Herbicide Drift Of Leaf Blotch On Small Grains?

May 05, 2015
Over the last week we have seen small grains with a small amount of damage to the upper canopy.  Damage includes sparse flecking or brown leasions forming in the upper canopy, with little to no injury lower in the plant.  Lesions are often light brown to white and do not have clorotic halos or black pin heads within the lesion.  Some individuals were worried that this may be Stagonospora, Septoria, or Tan spot lesions in wheat or net blotch and scald in barley.
 
gramoxone damage wheat 2015
 
Herbicide damage on wheat. Note the tan/brown lesion with no yellow margin.
 
In nearly all the cases I’ve seen this symptom is more likely due to herbicide drift (e.g. gramoxone) blowing in from corn fields being planted in the vicinity.  How can you tell herbicide drift from foliar diseases?
 
1) Lesions from herbicide drift will not increase over time
 
2) Lesions from herbicide drift will not contain brown (Stagonospora) or black (Septoria) structures within the lesions
 
3) Lesions caused by herbicide drift are often scattered and present on the top of leaves.  Leaf blotch diseases move from the residue up the plant.  Therefore, expect to see lesions in the lower, mid, and upper canopy.  Lesion incidence and severity are likely to be greater in the lower and middle canopy as well
 
4) Leaf blotch lesions can have distinctive shapes or characteristics.  Lesions caused by Stagonospora are elliptical.  Tan spot lesions often have a distinct yellow boarder.  Net blotch lesions are black with a stringy appearance.
 
gram spot wheat
 
Lesions caused by herbicide drift are sparse, often located in the upper canopy, and do not grow over time as opposed to leaf blotch diseases.
 
wheat_septoria tritici_far
 
Septoria leaf blotch on wheat.
 
DSC_0158
 
Spot blotch / net blotch on barley
 
barley scald
 
Barley scald with distinctive cream colored lesion centers and black/purple margins
 
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