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

Since my last report, I don’t believe a lot has changed with the disease situation.  Similar to what I indicated in mid-December, I have continued to find small pustules of powdery mildew and a few scattered leaf rust pustules in rank wheat around Stillwater.  The temperature and weather has been such that these two foliar diseases (powdery mildew and leaf rust) have been able to persist but have not increased in the wheat around Stillwater.  This seems to be the case for southwestern and south central OK as well.

Gary Strickland (Extn Educator & SWREC Dry-land Cropping Systems Specialist; Jackson/Greer Cnty in SW OK) indicated wheat in SW OK is quite a ways behind the rest of the state and mostly is just now establishing a solid root system.  He had sent us a sample last week in which he expected a root rot to be involved, but we were not able to confirm any root rot pathogens.  Gary also indicated that he has confirmed Hessian fly at damaging levels in at least a few fields in Jackson Cnty.  He is going out in the next few days to scout additional fields.  Aaron Henson (Extn Educator; Tillman Cnty in southern OK) indicated wheat in his area varies from quite small to well-established with the majority of the wheat not yet jointing.  He is aware of the earlier reports of scattered stripe rust showing in south-central OK, but has not heard anything to indicate increase in incidence or severity.  For additional information regarding early season foliar wheat diseases and possible control with an early fungicide application please see:

• E-Pest Alert (Dec 18, 2015; vol. 14, no. 46) at http://entoplp.okstate.edu/pddl/pdidl
• A fact sheet discussing split application of fungicides at www.wheat.okstate.edu
• A February 9th post on this blog

Also around Stillwater, I am beginning to see symptoms indicative of wheat soil-borne mosaic/wheat spindle streak mosaic in areas such as my WSBM/WSSM screening nursery.  At this point, it is somewhat difficult to differentiate between symptoms of WSBM/WSSM and discoloration resulting from cold.  The photo below shows the contrast between a variety susceptible to WSBM/WSSM and a resistant variety, but was taken about 2-3 weeks later than today.  Thus, over the next month as temperature rises and wheat greens up, symptoms will become more striking.  However, nearly 100% of varieties planted across Oklahoma are resistant to both these viruses, so this disease complex has not caused a problem to wheat in Oklahoma (or other states) for many years.  For more information on the WSBM/WSSM complex, go to: https://www.youtube.com/user/OSUWheat/videos and watch the video on “Wheat Soilborne Mosaic Virus and Wheat Spindle Streak Mosaic Virus.”

Wheat soilborne mosaic virus can cause yellowing in the spring in susceptible varieties such as the one on the left.

Powdery mildew

Wheat stripe rust

Wheat leaf rust

Mature Hessian fly larvae are brown in color and often referred to as flaxseed. Tillers with larvae will not recover and will eventually die and slough off.

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

Oklahoma:  Early this week I spent time around Stillwater, Lahoma (10 miles west of Enid), and Cherokee and Alva (north-central OK not far from the Kansas border).  Wheat ranged from soft dough around Stillwater to milk at Alva (but quickly approaching dough).  There is a big difference between the wheat in these areas; obviously wheat around Alva suffered more drought stress than Cherokee, which is worse than Lahoma or Stillwater.  However, lots of moisture and cool temperatures are allowing wheat to fill and mature.

As expected, leaf rust has exploded around Stillwater, with susceptible varieties such as OK Bullet at 80-90S.  Leaves are gone on varieties that were highly susceptible to stripe rust, but varieties with stripe rust resistance that are susceptible to leaf rust (e.g., Jackpot, Greer) are now hit hard with leaf rust.

Weather since the middle of April has been reminiscent of the weather in April and May of 2007.  As a result, wheat diseases favored by cool and wet weather are starting to occur with increasing frequency and severity.  These diseases are causing head discoloration, which can be caused by fungi or bacteria.  Around Stillwater and Lahoma, head discoloration due to a bacterium (Xanthomonas) has been observed.  l have had reports of similar symptoms on wheat in southwestern OK, where more rain has fallen then around Stillwater.  This bacterial disease is called black chaff when on the heads and is called bacterial streak when symptoms are expressed on leaves.  The fungi Septoria and Stagonospora also can cause head discoloration, and we have isolated Septoria from several samples during the last week or so.  For a full discussion of the various causes of head discoloration along with pictures, clicking on the following link will take you to the e-Pest Alert sent out in June, 2007.

http://entoplp.okstate.edu/pddl/pddl/2007/PDIA6-17.pdf

Barley yellow dwarf also was observed this past week, but it was not extensive and little to no stunting was associated with the BYD spots indicating infection most likely occurred in the spring.

Purple / yellow leaves associated with barley yellow dwarf

I have had reports of Fusarium head blight (scab) from southeastern KS, from Arkansas, and from eastern/northeastern OK, but have had no reports from anywhere else in Oklahoma.

