This article was written by Dr. Bob Hunger, Extension Wheat Pathologist

Department of Entomology & Plant Pathology

Oklahoma State University – 127 Noble Research Center

405-744-9958

Wheat around Stillwater varies but appears to be in various stages of grain formation. Based on talking to a number of producers across northern Oklahoma, wheat heads have fully emerged but has not yet flowered or just beginning to flower. Of course, as you go further west into the panhandle the wheat is not as far along.  I’m not sure as sure about central and southern/southwestern OK, but I talked to one grower yesterday from southwest OK that indicted his wheat varied from starting to flower to kernels being formed. This same grower indicated leaf rust is the most common disease he has seen, and in some fields it is fairly heavy. Across northern OK, producers indicated they are not finding severe levels of foliar disease (primarily rust), but it is there (again, especially leaf rust). Brian Olson (my A&P) spent all of Thursday rating wheat at Lahoma (15 miles west of Enid in northern OK). He indicated there were some hot spots/areas of heavy leaf rust, but that over all, leaf rust was light in the plots he was rating. He also indicated he saw quite a bit of what he assumed to be physiological leaf spot (Figure 1), which we tend to see more of in years with cloudy, cool and rainy weather such as this year. Around Stillwater, I observed both stripe rust and leaf rust, but more leaf than stripe. In some cases, there was a mixture of both rusts on the same leaf (Figure 2).

Figure 1. Physiological leaf spot observed on wheat in Oklahoma.

Figure 2.  Wheat leaf showing co-infection of leaf rust and stripe rust at Stillwater.

Samples testing positive for Wheat streak mosaic virus (WSMV) have continued to come into the Diagnostic Lab. To date, about 45 samples have been tested for WSMV, High plains virus(HPV), and Barley yellow dwarf virus (BYDV). Thirty-two of the 45 were positive for WSMV, 24 were positive for BYDV, and 2 were positive for HPV (co-infection with WSMV). Eleven of the samples were positive for WSMV and BYDV. Most of these samples were received from central and west-central Oklahoma, but it seems the area is expanding. For more information on mite-transmitted wheat viruses such as WSM, please see OSU Fact Sheet EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic: Three Virus Diseases of Wheat in Oklahoma) available at http://wheat.okstate.edu/wheat-management/insectsdisease/EPP-7328.

Reports/excerpts of reports from other states:

Dr. Stephen Wegulo, Professor/Extension Plant Pathologist, University of Nebraska, April 18, 2017:  “Bob Harveson, Extension Plant Pathologist at the University of Nebraska-Lincoln’s Panhandle Research and Extension Center located in Scottsbluff, Nebraska has informed me that he has confirmed stripe rust on wheat plant samples brought today to the diagnostic clinic at the center. The samples are from Sheridan County in the northern Panhandle. Due to the location, we think the stripe rust overwintered. We had widespread stripe rust on fall-planted wheat in Nebraska last fall, and it was most severe in the Panhandle. Currently wheat in Nebraska is mostly at the jointing growth stage. Growth stage in fields I surveyed on Thursday April 13 and Friday April 14 in the southern Panhandle and the southwestern and south central parts of the state ranged from Feekes 6 to Feekes 7. Some fields in the southeast are at Feekes 8.  Almost all wheat fields I looked at during the survey last week looked very green with little or no disease – very low levels of fungal leaf spots in the lower canopy in some fields. The exception was an area in Garden County in the southern Panhandle with fields that are hard-hit with wheat streak mosaic. Gary Hein, Entomologist here at UNL, has told me that this area had pre-harvest hail last year which resulted in volunteer wheat that was not controlled.

