Grateful for seasons, for fall, and that we have such beautiful fall colors this year! For whatever reason, it just doesn’t seem like we’ve seen colors like this for a few years and several have commented about the beauty this year. May we take time to notice the beauty around us each day! Also grateful for harvest being completed or nearing completion for many! Each day is one day closer to the end!

White Mold in Soybean: Sclerotinia sclerotiorum (the pathogen that causes white mold) impacts over 400 plant species from 75 families including soybean, dry bean, potato, sunflower, peas. Host weed species include: pigweeds, velvetleaf, henbit, lambsquarters, ragweeds, nightshades, mustards, sunflower. Cover crop hosts include: peas, lentils, turnips, radishes, collards, common vetch, (alfalfa and clover to a lesser extent). Just reading all that is discouraging. The fungus survives in a hard, black structure called a sclerotia that looks like mouse/rat droppings and can survive in the soil for 5 years (3 years in no-till fields). Frequent irrigation, plant wetness/fog/cool conditions at flowering (like we had in 2023), narrow rows, high plant populations, and cooler weather conditions of 46-75F allow the sclerotia to develop apothecia (look like circular tan mushrooms). Under a specific pressure, the apothecia shoot spores into the canopy where they infect the soybean plants whenever they land on senescing soybean flowers.

Flowering occurs from R1-R5 in indeterminant soybeans. Thus, why white mold is difficult to control and why we can see it develop so late into the year. The fungal infection moves from the flower into the stem, disrupting water transport. Thus, why you will often see plants that look wilted as an early symptom even before you see the signs of the white fungal growth. Wilting of plants leads to premature death impacting yields like we saw with large yield hits this past year. New sclerotia are formed within and also stick out of the plant stem and pods. They fall to the ground and the cycle continues when a susceptible host is grown.

So, what do we do about it? The following information is for fields that have a history of white mold. I’d suggest looking at this from a system’s approach. One piece is to consider varieties with disease resistance. I won’t argue that’s important. However, I’m honestly hesitant to start there as I’m unsure we have strong disease packages. And some defensive varieties give up too much yield. This is my perspective and I don’t expect people to agree. For now, I suggest finding the strongest yielding genetics first because there’s large yield variation in soybean.

From there, it becomes managing the other factors that can aid in disease. Manage weeds and avoid susceptible cover crops in the field. Crop rotation is not effective if it’s only a 2 year corn/soy rotation. While I realize they won’t work for everyone’s operation and they take more management, using small grains like cereal rye before soybeans is a tool that can help with both weed and disease suppression and doing so adds another crop to a 2 year rotation. Avoid irrigation at flowering (I realize 2023 was tough) and seek to irrigate deeper and less frequent.

There’s also tradeoffs within the system regarding row spacing for weed or disease control. Results from 18 site-years of research from Wisconsin, Michigan, Ohio, Indiana, and Iowa showed:

• If planting in row spacings of 15 inches or less and in fields with a history of white mold, use a seeding rate no greater than 110,000 seeds/ac.
• If planting in fields with a history of severe white mold, widen the row spacing to 30 inches and use a seeding rate no greater than 110,000 seeds/ac. UNL research shows you don’t give up yield with final plant stands at 100,000 plants/ac.
• Fungicide applications remain an effective tool for reducing white mold levels if applied between the R1 (most effective) and R3 (less effective) growth stages. Fungicides provide 0-60% control. They’re most effective if sprayed below the canopy.
• Not all fungicide products are equally effective at controlling white mold, with Endura® remaining the most effective product if applied between the correct growth stages.
• Consider downloading the Sporecaster app for white mold to better time fungicide apps based on weather conditions (found to be 81.8% accurate).

Most biological control agents should be applied at least 3 months before flowering for fungal colonization. Biological control agents and seed treatments such as Heads Up® were shown as effective tools to reduce severity of white mold and SDS based on Iowa State research. Plant nutrition is also showing promise and that’s something I’d like to try next year with a few growers; please let me know if you’re interested. Hydrogen peroxide products had little to no success in Wisconsin research (provided only 4 hours of activity killing spores). None of the things mentioned here are exclusive, but a combination of many of these factors as a system’s approach can help in the battle against white mold.

To view tables of the fungicide research results for white mold go to: https://cropprotectionnetwork.org/publications/modern-integrated-management-practices-for-controlling-white-mold-of-soybean