May 25, 2015: Wheat Pest and Disease Update

From Ned Tisserat, Retired Plant Pathologist, Bioagricultural Sciences and Pest Management

Stripe rust.

I guess I don’t need to  tell everyone that stripe rust is going to be bad this season.   It has been found in most areas in eastern Colorado with reports of rust on lower leaves even  in the northeastern corner early last week.   The continuing cool weather and rains have been ideal for development of this disease.  It looks like this weather pattern will hold this week and I would expect more infections and  the flag leaf to be severely impacted in many areas.  I suggest that anyone with fields with moderate to high yield potential spray with a fungicide if the crop is at flowering or just past this stage.    If you are way  past flowering (e.g. the southeast corner of the state) then the fungicides are not going to give you as much, if any yield boost.

I understand there may be shortages of certain fungicides.   If so, pick any fungicide available with very good to excellent control of stripe rust, whether it is a strobiluron, a triazole or a combination. I have been asked  if generic products are OK  Just as an example, tebuconazole is sold by different companies.   That is a hard one to answer because many of the generics have not been tested in replicated field trials.  One would expect that any product containing the same active ingredient and approximately the  same concentration would provide a comparative level of control.  But one also  needs to consider what level of technical support the company will provide if something were to go wrong.

Another question that comes up is how long after the rust penetrates the leaf can you spray a fungicide and still expect control?  That is, how good of an eradication component do these fungicides have?   That is a question I need to look up in more detail.  When I was in Kansas working with fruit growers, I would tell them the triazoles had about a 72 hour kickback.   That means you could spray 72 hours after the infection process had started and still get control of the fungus (on fruit trees) to prevent a lesion or spot from developing.  I just don’t know if this holds true for wheat, but I suspect it will be similar. This might be an important consideration. For example, if infections occurred more than a week ago, don’t expect your application to eradicate those developing lesions; they may only prevent further infections.

Other foliar diseases, such as tan spot and Stagonospora and Septoria leaf spots may also be impacting the leaves.  These diseases will cause necrotic lesions that will not develop bright yellow or red blisters as with the rust diseases.   Many of the fungicides used for stripe rust will also provide protection for these leaf spots.  Right now I would target stripe rust and hope to get residual control of the other leaf spots.

There is also talk about discoloration of the heads, especially in the southeast.  There could be multiple causes of this and we are not sure yet as to the culprit.  Dr. Scott Haley points out that some varieties carry a stem rust resistance gene Sr2 that will cause the heads to darken under certain environmental conditions .. Our major varieties right now (Byrd, Hatcher, Brawl, Ripper) do not carry this but Snowmass and Thunder CL both do.  The point is that fungicides may not prevent this type of discoloration.   However the Stagonospora fungus will cause a glume blotch and fungicides can help control this.  Finally there is a bacterial disease called black chaff (Xanthomonas) that will discolor the heads- this wet weather is perfect for this disease. Fungicides will also not control bacterial diseases.  Unfortunately right now I can’t tell you which scenario is correct.  I will get back as soon as we have more info. Please send samples to the diagnostic lab to verify.

 

Virus diseases.

And if rust wasn’t enough, we apparently are have lots of issues with virus diseases. Both barley yellow dwarf and the wheat streak mosaic complex are widespread both in eastern Colorado and on the western slope (BYD.  There is nothing to do for these – the damage is already done. But again, don’t sink a lot of money into  fungicides if fields are already impacted with these virus diseases.

Happy (soggy) Memorial Day.  I will be off to hunt for mushrooms.

Ned Tisserat

Retired Plant Pathologist

 

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