This question comes up regularly, which makes sense. Early season outbreaks of fungal leaf spot diseases such as tan spot do occur in Manitoba, such as in 2024. But to really answer this question, we need to dig into some western Canadian research.

There have been a few studies done in Western Canada over the last 15 years investigating this very practice in both wheat and barley.

1. The impact of fungicide and herbicide timing on foliar disease severity, and barley productivity and quality
2. Evaluation of disease, yield and economics associated with fungicide timing in Canadian Western Red Spring wheat

In both these studies fungicide application timings were evaluated on their impact on leaf spot disease severity, overall crop yield and economic returns of the practice. In both studies, it was found that a foliar fungicide application at herbicide timing (2-3 leaf or 5 leaf) did not lower upper canopy disease incidence and severity compared to the check (no treatment or herbicide only). It was also found that the foliar fungicide treatment at herbicide timing didn’t improve crop yield compared to the untreated or herbicide only check. This ultimately led to an economic loss. In both studies, researchers found that foliar fungicide application timings for cereal leaf diseases should be aimed at protecting the upper canopy and yield-contributing leaves.

There has also been some research on this topic out of North Dakota. Andrew Friskop from North Dakota State University recently compiled replicated foliar fungicide timing trial data from 2008-2024. He evaluated the yield response based on “disease risk” scenarios determined by previous crop, variety resistance, tillage, environmental conditions and disease onset.

Figure 1. NDSU disease risk categories for development of residue-borne foliar diseases.

His finding suggests that under high-risk scenarios, (where wheat was grown on wheat stubble, a susceptible variety was planted and the disease [tan spot] was established early and firmly in the crop), a fungicide application at tillering could provide a small benefit of 2.2 – 3.7 bu/ac. In basically all other production scenarios he found that a foliar fungicide application at tillering would provide very little yield response.

Figure 2. Summary of yield response by disease risk level for early season fungicide application.