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Small Grains Disease Update 06/15/2020

The first instances of stripe rust, crown rust, and barley yellow dwarf were confirmed in winter wheat and oats, respectively, this past week in southern Minnesota. Meanwhile, tan spot is prevalent in wheat following wheat in the northern half of Minnesota.  

These findings are all in line with expectations/risk models. The conditions for tan spot, for example, have been favorable across much of the northern half of the state for seven out of the last ten days.  
One of the characteristic symptoms of early-season tan spot infections is a yellowing discoloring of whole leaves. This is a more extreme expression of the same yellow halo that surrounds the tan spot lesions in more mature plants.  Be careful not to mistake this yellowing for a nitrogen deficiency.  

Research at both NDSU and the University of Minnesota has shown that the early onset of tan spot yield can results in yield reductions of 4 to 5 bushels if conditions continue to favor the development of the disease. Use half a labeled rate of a registered fungicide to halt/slow down the disease progression.  Most of the labeled fungicides can successfully be tank-mixed with the commonplace herbicides. Always check the label of both the herbicide and fungicide for tank mix restrictions.
Research at the Northwest Research and Outreach Center has shown that the combination of any of the EC formulations of fungicides in combination with common wild oat herbicides and bromoxynil + MCPA can result in bromoxynil injury in both wheat and wild oats. This injury generally did not affect grain yield of the wheat or the control of the wild oat. 

A nitrogen deficiency can readily be identified as the symptoms are worst on the oldest leaves and start at the tip of the leaves, progressing towards the base as the deficiency gets worse.  This is in contrast to the yellowing caused by tan spot, which will start from the initial lesions and migrate up and down the leaf blade from the initial point of infection.  The causes of the N deficiencies are several, all of which have a common denominator, namely excess precipitation.  Excessive rainfall causes leaching, denitrification, and the inability of the plants to take up available N

Leaching is a potential problem in coarser textured soils. Saturated soils/standing water will cause both denitrification and inability to take up available N.  As soils are saturated, the plant’s roots also are unable to take up N - even if available.  Often the crop recovers quickly if the growing conditions improve and the excess water has drained. If the N deficiency is severe, a supplemental application of N as either urea (46-0-0) or urea ammonium nitrate solution (28-0-0) can be advantageous. 

The earliest seeded spring wheat in the southern half of the state will likely reach anthesis sometime this week.  To date, the risk models Fusarium head blight have been trending relatively low.  This is largely the result of the lack of precipitation the past two weeks across a large swath of southern half of the State.  The lower dew points further help reduce the risk of the disease to develop.  Focus the scouting efforts on the presence of foliar diseases and still consider a fungicide application at anthesis if you detect tan spot, Septoria, and/or one of the rust on the flag leaf, penultimate leaf or the second leaf below the flag leaf.

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