Fusarium head blight (head scab) can partially or completely infect wheat heads

Fusarium head blight (head scab)

Fusarium head blight infected (top) vs. normal wheat kernels

The diagnostic lab continued to receive multiple samples that tested positive for Wheat streak mosaic virus, with several also testing positive for High plains virus (Wheat mosaic virus) and Barley yellow dwarf virus.  For information on mite-transmitted diseases, I refer you to EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic:  Three Virus Diseases of Wheat in Oklahoma) also available at http://osufacts.okstate.edu.

Reports/excerpts of reports from other states: 

Colorado:  Dr. Scott Haley (Wheat Breeder); Colorado State Univ, 13-May-2015:  “Stripe rust continues to develop in CO. Cool wet weather will likely favor continued development. Wheat is about at the heading growth stage.”

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

Oklahoma:

In addition to looking at wheat around Stillwater this past week, I also was at field meetings near Kingfisher (30 miles northwest of OKC), Kildare (10 miles north of Ponca City), and Lahoma (10 miles west of Enid).  Wheat was at full berry to borderline milk.

Stripe rust, leaf rust, and powdery mildew were present at all locations, with stripe rust by far the most prevalent.  Where a fungicide had been sprayed (e.g., in Dr. Jeff Edwards variety trial), the effect was striking in terms of green leaf tissue.  Difference in variety susceptibility to stripe rust was obvious, with Ruby Lee, Garrision, Pete, and Everest being some of the more common highly susceptible varieties.  There also seems to be varieties with high levels of resistance (e.g., Gallagher, Jackpot, etc), and intermediate resistance; however, I’ll wait until I have all ratings in to evaluate this in more detail.

This overhead shot of the Chickasha intensive and standard wheat variety trials illustrates the severity of stripe rust in the region. The intensively managed trials on the left was treated with a fungicide just prior to heading. The standard trial on the right has the exact same varieties but no fungicide. The “middle” replication between the two studies is a border of Ruby Lee that is 1/2 treated 1/2 non treated. Photo courtesy Brian Arnall.

Leaf rust can be found in some varieties at severe levels, but has not increased to a level comparable to stripe rust.  Most commonly, I am seeing it on leaves of varieties that are resistant to stripe rust, but susceptible to leaf rust (e.g., Jackpot).

Barley yellow dwarf also was observed at all locations, but little to no stunting was associated with the BYD, so infection most likely occurred in the spring.

Powdery mildew also was observed at every location, but only rarely was on the flag leaf or heads.

I have not seen any Fusarium head blight, but have had a few reports of it from eastern/northeastern OK.

The diagnostic lab continued to receive samples testing positive for Wheat streak mosaic virus, with several also testing positive for High plains virus (Wheat mosaic virus)  For information on mite-transmitted diseases, I refer you to EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic:  Three Virus Diseases of Wheat in Oklahoma) also available at www.wheat.okstate.edu 

Reports/excerpts of reports from other states: 

Kansas:  Dr. Erick De Wolf (Extension Plant Pathologist); Kansas State Univ, 4-May-2015:  “The past week continued to bring more finds and reports of rust diseases in Kansas. Stripe rust is the primary concern many growers and the disease is now established in many areas of the state.  The disease has moved onto the upper leaves in many fields in the southeastern and south central regions of the state.  The wheat in these areas of the state was at or near the heading and flowering stages of growth this past week. Infections on the upper leaves at these stages of growth places the crop at risk for severe yield losses.  Stripe rust was found at low levels in many counties in the central and north central regions.  The disease was also reported at low levels in western Kansas.  At this point the stripe rust was primarily low to moderate incidence (1-5%) and mostly restricted to the lower leaves of many fields in the central and western regions.  However, the weather this past week was very conducive for stripe rust and the disease may soon increase to damaging levels in more areas.

The risk of yield loss in these areas depends heavily on weather over the next 2 weeks.  Stripe rust is favored by temperatures in the 40-60’s, frequent rain or heavy dew deposition. The progress of stripe rust often slows when nighttime temperatures exceed 60 degrees F.  The weather outlook for the central region indicates that rain is likely this week but also suggests low temperatures may slow further disease development. Temperatures in north central and western Kansas may be more conducive for stripe rust.  So far the stripe rust is most severe on varieties known to be susceptible to the disease but there are some early indications of unusual disease reactions on varieties previously thought to be resistant.  I will gather more information and come back with reports soon.

Leaf rust has also been detected at trace or low levels at many of the same locations as stripe rust (Crawford, Clay, Ness, Riley, Saline, and Sumner counties).  These reports are significant because the presence of leaf rust increases the risk of disease related yield loss in the state. Many of the popular varieties grown in the state are susceptible to leaf rust and finding the disease prior to flowering indicates the leaf rust may also cause problems in some areas.  To date the leaf rust appears to be most common on varieties with the Lr39/41 resistance gene (Fuller and WB 4458 for example).

