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Wheat disease update – 26 April 2014

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

Oklahoma:  Wheat around Stillwater is mostly at GS 10.2 to 10.4 (heads ¼ to ½ emerged from boot).  Conditions are still dry, not only around Stillwater, but also around much (if not all) of the state.  Some rain fell this past week in the 1 inch range in a few areas, but in southwestern OK it was most likely too late to help the wheat.  Freeze damage also is becoming much more apparent.

Dr. Jeff Edwards (OkSU Small Grains Exten Spec) and I looked at wheat and attended a field day yesterday evening near Apache, OK (about 75 miles southwest of Oklahoma City).  The variety trial and field at Apache was lost because of freeze.  Wheat at Dr. Edward’s trial at Chickasha (30 miles northeast of Apache) also had some freeze damage but not as severe as the wheat around Apache.  No foliar diseases were seen at any field at which we stopped, and no reports of foliar diseases have come to me since my last update (10-Apr).  As you can tell from the reports below from Texas, there just is not much inoculum south of us to be carried northward, and what does blow up is likely not finding an environment conducive to infecting.

Our diagnostic lab has received few samples.  Of two recent samples from Garfield County (north-central OK), one was positive for Wheat streak mosaic virus (WSMV) and the other was positive for WSMV and High plains virus.  The diseases caused by these viruses, which are transmitted by the wheat curl mite, were fairly widespread in Oklahoma in 2013 and probably will be again in 2014.  However, I suspect that the drought and freeze will mask these infections.

Reports/excerpts of reports from other states:

Texas  Dr. Ron French (Ast Prof, Extn Plant Pathologist, Texas A&M AgriLife Extn Service) 25-Apr-2014:  Was driving all day and night (600 miles) yesterday looking at crops from Castroville to the Lower Rio Grande Valley and back. Today heading back to Amarillo.  Last Tuesday April 15 I drove from Amarillo to Wichita Falls (Northern Central part of the state right along the Oklahoma border) and back to  Amarillo (220 miles NW of Wichita Falls). Looked at commercial wheat and some trial plots in Chillicothe.

Did not see any rust at all and brought samples back just to make sure.  In the Wichita Falls area, wheat was at Feekes 10.4 (3/4 fully headed) on average. So now they should be at least flowering. In the Chillicothe area (65 miles NW of Wichita Falls towards Amarillo) wheat was on average at Feekes 10.3 (heading half complete).  Have not heard of any rust in this area (Rolling Plains and Panhandle) this week.

I have not received any feedback of stripe rust moving north of Ellis County (from my April 10 report), although now it is also in Hill County, adjacent to Ellis, but in a SW direction. In Hill County, wheat with stripe rust was past pollination and into grain fill.  Even in Bell county (Temple area, 120 miles south/southeast of Dallas) there was trace levels of stripe rust and none in the flag leaf.  Except for the Texas Panhandle, all other areas will soon (early next week for the Vernon area) be out of the window for spraying any fungicides.

So the good news is that rust is not moving north, yet. With warm temperatures during the day but cool at night, this might be affecting continuous fungal growth for both stripe and leaf rust fungi.

Texas (from a report issued by Dr. Erick DeWolf at KSU 25-Apr-2014:  Texas has reported some stripe rust activity just south of Dallas but warm temperatures have slowed the progress of that disease. Bob Bowden, USDA Plant Pathologist, reports that leaf rust remains active in research plots near San Antonio, Texas. However, the disease remains at low levels in commercial fields according to Tom Isakeit, Extension Plant Pathologist for Texas A&M. Wheat fields in southern Texas are nearly ready for harvest.

Kansas  Dr. Erick DeWolf, Extension Wheat Pathologist, Kansas State University 24-Apr-2014:  The risk of severe leaf diseases remains low throughout Kansas. My own scouting and reports from K-State agronomists indicate that leaf rust and stripe rust are not present in the state. Tan spot, septoria leaf blotch, and powdery mildew were absent in most fields; however, we did find a small number of fields with low levels of tan spot in Saline, McPherson, and Sedgwick counties. These fields all had wheat residue from previous crops on the soil surface. This residue is important because it often harbors the fungus that causes tan spot.