Dr. Amir Ibrahim, Professor/Small Grains Breeder & Geneticist, Texas A&M AgriLife Research, April 19, 2017:  “I detected wheat stem rust at Castroville, TX on April 8, 2017. Samples sent to Dr. Yue Jin and genotyped by Dr. Les Szabo shows this to be conventional stem rust. Presence of rusted tissue across the plant, especially near the base shows a high probability of overwintering foci, which is uncommon in this region, and might have been triggered by the very mild winter. I visited the site again on April 17, 2017 and detected infections in a 40 X 60 foot area with the original overwintering focus in the middle. I have also found stem rust on susceptible ‘Morocco’ wheat in sentinel plots that are far from the aforementioned foci.”

Pustules of wheat stem rust in southern Texas (courtesy of Dr. Ibrahim)

Dr. Erick DeWolf, Extension Plant Pathologist, Kansas State University, April 21, 2017:  “This week has brought more reports of stripe rust in Kansas. Stripe rust can be found in the lower and middle canopy of many fields in central Kansas, but the severity remains low. Stripe rust is more severe in the southeast region of the state and has moved to the upper leaves in some fields. The weather conditions the past 14 days have not favored the rapid spread of stripe rust. Stripe rust is favored by cool, wet weather and temperatures in recent weeks were too warm for the stripe rust fungus to function efficiently. For example, most areas of the state had more than 30 hours of temperatures above 75 F in the last two weeks. Some areas of southwest and south central Kansas had more than 50 hours of unfavorable temperatures. The threat of stripe rust has not passed, however. We know stripe rust is present at low levels in many fields in the state. The disease could increase rapidly if we get into another period of favorable weather with frequent rainfall and temperatures in the 40-50F range at night. I still think there is a moderate risk of Kansas having a serious problem with stripe rust this season.

Leaf rust was reported previously in south central and southeastern Kansas. This week brought a few new reports of leaf rust and indications that leaf rust has moved to the upper leaves in few areas. This movement of rust to the upper leaves is important because these leaves provide most of the resources the plants will use produce grain. Any damage done to the upper leaves increases the risk of yield loss.

Powdery mildew is becoming severe in fields planted to moderately susceptible and susceptible varieties. 1863, Gallagher, KanMark, LCS Pistol, SY Flint, WB4458, WB-Grainfield, and WB-Redhawk are vulnerable to powdery mildew. In some fields, the powdery mildew has moved to the leaf just below the flag leaf prior to heading. This early establishment of the disease is cause for concern and growers should consider both rust and powdery mildew into their fungicide decisions. Fields with multiple diseases in the middle canopy and those where disease has moved to the upper leaves prior to heading have a more than 80% chance of experiencing a yield loss of >4.0 bu/a.”

This article was written by Dr. Bob Hunger, Extension Wheat Pathologist

Department of Entomology & Plant Pathology

Oklahoma State University – 127 Noble Research Center

405-744-9958

As expected, over the last week the disease situation has change significantly.  Wheat around Stillwater is mostly nearing complete heading to starting of flowering (Feekes’ GS 10.1 to 10.5.1).  In Jackson County (far southwestern Oklahoma), Gary Strickland (Extn Educator; Jackson County) indicated he has seen wheat as far along as ¼ berry, but most of the wheat is in flowering.  Across southwestern Oklahoma, Heath Sanders (Area Extn Agron Speclst, SW District) indicated wheat he has seen mostly is in the flowering stage.  Wheat at the Lahoma Station (15 miles west of Enid) was mostly between heads emerging to 3/4ths emerged, and according to Greg Highfill (Extn Educator; Woods County) wheat in the Alva area (northwest OK) was mostly just emerging from the boot as of last Tuesday (11-Apr).

Around Stillwater, powdery mildew (PM) is the primary foliar disease, but it is staying low in the canopy.  A sparse scattering of stripe rust can be found, but it is extremely sparse.  Leaf rust also is sparse, but is at a higher level than stripe rust.  Barley yellow dwarf (BYD) symptoms are common, but it seems to me there is more yellowing than can be totally attributed to BYD (but I’m not sure of the cause).