TEXAS:  Dr. Clark Neely (Small Grains & Oilseed Extn Specialist; Texas A&M AgriLife; College Station; 7-May-2015:  “I spent the past several days attending wheat field days in northeast Texas. Stem rust was found in susceptible soft wheat varieties in Ellis County. After fading away for the most part from warmer temperatues, stripe rust was re-establishing itself on flag leaves of susceptible varieties in due to the cooler, wet weather affecting the region. Most wheat in the region is in the milk-soft dough stage. Leaf rust was moderate to severe on susceptible lines at Howe, TX including TV 8861, however, leaf rust was much lighter at Leonard and Farmersville, TX locations.”

Georgia:  Alfredo Martinez, John Youmans, James Buck; University of Georgia; 6-May-2015:  “Leaf rust infections have been observed in commercial wheat fields in Southwest Georgia (Seminole Co. Mitchel Co. Taylor Co.). The incidence of leaf rust seems to be localized and the severity was low. Stripe rust was confirmed in northwest GA (Floyd Co.). The incidence and severity was minor. Commercial fields surveyed near UGA College of Agricultural and Environmental Sciences Research Station in Plains, GA and around the UGA CAES Bledsoe Farm near Griffin GA yielded no stripe, leaf rust or stem rust infections.  Low rust and powdery mildew on wheat in GA was probably due to the use of resistant varieties and /or timely applied fungicides. However, for a second year in row, Fusarium Head Blight (FHB/Scab) incidences were numerous (albeit not as prevalent as in 2014), in some fields the severity was high. Surveyed fields in Sumter Co. had severity of 50%-60%. Environmental conditions at the time of wheat flowering provided conducive conditions for FHB infections especially in the southernmost part of the state. Stagonospora spot blotch and tan spot were observed throughout the state and seemed more prevalent than previous years.”

Wheat leaf rust

Wheat stripe rust

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

Oklahoma:  This past week I spent in fields/nurseries around Stillwater and also attended field days in central OK (Caddo Cnty) and southwestern OK (Jackson Cnty).  Most of the wheat I saw was at ¼ to nearly full berry.  Foliar diseases have definitely increased.

Around Stillwater, powdery mildew is evident on the lower leaves in my fungicide trial (approx. 25% severity).  In the variety demo strips at Stillwater, I can find severities >65% on lower leaves.  However, stripe rust is still the most evident foliar disease with mid-canopy leaves of susceptible varieties often completely or nearly completely infected.  Flag leaves of susceptible varieties around Stillwater are beginning to show stripe rust pustules, but that is still not uniform in all fields/locations.  Also this past week I started to see leaf rust pustules developing on lower leaves; I only occasionally saw pustules on flag leaves with the exception of ‘Jagalene’, which was at 90% or so.

In central OK I saw mostly the same thing.  At the variety trial near Kingfisher (Kingfisher Cnty), disease was surprisingly light with stripe rust the most evident.  Near Apache OK (southern Caddo Cnty), disease was more prevalent with stripe rust the most severe.  Varieties resistant to stripe rust but susceptible to leaf rust (e.g., ‘Jackpot’ were beginning to show more leaf rust pustules).  Tan spot also was evident at this variety trial as it is a no-till field.

This overhead shot of the Chickasha intensive and standard wheat variety trials illustrates the severity of stripe rust in the region. The intensively managed trials on the left was treated with a fungicide just prior to heading. The standard trial on the right has the exact same varieties but no fungicide. The “middle” replication between the two studies is a border of Ruby Lee that is 1/2 treated 1/2 non treated. Photo courtesy Brian Arnall.

In southwestern OK (near Altus, OK), stripe rust was severe (90% or so) on the flag leaves of susceptible varieties such as ‘Ruby Lee’, ‘Everest’, and ‘Garrison’.  Varieties with resistance to stripe rust such as ‘Gallagher’ and ‘Billings’ showed little sporulation but loss of some green tissue due to the hypersensitive reaction (HR).  ‘Greer’ showed no stripe rust and no dead tissue due to the HR.  Here again, a variety like Jackpot showed no stripe rust, but leaf rust was at a moderate level.  An interesting observation was made by Dillon Butchee (Helena Chemical Rep), who noticed stripe rust sporulating inside the glumes of susceptible varieties, which I have seen only occasionally in Oklahoma.

In more northern OK, Greg Highfill (Extension Educator, Woods Cnty – near Alva, OK) indicated he has seen small amounts of stripe rust in the border of the test plot near Alva.  Although temperature is increasing, the forecast for next week is highs only in the low to mid 80s with Tuesday-Thursday being rainy again.  These temps will be lower in northern and northwestern OK.  Hopefully the rain will continue, but these conditions will favor continued spread and increase of particularly leaf rust.  For most of Oklahoma, I believe wheat is past or quickly approaching the point where a fungicide can no longer be applied.  For a discussion of this, see “CR-7668 Foliar Fungicides and Wheat Production in Oklahoma – April, 2015,” which is available at www.wheat.okstate.edu.

