Timing of fungicides for scab in wheat and barley
Early planted barley is approaching the stage of greatest risk for Fusarium Head Blight infection (scab). Spring wheat is not far behind. High humidity during this critical stage increases the risk of scab development, so the recent wet weather has increased the likelihood that scab will develop. Refer to the small grains disease-forecasting model for further guidance on determining the risk for scab development for your particular area as your crop approaches heading. If conditions are conducive for scab development, recommended fungicides can be effective in reducing scab development, particularly if applied at the appropriate stage of development. If wet conditions continue, there is a high likelihood that fungicide use will be profitable. Given that we are approaching the optimum stage for fungicide applications for the management of scab, we have felt it would be useful to review information on the most effective stage for applying fungicides for the control of scab. This is a revision of an article previously published.
Timing in wheat – The optimum time to apply recommended fungicides for FHB control in wheat (winter, spring and durum) is at early flowering. Applying fungicide at this stage helps to protect vulnerable florets from Fusarium damage during fertilization and the kernel during early grain-filling. The center spike in the accompanying photo is at the ideal stage for applying fungicides. The spike on the left has emerged from the boot, but has not yet started to flower (there are no visible anthers extruded from the glumes) and will likely be at the optimum stage in about two or three days depending on the temperature during the day. The spike on the right is past the optimum stage; the anthers are bleached and dried, unlike the turgid, yellow anthers in the center spike. The period between head emergence and flowering is usually about three days. Since not all spikes emerge at the same time, fungicides are best applied when most of the main stem and first tiller spikes have reached early flowering. Experience has shown that it is better to apply fungicide too early rather than too late.
Timing in barley – Flowering in barley begins just before the spike emerges from the boot, so barley florets are not overly susceptible to scab infection. Therefore, scab infections do not generally impact yield in barley. The scab fungus, however, is able to infect the glumes of barley and produce DON which impacts its market value, particularly if it is being sold for malt. The malting and brewing industry is sensitive to very low levels of DON. The optimum stage for applying fungicides to protect the glumes of barley from FHB infection is when the spike is fully emerged from the boot. In the accompanying photo, the spike third from the left demonstrates the optimum stage for treating barley with fungicides, with those further to the left too early and the one on the right too late. With barley the appearance of the first spikelet from the boot is a good indication that the best stage for spraying is only a few days away.