Gary Strickland indicated that leaf rust is predominate in southwestern OK, but that stripe rust also can be found.  He also is seeing widespread BYD symptoms and higher levels of tan spot (in no-till fields) up higher on the canopy than he has observed in previous years.  Southwestern OK has received more moisture over a longer period of time than the rest of Oklahoma, which explains the higher incidence of foliar diseases.  Gary did not see many aphids over the fall and winter, and so like me, he is surprised at the amount of BYD symptoms.  Heath Sanders indicated he has seen a lot of powdery mildew but again, so far restricted to the lower canopy.  He does feel that the PM has resulted in thinner wheat in some fields due to secondary tiller death/sloughing, which PM can do.  He also indicated as did Dr. Brett Carver (OSU Wheat Breeder) that wheat at the Chickasha Station (about 50 miles southwest of OKC – central OK) is showing severe levels of leaf rust and (especially) stripe rust (Figure 1).  In more northwestern/north-central OK, only light levels of stripe and leaf rust were observed at the Lahoma Station, and Greg Highfill indicated that the variety trial near Alva, OK was “clean.”  He did indicated that west of Alva he had received reports of light levels of stripe rust.

The other disease that is definitely making a presence this year is wheat streak mosaic (WSM).  So far the lab has received 16 samples that have tested positive for Wheat streak mosaic virus (Figure 2).  These samples have primarily come from central and west-central Oklahoma, and typically have been associated with lack of controlling volunteer wheat in the field itself or in an adjacent field. Controlling volunteer wheat prior to the emergence of seedling wheat in the fall is critical to limiting WSM.  Once infection occurs, especially if it is a fall infection, wheat likely will be damaged in the spring (Table 1).  For more information on mite-transmitted wheat viruses such as WSM, please see OSU Fact Sheet EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic: Three Virus Diseases of Wheat in Oklahoma) available at http://wheat.okstate.edu/wheat-management/insectsdisease/EPP-7328.

Figure 1. Stripe rust and leaf rust on wheat in experimental trials at the Experiment Station at Chickasha, OK (photo courtesy of Dr. Brett Carver).

Figure 2.  A commercial field of wheat showing severe WSM symptoms (A), a close-up view of WSM on an individual leaf (B), and a photo of the field immediately adjacent to the commercial wheat field in which abundant volunteer wheat had not been controlled until spring (C).

Table 1.  Effect of time of infection by Wheat streak mosaic virus on the severity of wheat streak mosaic (WSM) and yield of three wheat varieties.  [Taken from Hunger, et al. 1992. Effect of planting date and inoculation date on severity of wheat streak mosaic in hard red winter wheat cultivars. Plant Disease 76:1056-1060.]

Reports/excerpts of reports from other states:

Dr. Erick DeWolf, Extension Plant Pathologist, Kansas State University, April 4, 2017:  “The Kansas wheat crop is currently at the boot and heading stages of development in the Southeast corner of the state. The Central and Western regions of the state are between jointing and flag leaf emergence.

Stripe rust was found in more fields this week with multiple reports in Southeast and South Central regions of the state. Most reports have come from the southern tier of counties bordering Oklahoma (see that attached map).  The disease was present in a lower and middle canopy; however, a few fields had infections of stripe rust on the leaf just below the flag leaf (F-1).  The disease was found primarily on the cultivar “Everest” which is known to be susceptible to stripe rust in previous years.  Weather conditions the past two weeks was very conducive for further spread of stripe rust with cool temperatures and frequent rainfall.  Therefore, additional finds of the disease are very likely.  The severity of the disease and risk of yield loss will be influenced by weather conditions over the next month.  Continued cool, wet weather will increase the risk of a severe problem with stripe rust in Kansas.  However, multiple days with high temperatures above 80 F, particularly with night time temperatures above 60 F, could slow the development of the disease.

