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Fungicide Efficacy and Timing Questions on Fusarium Head Blight (Scab) (6/29/17)

Small grains in western ND have headed with most of the crop past the flowering growth stages.

Fungicide Efficacy and Timing Questions on Fusarium Head Blight (Scab)

Small grains in western ND have headed with most of the crop past the flowering growth stages. In the northcentral and northeastern parts of the state, the small grain crop is near heading or flowering. This area of the state currently has the most amount of Fusarium head blight (FHB) risk. Several questions have been asked about fungicide use for FHB and here is some information on the three most commonly asked questions.

What to spray?

Fungicides in the FRAC 3 (DMI or triazoles) are the only labeled group of fungicides that provide adequate suppression of FHB and deoxynivalenol (DON). However, efficacy differences are observed among this group. Specifically, prothioconazole + tebuconazole (Prosaro) and metconazole (Caramba) offer the most suppression (~50-60%) of FHB and DON. Tebuconzole (Folicur, generics) provides some suppression (~20-30%), while propiconazole (Tilt, generics) does very little for scab suppression (~10-15%). With regards to fungicide chemistries, do not apply a product containing a FRAC 11 (QoI or strobilurin) for FHB and DON suppression. Products containing FRAC 11 chemistries have been shown to increase DON levels.

When to spray?

The best time to apply a fungicide for FHB management in wheat (and durum) is at early-flowering and for barley at full-head. However, there are several field situations where the optimum timing cannot be completed due to environmental conditions, logistical issues, etc. Therefore, the question arises on whether you should err on the side of being “too-early” or “too-late”. Too-early is referred to growth stages that occur before early-flowering (yellow anthers on the wheat head) in wheat (Figure 1) and growth stages prior to full-head in barley (Figure 2). Too-late is referred to applications made after early-flowering in wheat and after full-head in barley. Although FHB suppression occurs with a “too-early application, recent research suggests that “too-late” applications provide greater FHB and DON suppression (Figure 3). Studies conducted by the US Wheat and Barley Scab Initiative across several states have shown that “too-late” applications show just as good or sometimes better management of FHB and DON when compared to recommended timings. Potential explanations on why this occurs could be attributed to a greater FHB risk later in the growing season and/or the fungicide application is protecting more small grain heads (i.e.: tillers, delayed main stems).

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What about double/split applications?

The recent epidemics in northcentral and northwestern ND in the past couple years have prompted more trials on the use of double or split fungicide applications. This includes applying a half-rate of a fungicide twice in the flowering process or applying a full-label rate of two fungicides belonging to the FRAC 3 group. Last year, 12 research trials at 5 locations (Carrington, Fargo, Langdon, Prosper and Williston) were conducted using double/split applications for FHB and DON management in barley, winter wheat, spring wheat and durum. Table 1 includes the mean level of DON reduction for treatments included across all locations and small grain market classes. Notice that double applications often had greater DON suppression. However, keep in mind that some of these fungicide treatments are not economical and the cost of an additional application should be weighed before employing this across a farm. These trials also show that sequential fungicide applications using two different manufacturer’s triazole chemistries do not provide complete additive control. For example, a single Prosaro application giving 44% DON production, followed by a Caramba application 4-7 days later does not double the DON reduction to 80-90%. These trials are being conducted again this year and results will be analyzed and communicated at several Extension events.

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Andrew Friskop

Extension Plant Pathology, Cereal Crops

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