Drought stress was evident in most fields and the dry conditions are holding disease in check for now. Recent rains have brought some temporary relief to the dry conditions in a few areas of the state. We will continue monitoring the disease situation as this moisture may stimulate some disease. The symptoms of any new infections would not become evident for 7-10 days.  The current risk of severe disease in Kansas and the need for foliar fungicides is low.

Freeze injury update 22 April 2014

Injury symptoms from the April 14th freeze are now showing in the Oklahoma wheat crop. Robert Calhoun and Matt Knori madeTh a trip through north central Oklahoma yesterday splitting stems (some pictures are posted below). Their first stop was our wheat variety trials at Marshall Oklahoma where they found 20% injury in our grazed wheat plots and 51% injury in our non-grazed wheat plots. While planting date and management system clearly affected the level of injury, variety did not seem to have much effect.

Next stop was a grazed field north of Hennessy where they found little injury. The same was true for a field in the Waukomis area and the Lahoma variety trial where they found less than 5% injury. Not too far to the north, however, our Lamont variety trial sustained over 80% injury. I received similar reports of severe wheat freeze injury from Curtis Vap in the Blackwell area.

Late last week our team traveled to Apache to apply fungicides to the wheat variety trial, but never unloaded the sprayer. Freeze injury was severe and clearly visible without splitting stems. Our wheat at the Chickasha research station had little to no damage, and most wheat in the area seemed to dodge the freeze bullet. I will make a bigger loop into southwest Oklahoma later this week and report findings.

Injury symptoms should now be easily identifiable and growers can assess damage to individual fields. I recommend splitting 10 stems at four or five locations throughout the field and determining % injury from these numbers. If injury is extremely variable, increase sample size. While it is fairly easy to determine the extent of injury on individual fields, the hit or miss nature of freeze injury this year makes it difficult to estimate the total impact on the Oklahoma wheat crop as a whole.

The drought has severely limited resilience in our crop and we are entering late April, so I do not anticipate there will be much of a recovery or rebound in fields that were severely damaged. It is important to note that 50% injury does not necessarily mean 50% yield loss. In most cases the actual yield loss will be less than the % injury. So, it is reasonable to expect that 50% injury might only result in a 35 or 40% yield loss. Of course, this depends on several factors such as soil moisture and temperature.

Finally, a word on foliar disease and fungicide application. I would make decisions regarding fungicide application based on variety, current disease reports, and the yield potential of the crop as it stands right now. Our long-term data shows that fungicides protect yield potential to the tune of about 10%. Of course individual variety responses can deviate from this number but 10% is a good rule of thumb. I do not, however, recommend applying a fungicide to “assist the crop in recovery from freeze”. Again, make these decisions based on the remaining yield potential rather than an effort to attempt to nurse the crop back to health after freeze.

 

Freeze injured wheat from Marshall, OK

Freeze injured wheat from Marshall, OK

As evidenced by this picture from Marshall, Oklahoma, freeze injured wheat can still have a green appearance. You must split stems to accurately assess injury.

As evidenced by this picture from Marshall, Oklahoma, freeze injured wheat can still have a green appearance. You must split stems to accurately assess injury.

Freeze injured wheat at Lamont, Oklahoma. Freeze might have finished this plot, but drought had it down for the count prior to the freeze.

Freeze injured wheat at Lamont, Oklahoma. Freeze might have finished this plot, but drought had it with a standing eight count prior to the freeze.

Freeze injury in Kay County Oklahoma. Photo courtesy Curtis Vap.

Freeze injury in Kay County Oklahoma. Photo courtesy Curtis Vap.

