As soybean crops move into flowering, questions are sure to be raised about whether fungicide applications are needed after the white mould levels we saw in some parts of the province in 2025. In anticipation of the questions, let’s take a closer look at the potential disease threats and what Manitoba On-Farm Network research has told us.
What Should We Be Concerned About?
- Foliar diseases infecting soybeans in Manitoba typically occur at low severity levels and are not expected to impact yield most years. These usually include bacterial blight, downy mildew and Septoria brown spot.
- Stem diseases generally have a greater impact on yield.
- White mould: infections begin at nodes along the main stem. Wilted plants may be spotted from afar, within a humid canopy may find white mycelial growth and black sclerotia bodies later in the season. This disease is the main target of fungicide applications. Cool, wet conditions throughout July and August favour white mould. For every 10% increase in the percent of plants infected with white mould, 2-5 bu/ac of yield are lost in soybeans. Early infections are the most damaging.
- Northern stem canker: small, reddish-brown lesions at the base of a branch or petiole in the lower third of the canopy after flowering. Lesions expand to form sunken cankers with reddish margins. May be confused with Phytophthora root and stem rot.
- Phytophthora root and stem rot: late infection appears as a chocolate brown lesion on the stem that extends upwards from the soil line. If roots are rotten, suspect PRR. If roots appear healthy, northern stem canker may be suspected.
- Pod and stem blight: distinct lines of raised, black dots on infected stems and pods.
- Anthracnose: reddish-brown, irregularly-shaped blotches on leaves and stems. Black fungal bodies develop in these blotches later in the season.
What Does the Research Tell Us?
From 2014 to 2024, 71 on-farm trials compared a single application of fungicide at flowering to none in soybeans. The main disease target to manage has been white mould.
In these trials, a single foliar fungicide application increased soybean yield in 15% of the trials by an average of 1.3 bu/acre. Overall, only six of 71 trials (8%) resulted in increased profits because the yield increase observed was worth more than the cost of a fungicide application. The research demonstrates that, though rarely occurring at severe enough levels, when conditions are optimal for white mould to develop, soybean yield will be protected with a fungicide application.
Bottomline in 2026 – when it comes to fungicide applications in soybeans we don’t see our recommendations in Manitoba changing. Although white mould reached yield impacting levels on some fields or field areas in 2025, we don’t consider that sufficient evidence to make a change to recommendations in 2026. Obviously agronomists will need to be vigilant this year paying close attention to daily weather and canopy conditions as the season progresses. But, we should all bear in mind that, given how soybeans flower, it is very difficult to accurately time a single fungicide application for good, consistent control and hard to justify multiple applications given product costs and soybean prices.
Read through the Results Summary – Click Here→ Soybean Fungicide Trials – Long Term Results (2014-2024)
Watch the 2024 Results Summary Video- Click Here→ Soybean Fungicide – 2024 On-Farm Network Results (10:33 minutes)