By Andrew Friskop, Extension Plant Pathologist
Winter wheat and spring wheat are approaching (or have approached) the growth stage when producers will consider tank mixing a fungicide with a herbicide for management of early-season fungal leaf spots (tan spot, Septoria leaf spot) and weeds. There are several factors that will influence the value of an early-season fungicide and I will review some of these below.
1. Crop rotation – Planting wheat into last year’s broadleaf ground will reduce the risk of residue-borne diseases from flaring up in a field and delay the onset of fungal leaf spot development. Conversely, a wheat on wheat rotation will increase the risk for leaf spot development.
2. Tillage – No-till systems will have more wheat residue on the soil surface that harbor the overwintering structures (fruiting bodies) of fungal leaf spot pathogens (Figure 1).
3. Weather – Cool weather and prolonged moisture periods (dews) favor the development of fungal leaf spots. Given that North Dakota experiences cool weather in the beginning of the season, moisture and dew are often the driving factor for fungal leaf spot development (Figure 2).
4. Varietal Selection – Resistance to fungal leaf spots varies among wheat varieties and ratings are available in NDSU variety selection guides.
5. Scout – One of the most important disease management practices is scouting. If fungal leaf spots are appearing on the oldest leaves at the time of an early-season application, a fungicide is likely recommended. However, if a wheat on broadleaf rotation is used and no fungal leaf spots are observed on the oldest leaves, you may want to reconsider applying a fungicide at this time.
6. Fungicide Selection – There are several products labeled for management of fungal leaf spots. Most of them are either excellent or very good in managing tan spot and Septoria. For efficacy ratings of wheat diseases, please review the fungicide efficacy table compiled by wheat plant pathologists: NCERA184 – Wheat Fungicide Table.
7. What to expect from an early-season fungicide application? – Studies conducted by NDSU over the last 20 years have shown that a 2 to 6 bushel response occurs when an early-season fungicide was used in a wheat-on-wheat production system with minimum tillage when favorable weather was present.
The incorporation of other management tools such as crop rotation and tillage will reduce the risk of tan spot development and reduce the expectant yield response.
Also, remember an early-season fungicide will protect the leaves available at the time of application, but as the wheat crop matures, newly developed leaves will be left vulnerable to leaf spot and rust pathogens.