By Darren Hefty

What happened in 2017 that made it the worst white mold year I’ve seen in 20 years? There are several factors that played into our issues with sclerotinia white mold, and there are also a number of control methods that showed some promise. Here’s what we learned this year.

Conditions that led to sclerotinia white mold outbreaks included cool, wet weather in August. Sclerotia from previous white mold infestations can lay in the soil for up to 10 years. With favorable conditions, the sclerotia will form small mushrooms called apothecia. The mushrooms will literally shoot millions of spores into the air. When those spores land on dead plant tissue (most notably dried up flower petals on blooms of crops like soybeans) the disease can infect the plant.

Lessons from 2017

  1. Variety differences: There are certainly differences in the tolerance level soybean varieties have for sclerotinia white mold. While there is no genetic resistance available on the market today, varieties with better standability and a thin, upright stature typically have less white mold pressure.
  2. Maturity differences: Some years the earlier maturing beans have more white mold. Other years the later maturing ones are more affected. It all depends on when the white mold spores hit your field and the plant growth stage at that time. Don’t rush to judgment about a single soybean variety without multiple data points to make the decision.
  3. Biological controls: Contans gets the most attention here as it is a naturally-occurring fungus that actually eats the sclerotia. It has been shown to reduce white mold pressure and is best used immediately following harvest of a white mold-infested crop. It’s been a good helper, but is not a total solution. There are other biologicals on the market and in development that are showing promise in reducing white mold pressure, as well.
  4. Cultural controls: Row spacing can have an impact on white mold as a thinner plant canopy allows more air movement and sunlight to naturally hold back a fungus like white mold. Cutting plant population can help a little bit, but not as much as widening the rows. However, you don’t want to cut back population or widen out the rows to the point of costing you yield and reducing weed control. Crop rotation has little downside risk and has proven to lessen white mold pressure to future crops. Just keep in mind sclerotia can survive up to 10 years.
  5. Cobra: Cobra sprayed just before the plant reaches reproduction can help control emerged broadleaf weeds. It also performs fairly well in white mold trials. The exact mode of action for stopping disease is not known, but the fact that it burns and often drops some leaves certainly leads to more air movement and sunlight penetrating the canopy.
  6. Foliar fungicides: Fungicides are good at preventing diseases from forming if sprayed at the proper timing. First bloom is a critical time to protect soybean plants from white mold. The residual of most fungicides runs out after a couple of weeks, and retreatments at R3 (first pod) and even R5 (beginning fill) can prove beneficial by extending the window of protection for your plants. Not all fungicides work on white mold. Endura is the best, followed in performance by Topsin, Proline, and Domark. Acropolis may also be a good choice, since it is a premix of Topsin and Domark. Just keep the rate up and spray early and often if you want better suppression from any fungicide.

Just because 2017 was a bad white mold year does not mean 2018 will be bad, too. For white mold or any disease to infest your crop, you need the presence of the disease, a susceptible host, AND the right environmental conditions to all happen at the same time. Take steps necessary to protect your fields if these conditions look like they will again converge in 2018.