By Darren Hefty

Why do most diseases wait to show up until plants get to the reproductive stages of growth?  It’s simple.  There is a lot of stress on the plants at this stage.  Think about it.  Plants are completely switching what they’re doing.  They have been feeding leaves and stems and roots.  Now the main focus of the plant is to send resources to the seed and everything that goes into making it.  When that happens, the plant’s defenses are down.  That’s usually the key time to protect your crop.

Let’s hone in on one particular disease, sclerotinia white mold.  Growing up on a farm that raised soybeans, I was well aware of white mold, but had no idea it impacted other crops like canola, dry beans, sunflowers, peas, chickpeas, lentils, and more.  Fortunately, my lessons in soybeans held true for the most part with other crops.

  • Environmental Conditions – Sclerotinia white mold is a fungus that thrives in cool, wet conditions.  It is soil-borne and spreads under the canopy in susceptible crops.  It often starts in low-lying areas of a field.  Continuous cropping with susceptible crops and a history of previous infections are factors leading to a greater severity of white mold in a field.
  • Prevention – Control of white mold after it’s already developed on the plant is possible to some degree, but the yield loss has already occurred.  Fungicides must be sprayed BEFORE there is disease present to maximize yield.
  • Coverage – Fungicides must completely cover the plants to protect them.  White mold enters through dried up flower buds on soybeans, for example, so using more gallons of water and more pressure generally is recommended.
  • Product Selection – Not all fungicides work on white mold, so choose carefully.  Endura (boscalid), Proline (prothioconazole), Talaris (thiophanate-methyl), and Domark (tetraconazole) are examples of products (listed in order of effectiveness) that have a good level of control or suppression on white mold, but check first to see which are labeled in the crop you are raising.  Contans is a soil applied biological fungicide that specifically eats the sclerotia, which are the seeds for white mold.  While it’s not cheap to try, this is a product that can be used in a variable rate application to hit the white mold hotspots in your field and reduce pressure.  Treatment with Contans works best as soon as possible after harvest to give this product more time to work in your soil.
  • Varietal Tolerance – There is a difference from one variety to another in terms of tolerance to white mold.  There is also a difference in crops.  For example, soybeans have had much more breeding work done than the other crops listed above.  With all crops, much of the difference in varietal tolerance comes down to plant type.  The number one characteristic needed to reduce white mold pressure is standability.  The more upright the plant stands, the less likelihood of a disease like white mold.
  • Cultural Practices – Row spacing, population, and other factors that lead to a slower or less dense canopy can reduce white mold pressure.  Improving field drainage is also a big help.
  • Timing – Depending on the crop, spraying a fungicide as reproduction begins is generally the recommendation that achieves the best results.  Since fungicides often only have 10 to 14 days of effective residual, you likely need to re-apply every couple weeks as the threat exists.  In our First Steps plots at the Ag PhD Field Day, top soybean producers from across the country typically spray fungicides three times to protect the plants all the way through the reproductive stages of growth.
  • Spray Nozzles – Use the Ag PhD Spray Tip Guide to look up your product of choice and find the best tips for that product.  I personally choose the Guardian Air Twin tips and size them appropriately to deliver a medium-sized droplet at 15 to 20 gallons per acre.  DON’T USE ROUNDUP TIPS when spraying a fungicide.  While big droplets reduce drift, smaller droplets improve coverage, and that’s key to making fungicide work well.

Sclerotinia white mold can be a tough disease to fight.  Get out in front of it this year with a timely fungicide application and utilize additional applications as needed.