 

 

Freeze injury update 15 April 2014

I have posted a few images from the Oklahoma Mesonet below. Most of Oklahoma spent at least four hours below freezing last night and some areas spent an extended period of time below 28F. While temperatures in the wheat canopy might have remained slightly higher than reported air temperatures, they were still probably low enough to result in significant injury to wheat.

Over the next few days growers will need to inspect fields closely to determine the extent of injury. Symptoms may start to appear later this week and will likely be clearly identifiable by early next week. Healthy wheat heads will remain turgid with a green color. Damaged wheat heads will be bleached, yellow, or brown and will easily break when pushed against. I anticipate that we will not have any partial “blanking” of wheat heads and that most wheat heads will either be okay or a complete loss.

What about new tillers? New tillers might emerge, but it is already April 15. In addition we have very dry soil conditions. For these reasons I am doubtful that newly emerging tillers will have much yield potential in areas south of I-40. IF (and that is a big if) weather conditions remain favorable, late emerging tillers in northern Oklahoma might still have a shot at producing grain.

I will survey some fields in a few days and report back with my findings. If you are interested in receiving weather maps and updates such as the ones posted below, subscribe to the OCS Mesonet Ticker by emailing ticker@mesonet.org

Hours below freezing on April 15, 2014

Hours below freezing on April 15, 2014

Hours below 28 F on April 15, 2014

Hours below 28 F on April 15, 2014

Hours below 24F on April 15, 2014

Hours below 24F on April 15, 2014

 

Potential for freeze injury

Temperatures are predicted to drop well below freezing tonight (14 April 2014), and there is high potential for freeze injury to Oklahoma wheat. I have posted an excerpt from K-State Extension Publication C-646 Spring Freeze Injury to Kansas Wheat along with a map which provides some rule of thumb temperature thresholds for the current Oklahoma wheat crop. Keep in mind these temperature thresholds are not exact, and temperatures closer to the soil surface might be higher than those reported by weather stations one meter above the soil surface, especially if moisture is present. Wheat in Oklahoma ranges from just past jointing to late boot and if forecasts are correct we will drop below the threshold temperatures where injury might be observed. The extent of injury will depend on how cold we get and how long we stay there. We can lose a few main tillers at this stage and still recover. Given our limited moisture and limited time prior to harvest, though, it is not likely that we will recover from a complete loss of tillers as we have after some March freezes in the past.

Excerpt from KSTATE publication C-646 Spring Freeze Injury to Kansas Wheat

Excerpt from KSTATE publication C-646 Spring Freeze Injury to Kansas Wheat

 

Approximate temperature thresholds for freeze injury to Oklahoma wheat on 04/14/2014

Approximate temperature thresholds for freeze injury to Oklahoma wheat on 04/14/2014

Freeze injury is not clearly identifiable until 7 – 10 days after the freeze event. So, the best advice for a wheat farmer after a freeze event is to find something else to do for a week or two and then check your crop. I have provided some pictures below with typical injury symptoms and rules of thumb regarding the extent of the injury. Fields should be checked at several random locations by splitting 10 – 20 stems at each location and looking for injury. Don’t focus solely on the large stems. Split a random sampling and determine the percent damage. A good reference for evaluating freeze injury to wheat is K-State Extension Publication C-646 Spring Freeze Injury to Kansas Wheat (access online by clicking here).

This is a healthy wheat head at approximately growth stage 6 - 7. Note the light green color and healthy, turgid appearance.

This is a healthy wheat head at approximately growth stage 6 – 7. Note the light green color and healthy, turgid appearance.

Freeze injury just after jointing. Note the pale, milky color of the head.

Freeze injury just after jointing. Note the pale, milky color of the head. Freeze injury to wheat heads at this growth stage is all or none, so this head is a complete loss.

Leaf tip burn from freeze injury will have no impact on final grain yield

Leaf tip burn from freeze injury will have no impact on final grain yield

Yellowing is a common reaction to light freeze injury. Wheat will recover quickly from this injury.

