Wheat disease update – 17 April 2015

Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Plant Pathologist

Oklahoma:  This past week has been wet and cool across most of Oklahoma, which has greatly helped the wheat but also should lead to more foliar diseases – especially stripe and leaf rust.  Around Stillwater, we have not seen a big increase in stripe rust yet, but can find many leaves beneath the flag leaf on which there are actively sporulating pustules.  There also are leaves on which the black spore structure stage of stripe rust has appeared (called the telial stage – see photo – note actively sporulating pustules towards the base of the leaf).  Typically this indicates the disease is “shutting down,” which it was prior to this week.  However, with the cool and wet weather since Monday, there likely will be a “reactivation” of stripe rust – especially given that cool and wet weather is forecast for the next 5-10 days as well!

The black spore structure stage (telial stage) of stripe rust. Note actively sporulating pustules towards the base of the leaf.

The black spore structure stage (telial stage) of stripe rust. Note actively sporulating pustules towards the base of the leaf.

I get the impression from talking to Aaron Henson (County Educator; Tillman County) as well as other growers and consultants around the state that this is similar to what they are seeing.  Wheat seems to range from boot to heads emerging, so if you are contemplating applying a fungicide to protect a promising wheat yield, I suggest it be done soon.  Remember, GS 10.5 (heads fully emerged but not yet flowering) is the cut-off for applying fungicides for wheat foliar diseases.  That point likely is not far away for much of southern and central Oklahoma.  It likely is a bit farther away for northern and northwestern Oklahoma, but it is much better to apply a fungicide a little early rather than after the flag leaf is infected.

Most of the samples that came into the Diagnostic lab last week tested positive for wheat streak mosaic (WSM), including three from the panhandle.  However, there is nothing that can be done about WSM at this point.  I would refer you to EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic:  Three Virus Diseases of Wheat in Oklahoma) available at www.wheat.okstate.edu for more information on these mite-transmitted viruses.

Reports/excerpts of reports from other states:

Colorado  Dr. Scott Haley (Professor & Wheat Breeder, Colorado State University) 16-Apr-2015:  “FYI, stripe rust was confirmed for a field site near the Denver International Airport in Colorado this week. Infection was moderate but cool wet weather we are experiencing now could cause this to increase. This is pretty early for stripe rust sightings in eastern Colorado.”

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