Leaf rust was also reported this week south central Kansas.  The disease was at low levels but it is significant to leaf rust prior to heading in this region of the state.  Many popular varieties are susceptible to leaf rust including WB4458, T158, TAM111, TAM112, and Winterhawk. Growers should not overlook the potential threat of leaf rust in their fields.”

This article was written by Dr. Bob Hunger, Extension Wheat Pathologist

Department of Entomology & Plant Pathology

Oklahoma State University – 127 Noble Research Center

405-744-9958

Yesterday (04-April) wheat in my foliar fungicide trial at the Plant Pathology Farm located just west of Stillwater appeared to be at GS 10 to 10.1 (head in the boot to awns just emerging from the boot).  The field was too wet to go out into, which seems to be the situation around much of the state (with more rain coming today and tonight).  Leaf rust can be found, but I have not seen any stripe rust.  Powdery mildew is heavy on lower leaves and spreading to mid-level leaves in wheat that has a heavy canopy.  Dr. Misha Manuchehri (Ast Prof & Weed Scientist, OSU) reported finding some heavier leaf rust in her experimental plots at Perkins, OK (about 15 miles south of Stillwater), but no or little leaf rust in surrounding trials/fields (Figure 1A).  Heath Sanders (Area Extn Agron Speclst, SW District) indicated to me on 04-Apr that wheat varied from boot to heads emerging across SW Oklahoma depending on planting date and management (primarily grazed or not grazed).  He has seen some leaf rust on lower leaves, but what has caught his attention more is the level of powdery mildew on lower and mid-level leaves in some wheat fields (early planted and not grazed).  This powdery mildew observation fits with what I have seen and with a photo received by Josh Bushong (Area Extn Agron Speclst, NW District) (Figure 1B).  Dr. Stephen Marek and his student Salome Suarez took a trip last Thursday to SW OK on a tan spot scouting trip and found plenty of tan spot in no-till fields.  He also found light leaf rust in some fields (Figure 1C), but again, no stripe rust.  Aaron Henson (Extn Educator, Tillman County, Southcentral OK) indicated quite a bit of the wheat in his area is headed and that he had seen some light leaf rust before the recent rains, but no stripe rust.

Figure 1. (A) Leaf rust pustules found on wheat in experimental trials near Stillwater, OK; (B) Severe powdery mildew as observed on lower and mid-level leaves in wheat in Custer County; (C) Scattered leaf rust pustules on a wheat leaf in southwestern Oklahoma.

The take-home message from these reports is that with the recent and forecast rain, foliar diseases (especially leaf rust and powdery mildew) likely will be increasing over the next couple of weeks.  Be prepared to apply a fungicide if necessary to protect a high yielding field, especially if that field is not highly resistant to leaf rust (and perhaps at least moderately resistant to powdery mildew).  More information about foliar fungicide use in Oklahoma wheat production can be found in the current report CR-7668 and is available at http://wheat.okstate.edu/wheat-management/insectsdisease/CR-7668web2017.pdf. Although a few citings have been made, it does not appear that stripe rust will be much of a disease factor this year.  Josh Bushong observed some active stripe rust in central OK near Hennessey (Kingfisher County; Fig 2A), and Dr. Manucherhi found some stripe rust in southwest OK near Altus (Jackson County) in the “telial” stage (Figure 2B).  The telial stage indicates that warm/hot temperature has caused the stripe rust fungus to go from the active state (Figure 2A) to a more dormant state (Figure 2B).

Figure 2. (A) Stripe rust active pustules found on wheat near Hennessey in central OK; (B) Stripe rust “dormant” pustules (telia) found in southwest OK.

I also should reiterate that the Plant Disease and Insect Diagnostic Lab has received quite a few wheat samples over the past two weeks that have tested positive for either wheat streak mosaic (WSM) and/or barley yellow dwarf.  The number of positive samples for WSM especially has seemed somewhat high for this early in the spring.  In talking to producers or educators that submitted these samples it seems as though they typically are associated with volunteer wheat not being controlled in the field, or in a neighboring field.  For more information on mite-transmitted wheat viruses such as WSM, please see OSU Fact Sheet EPP-7328 (Wheat Streak Mosaic, High Plains Disease, and Triticum Mosaic: Three Virus Diseases of Wheat in Oklahoma) available at http://wheat.okstate.edu/wheat-management/insectsdisease/EPP-7328%20three%20virus%20diseases%20of%20wheat.pdf.