Yellowing is a common reaction to light freeze injury. Wheat will recover quickly from this injury.

Severe freeze injury at or just after jointing can turn the entire plant brown and fields can exude an odor similar to fermenting silage. If conditions are favorable, the plant can produce new tillers (as shown here) and make a partial recovery. It will take a few weeks after a freeze event to determine if the plant will recover from this type of injury

Severe freeze injury at or just after jointing can turn the entire plant brown and fields can exude an odor similar to fermenting silage. If conditions are favorable, the plant can produce new tillers (as shown here) and make a partial recovery. It will take a few weeks after a freeze event to determine if the plant will recover from this type of injury

It is common for sub-lethal freeze injury to result in bent or weak lower nodes. These plants might look fine, but will lodge during grain fill.

It is common for sub-lethal freeze injury to result in bent or weak lower nodes. These plants might look fine, but will lodge during grain fill.

Wheat disease update – 10 April 2014

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

Oklahoma:  Wheat around Stillwater is mostly at GS 7 (2 nodes detectable).  In a few places the flag leaf has emerged but only in one field.  I still have not received reports of significant foliar diseases in Oklahoma, which is not surprising given the dry conditions.  Around Stillwater I have seen some powdery mildew on low foliage in scattered spots.  I and my technician Brian Olson also found tan spot in a no-till field, but only on low foliage and not severe.  The one find of a different disease that is concerning was wheat streak mosaic (WSM) in Dr. Jeff Edward’s variety trial in Kay County near Kildare.  Visiting the trial on April 2nd, yellowing and streaking were present in all varieties but some were much worse than others.  I was not thinking about WSM at the time, but 5 samples I brought back to the lab all tested positive for the virus that causes WSM, so I believe that is what is present.  On a trip today to northern Oklahoma and over to Lahoma in north central Oklahoma, I saw some good and some bad wheat that ranged from GS 6 to GS 7 (or close to it).  However, all the wheat seemed short to me – some not much more than 10-12 inches tall.

Powdery mildew

Powdery mildew

In northwestern Oklahoma, Rick Kochenower (Area Res & Extn Agron Spclt) indicated, “I see a lot of dryland wheat dying but not from disease.”  He said that wheat was just starting to tiller.  In southern/central/southwestern Oklahoma, Mark Gregory (Area Extn Agron Spclt) reported that today he was in wheat towards the eastern side of the district and saw no diseases; also that the wheat furthest along had flag leaves fully extended (GS 9).  Gary Strickland (Extn Educator, Jackson County – southwestern OK) indicated wheat in his area was in the flag leaf stage – anywhere from flag leaf just emerging to fully-emerged.  Drought is the problem; no rust, powdery mildew or other diseases, but brown wheat mites have exploded.

Reports/excerpts of reports from other states:

Texas (southern) Dr. Amir Ibrahim (Prof, Small Grains Breeding & Genetics, Texas A&M Univ) 09-Apr-2014:

Wheat rust conditions at Castroville, TX – Wheat looks very good here and most of it is at the ripening stage. We have been applying a ½ of irrigation per week for the past two weeks. We may put another ½ next week, depending on the conditions. The morning fog at this location is also keeping the rust going.

Wheat stripe rust (P. striiformis)- This week provided an opportunity for taking notes on infection type and severity of wheat stripe rust (P. striiformis). Evidence points to a milder repeat of 2012 virulence that attacked ‘TAM 111’, ‘Garrison’ and other HRW. There is no indication of Yr17 vulnerability, which seemed as a one‐time incident that took place in 2010. Stripe rust is up to 70S on susceptible ‘Patton’.  We don’t expect infection types to be distinguishable next week but ratings for green leaf area duration can be taken. A lot of guessing has to be made, though, as both P. striiformis and P. triticina have been competing for the upper leaf surfaces. The former has started to dry up and newer infections are not likely. This does not overrule establishment farther north of here where conditions are conducive.  ‘Armour’ received a 40 S rating at this site, but ‘Redhawk’ looks very vulnerable with a rating 50S on the flag leaf and homogenous spread across the plot.

