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Small grains disease update 06/09/2022

 Jochum Wiersma, Extension small grains specialist

I now also found tan spot on some of the earliest seeded spring wheat. In this case, it was at one of the on-farm trial locations and some of the varieties in the trial were not only showing tan spot but also turned very pale to almost yellow. Early infections of tan spot can indeed turn susceptible varieties yellow as the young seedlings respond almost systematically rather than just locally to the toxin that the tan spot fungus produces. In older plants the toxin's effect is limited to just the leaf tissue surrounding the tan spot lesion, yielding the telltale yellow halo. The disease models for tan spot indicate that the weather is not been favorable for initial infection since the first of the month, meaning that the tan spot infections in this trial occurred in the last week of May and when teh crop was just reaching the two-leaf stage.

I encourage you to scout the spring wheat before you do your weed control and determine whether you have tan spot present in your field. if you find tan spot it is a good idea to add half a labeled rate of a fungicide to your weed control program. If you can not find any tan spot, do not add a fungicide and you can save yourself some money. Research from NDSU has shown again and again that you will not see a return on your fungicide application if no disease present at the time of the herbicide application.

Meanwhile, winter rye is either fully headed and at anthesis across much of the state, and winter wheat in the southern part of the state is nearing heading. The risk of Fusarium head blight was low all of last week. It has started to trend higher in the southeastern corner of the state. East of US Hwy 169 from the Iowa border to the Twin Cities is now at moderate risk while the area east of US Hwy 63 is now at moderately high to high risk.

This means making a decision on whether or not to spray a fungicide to suppress Fusarium head blight sooner rather than later. The weather forecast for the southern half of the state also suggests that the risk will not be waning and likely be spreading westward. The optimum timing to suppress scab (and control any leaf diseases that also may be present) remains at Feekes 10.51 for winter wheat and rye for all fungicides labeled for use in the respective crops. Miravis Ace provides a wider window of application as it maintains adequate suppression up to 5 days after the beginning of anthesis compared to the other fungicides labeled for suppression of FHB.
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