Reports/excerpts of reports from other states:

Dr. Erick DeWolf, Extension Plant Pathologist, Kansas State University, April 4, 2017:  “Stripe rust was reported in Southeast Kansas today (April 4th). This is the first report of stripe rust in Kansas for the 2017 growing season.  The find was made by Josh Coltrain, KSU Agronomy Agent in the Wildcat Extension District.  The stripe rust was found in Montgomery county that borders OK in the Southeast corner of the state.  The wheat variety was “Everest” which is known to be susceptible to stripe rust.  The crop was planted in September (early for this part of the state) and is now at the heading stages for growth.  The weather conditions have been highly conducive for the disease in recent weeks and wheat growers in the state should intensify their scouting efforts. As of late-March, reports of pests and diseases included active brown wheat mites in many fields in Oklahoma as well as in some fields in southwest Kansas, symptoms of wheat streak mosaic in some counties in west central Kansas, barley yellow dwarf virus in central and south central Kansas, and tan spot in some fields in south central Kansas.”

Dr. Kirk Broders, Plant Pathologist, Department of Bioagricultural Sciences & Pest Management, Colorado State University, April 1, 2017:  “This years first wheat disease newsletter is coming a later than last year for several reasons, but the primary one is wheat disease in Colorado have been sparse. I was able to visit several wheat fields in Colorado along I-70 as well as along Hwy 34 on March 29 and 31. The rains have provided much needed moisture to the crop and overall the wheat is looking good. However, with the rain and moisture also comes the increased likelihood of foliar diseases caused by fungi such as stripe rust, powdery mildew and tan spot. In regards to stripe rust, there is no evidence the fungus survived the winter in Colorado and there have been few reports from south of us. So far this season, there has been sporadic reports of stripe rust from Texas, Louisiana, Mississippi and most recently from Oklahoma and further east in North Carolina and Virginia. However, there does not seem to be significant levels of pathogen inoculum, which bodes well for us in Colorado as it should require more time for the pathogen to reach Colorado. That being said with cooler temperature and more moisture in the forecast for much of the Central Plains next week, these conditions could give stripe rust the jump start it needs. I will continue to keep you posted on the progress of stripe and leaf rust in the central plains. The other foliar fungal diseases commonly survive in wheat residue and crops planted into a previous wheat crop are at a greater risk for disease development if the weather stays wet and overcast. Given all these considerations, I do not recommend an early season fungicide application as there is simply very low fungal pathogen pressure. The only exception might be wheat planted into wheat. I recommend these fields be scouted over the next couple weeks, to see if any disease develops after these rain events.

The diseases I did notice sporadically throughout the eastern Colorado, were viral diseases. I found both Wheat Streak Mosaic Virus (WSMV) and Barley Yellow Dwarf Virus (BYDV) in several fields. The incidence was lower than what has been reported in central and southwestern Kansas, where the disease seems to be particularly widespread. We may start seeing more symptoms of virus infection as the wheat breaks dormancy and starts growing more quickly. Many of these viral infection likely occurred last fall after the wheat germinated, and then we continued to have very mild temperatures until late November. This allowed the insects that vector both of these viruses to be more active in the fall for a longer period of time resulting in a greater number of infections in some areas. Once wheat is infected with either WSMV or BYDV there is no chemical treatment that can eliminate the pathogen. In fields where virus diseases are present it will be important to ensure volunteer wheat and weeds are managed, as these represent “green bridges” for the wheat curl mite, which vectors WSMV, to survive from one wheat crop to the next.”