Wheat leaf rust (P. triticina): While stripe rust infections are fading, leaf rust (P. triticina) is now moving into the flag leaves of susceptible wheat. I have rated TAM 112 as 15S (FL) today. We have good diversity of races, but the Lr24 virulence is moving faster as evidenced by the rating of 60S on the flag leaves of ‘Jagalene’ as opposed to Lr17Lr39, and Lr41. We plan to come back here on April 21st to take notes on leaf rust, tag plants, and make selections.

Texas  Dr. Ron French (Ast Prof, Extn Plant Pathologist, Texas A&M AgriLife Extn Service) 10-Apr-2014:  I have spent many of the last 14 days traveling the Northern Central part of the state, from Wichita Falls (right along the Oklahoma border) and towards Amarillo (220 miles NW of Wichita Falls), and From Amarillo down to the Plainview area (70 miles south) and the Lubbock area (120 miles south of Amarillo). I have not seen any rust at all in commercial fields. Visual observation but also sampled just in case rusts were latent and not quite rupturing the epidermal tissue. No reports of there being any rusts from consultants as well.

From what I have gathered from consultants, there is/has been stripe rust as far north as Waxahachie, TX (Ellis County), 30 miles south of Dallas. Other locations with stripe rust are around Elgin, TX (Bastrop and Travis Counties, approximately 25 miles East of Austin, TX) and Taylor, TX (Williamson Co, approximately 25 miles south of Austin and approximately 75 miles West of College Station).

As far as leaf rust is concerned, these and other locations in that area have also had leaf rust, and in varieties such as Greer, Cedar, and Coronado.  Low levels but enough to warrant sprays as much of that crop is fully headed. In some locations, 7-10 before getting into the milk stage for Cedar.  I have also been told that Fannin had low levels of rust, something out of the usual.  I need to confirm which rust it is (leaf or stripe).

We are a bit behind other years where we have seen leaf rust by late March/early April as far north as Wichita Falls (a few miles from the Oklahoma border) and early April for stripe rust, North and Northeast of Dallas and close to the Oklahoma border.

With temperatures expected to be in the high 80s and low 90s Fahrenheit in the Texas High Plains sometime this weekend but lower next week but with little to no rain, disease pressure is still not there (humidity-wise) although common heavy winds could bring inoculum our way.

Texas (northern – blacklands area north of Dallas)  Jim Swart (IPM, Texas A&M AgriLife Extension 31-Mar-2014:  Wheat is rapidly recovering from the hard freeze we received in early March.  Winterkill does not appear to be an issue in any of the commercial varieties planted across the region, but some early maturing varieties in our research plots were damaged by the freeze.  Wheat planted in October is jointed (Feekes 6), and some plants are in Feekes 7 (two nodes above the ground).  Wheat planted in November is just beginning to joint.  We have identified a few bird cherry oat aphid infestations in central and southern Hunt County, but most of the area wheat crop has very low numbers of aphids (greenbug and bird cherry oat aphids).  We have not observed any stripe or leaf rust yet.

Louisiana Trey Price (Field Crops Pathologist LSU Ag Ctr, Winnsboro, LS) 31-Mar-2014:  There’s really not much to report at the moment in Louisiana.  Steve spotted leaf rust at Ben Hur Research Station in Baton Rouge a couple of weeks ago, and incidence and severity were low.  I’m unsure of the current status.  We have stripe rust developing in GACT7, a susceptible variety, at the Dean Lee Research Station in Alexandria.  Incidence and severity were less than 1% last week.  In Winnsboro, there are no rust issues on wheat or oats at the moment.  Low incidence and severity of leaf and stripe rust at Crowley (Rice Station, Southwest Louisiana).

