. Approach – we have measured significant reductions in white mold when we applied this fungicide at Western branch right before flowering followed by a second application 10 days later.
  5. Endura – we have measured significant reductions in white mold with this fungicide with one application timing (R1 – a few plants are beginning to flower in the field).
  6. Phoenix and Cadet Herbicides – both have reduced the incidence of white mold in trials in northeast Ohio.  If you are also going after weed escapes, this may also be a tool to consider.
  7. Topsin M – this has been the stand by white mold fungicide, but for the past 3 years, we have not been able to measure reductions in disease.
  8. Some cautions, we have not been able to reduce white mold with a fungicide nor with a herbicide if the field is planted to a highly susceptible variety and the crop is in full flower and infections have already occurred.  These materials mainly work as protectants and have to be on the plant at those lower nodes to protect it prior to the arrival of the pathogen.

In summary, for foliar pathogens there is time to let the plants recover and take a look later in the growing season to determine if the pathogens are present.  This is the year to focus those scouting efforts on highly susceptible varieties.  For historic white mold areas, this will be another year to implement measures on those highly to moderately susceptible varieties.

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