Soybean First Flower and Soybean White Mold

BY Dairyland Seed Agronomy Team

At this time of year, I start looking for flowers on soybean plants. This year I spotted flowers on June 17^th, which is one day later than the 2023 flower date of June 16^th. (The years 2023 and 2012 are tied with being the earliest I have seen flowers on soybean plants).

This brings up the point that soybean plants can start the flowering process from V2 -V8 (2^nd to 8^th trifoliate), and, with early planting dates, can start flowering prior to summer solstice. Other factors, such as drought stress, can induce plants into flowering. Conversely, soybeans that are planted early and have good fertility are also more prone to flowering early. The photograph below shows soybean growth stages VE to V3.

Once a soybean plant starts to flower, it is in the reproductive phase or R1 which is Beginning Bloom when one open flower on any node on the main stem (shown below and on the left).

The photo above and on the right shows a soybean plant in the R2 growth stage which is Full Bloom. Full Bloom is when an open flower is on one of the two uppermost nodes on the main stem. Peak flowering occurs at Beginning Pod development (R3), with flowering continuing until Beginning Seed (R5) or shortly thereafter. Soybean plants will flower and put on vegetative growth at the same time. Not every flower or blossom will produce a pod. In fact, it is not unusual for soybeans to abort 60-70 percent of its flowers. The issue with aborted flowers, besides reduced yield potential, is that it provides an opening for a fungus Sclerotinia sclerotiorum (which causes white mold) to enter the plant.

If you have fields that have a history of white mold (SWM) that are in soybeans this year and are looking at applying control measures, many fungicides such as Approach®, recommend applications at R1 followed by another application at R3. This tends to make sense in that most fungicides do not move into new growth, and an additional application at R3 would provide supplementary coverage to new flowers and vegetative growth on the plant.

Another factor that starts coming in to play when soybeans start flowering is, many herbicide labels place restrictions on application timing for crop safety, starting when soybeans reach the Reproductive or R stages. These restrictions are not always about crop safety. They may also have environmental and or rotational crop concerns for following years.

If you have any questions regarding this or other agronomic topics, contact your Dairyland Seed Regional Agronomist.