Stripe rust at Alva, Oklahoma. Photo courtesy Greg Highfill

Finally, the diagnostic lab continues to receive samples testing positive for Wheat streak mosaic virus.  For information on mite-transmitted diseases, I refer you to EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic:  Three Virus Diseases of Wheat in Oklahoma) also available at www.wheat.okstate.edu

Reports/excerpts of reports from other states: 

Nebraska:  Dr. Stephen Wegulo (Extension Plant Pathologist); Univ of Nebraska, 28-Apr-2015:  “Yesterday I surveyed wheat fields in south central and southeast Nebraska.  Stripe rust was widespread (prevalence of >70%) in the southernmost tier of counties.  Incidence ranged from about 15% to > 80% in some fields.  Severity was mostly trace to low (< 10%), although a few isolated leaves had >50% severity (see attachment).  Growth stage was mostly Feekes 6; in a few fields it was Feekes 6 to 8, and in two irrigated fields wheat was in the boot stage.  These two irrigated fields apparently had been sprayed and there was no stripe rust in them, but I was able to find some leaves on which stripe rust development was stopped by the fungicide spray.  There was severe winter kill in some fields to the extent that the wheat was sprayed with herbicide and another crop will be planted.

Last week (Thursday and Friday) I surveyed wheat fields in the southern and northern Panhandle of Nebraska (the far northwest of the state) and in southwest and west central Nebraska.  I found no foliar diseases, but there was root rot in one field that also had some winter kill in the northern Panhandle.  In the west central part of the state, there was severe winter kill in some fields (see slide #3 in the attachment).  Growth stage ranged from Feekes 5 in the Panhandle to Feekes 5 to 6 in the southwest and west central parts of the state.”

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

Oklahoma:  Again, a cool and moist week in Oklahoma with rain and cool temperatures in the forecast through the weekend.  Wheat is mostly at flowering around Stillwater but by early next week will likely be past flowering.  From what I hear across the state, wheat is quickly approaching, at, or will be quickly past flowering (depending on what part of the state, when planted, and how much drought stress was endured).

Around Stillwater there has been an increase in stripe rust and powdery mildew (especially stripe rust).  Leaf rust also has increased, but not to the same extent (incidence or severity) as stripe rust.  Dr. Brett Carver (OSU Wheat Breeder) indicated he saw significant stripe rust in his nurseries at Lahoma (north-central OK).  Gary Strickland (Jackson County Extn Educator) also has observed an increase in stripe rust (and to a lesser extent, leaf rust) in far southwestern Oklahoma.  My impression is that stripe rust has activated again with the cool and wet weather, and continues to spread across Oklahoma.  This Thursday and Friday there will be field days in central Oklahoma, so look for a more extensive report next week.

To the south of us in Texas (far southern Texas), Dr. Amir Ibrahim (Professor, Small Grains Breeder/Geneticist; Texas A&M AgriLife Research; College Station, TX) has indicated in breeder nurseries near Castroville, TX, that there is a mixture of stripe rust, leaf rust, bacterial leaf streak and some septoria.  This has occurred because of back-and-forth switching between cool and warm temperatures mixed in with lots of rain.  To some extent, this also has occurred across Oklahoma during April.

Mite-transmitted viruses also are prevalent in Oklahoma this year.  Yesterday when in nurseries here at Stillwater I noticed occasional trapped heads scattered throughout the field.  In observing these in the lab using a dissecting scope, I found wheat curl mites associated with these trapped heads.  Although only sporadic around Stillwater, the Diagnostic Lab has received multiple samples that they confirmed for presence of Wheat streak mosaic virus, Wheat mosaic virus (High plains virus), and/or Barley yellow dwarf virus.  This includes samples from Grady, Noble, Grant, Texas and Woods Counties.  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 http://osufacts.okstate.edu for more information on these mite-transmitted viruses.

Reports/excerpts of reports from other states: 

Kansas:  Doug Shoup (SE Area Crops/Soil Specialist) & Erick DeWolf (Extn Plant Pathologist), Kansas State University, 22-Apr-2015:  “Stripe rust has been found in several fields of wheat in southeast , south central, and north central Kansas. Given the forecast of cool and relatively wet conditions for the next seven days, this is a potentially significant situation for fields anywhere in Kansas planted to varieties susceptible to stripe rust. Intensive scouting of fields should begin now.”  [This was the first paragraph of their report, which can be found at:

https://webapp.agron.ksu.edu/agr_social/eu_article.throck?article_id=532

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.

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.”