Georgia  Dan Bland (Crop & Soil Dept, Univ of Georgia-Griffin) 3-Apr-2014:  We were down visiting our nursery in Plains, GA yesterday and found a lot of leaf rust in a strip of wheat planted about 2 months earlier than the recommended date for the purpose of spreading hessian fly to a small replicated test.  This is done every year so that David Buntin can get pupa and larva counts on the state variety test.  We normally see leaf rust in this area especially if the strip planted is susceptible.  This year Jerry Johnson says it’s the most he’s ever seen for this time of year.  We also saw leaf rust on the lower leaves of the most susceptible lines in another test about 300 yards away.  Leaf rust also has been reported in southeast Georgia.  Plains is in southwest Georgia.

Oregon  Dr. Michael Flowers (Extension Cereal Specialist, Oregon State University, Corvallis, OR) 3-Apr-2014:  After I sent out the note yesterday, reports have been coming in from both the north and south Willamette valley about stripe rust.  Currently there are reports of stripe rust found on Goetze, Kaseberg, Sy Ovation and Tubbs 06.  It appears that maybe we were not as lucky as hoped and that stripe rust did overwinter in some fields throughout (north and south) Willamette Valley

Wheat streak mosaic virus showing up

Add wheat streak mosaic virus to the list of possible causes of yellowing wheat in Oklahoma. Wheat streak mosaic virus is transmitted by the wheat curl mite, which oversummers on grasses such as volunteer wheat and corn. The wheat curl mite cannot survive more than two weeks without a green host, hence the recommendation to make sure that all grass plants are dead two weeks prior to planting. You can find more information on the wheat curl mite and wheat streak mosaic in OSU Fact Sheet EPP-7328 – Three virus disease of wheat in Oklahoma

The photo below is from our wheat variety trial at Kildare. As you can see there is significant yellowing in some of the plots. Our first thoughts were that either wheat soilborne mosaic virus and/or wheat spindle streak mosaic virus were causing the symptoms; however, the yellowing was present in many varieties that are resistant to both these diseases. The Disease Doctor, Bob Hunger, collected samples for analysis in the OSU Plant Diagnostic Lab. Tests showed that wheat streak mosaic was the culprit.

Wheat streak mosaic virus is responsible for yellowing at our Kildare variety trial. All varieties are affected by the disease, but as shown in this picture the severity of the reaction differs somewhat by variety.

Wheat streak mosaic virus is responsible for yellowing at our Kildare variety trial. All varieties are affected by the disease, but as shown in this picture the severity of the reaction differs somewhat by variety.

We are facing this problem because I did not follow my own recommendations. While the plot area was mostly clean at the time of planting, there was some volunteer wheat present. We planted anyway and sprayed glyphosate right after planting. In the past wheat streak mosaic virus was primarily a northwestern Oklahoma issue and we could get by with late burndown on wheat ground in central Oklahoma. Our Kildare plots are a prime example that this is no longer the case. We have to control volunteer grasses (wheat, corn, grain sorghum, etc.) in a manner that will break the green bridge for at least two weeks prior to planting.

So, what is next for our plots at Kildare? There are some variety differences in reaction to the wheat streak mosaic virus. We will rate plots and include this information in our wheat variety comparison chart. I anticipate the plots will continue to go downhill and it is yet to be determined whether or not we will be able to harvest useable data from the location. We will certainly try again next year and apply our turndown earlier.

 

Pictures from southwest Oklahoma

I wanted to share a few pics from Rocky Thacker, Station Superintendent at the OSU Southwest Research and Extension Center near Altus, OK. These pictures were taken on April 8 and clearly show the extent of the current drought’s damage to the southwest Oklahoma wheat crop.

Altus Area Wheat-April 2014 001 Altus Area Wheat-April 2014 002 Altus Area Wheat-April 2014 004 Altus Area Wheat-April 2014 003