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Amanda de Oliveira Silva, Small Grains Extension Specialist and Tom Royer, Extension Entomologist
Planting date: Much of the winter wheat sown in Oklahoma is used as a dual-purpose crop. In such a system, wheat is grazed by cattle from late fall through late winter/early spring and then harvested for grain in early summer. In a grain-only system, wheat is generally planted in October, but in a dual-purpose system wheat is planted in early to mid-September to maximize forage production. Planting wheat early significantly increases the likelihood that diseases and insect pests such as mite-transmitted viruses, the aphid/barley yellow dwarf complex, root and foot rots, and Hessian fly will be more prevalent and severe. For more detailed information on planting date and seed treatment considerations on wheat, see CR-7088 (Effect of Planting Date and Seed Treatment on Diseases and Insect Pests of Wheat).
Mite-transmitted virus diseases: These virus diseases are transmitted by wheat curl mites (WCMs) (Figure 1), and include wheat streak mosaic (WSM), high plains disease (HPD), and Triticum mosaic (TrM). Of these, WSM is the most common. WCMs and these viruses survive in crops such as wheat, corn, and sorghum as well as many grassy weeds and volunteer wheat. In the fall and spring, WCMs spread to emerging seedling wheat, feed on that seedling wheat, and transmit virus to the young wheat plants.
Given this disease cycle, it is easy to see several factors that determine the incidence and severity of these diseases. First, controlling volunteer wheat and other grassy weeds that serve as alternative hosts for the mite and the viruses is imperative to help limit these diseases. Often an infected field of commercial wheat is growing immediately adjacent to a field left fallow during the fall and winter (Figure 2). The fallow field contained abundant volunteer wheat and grassy weeds from which WCMs carrying Wheat streak mosaic virus (WSMV) spread into the commercial field. Wheat infected in the fall will be severely damaged the next spring. Wheat infected in the spring also is damaged, but not as severely as wheat infected in the fall. Hence, it is imperative to do yourself and your neighbors a favor by controlling volunteer wheat and grassy weeds in fields left fallow – especially, if they are adjacent to commercial wheat fields.
A second factor linked to the severity of these mite-transmitted virus diseases is planting date. Early planting dates associated with grazing provides for a much longer time period in the fall for mites to spread to and infect seedling wheat. Planting later in the fall (after October 1 in northern OK and after October 15 in southern OK) and controlling volunteer wheat are the two practices that can be employed to help manage these diseases. It is extremely critical that volunteer wheat is completely dead for at least two weeks prior to planting wheat because WCMs have a life span of 7-10 days. Thus, completely killing or destroying volunteer wheat for a period of at least two weeks prior planting will greatly reduce mite numbers in the fall.
The incidence and severity of these mite-transmitted virus diseases as affected by planting date can be illustrated by the number of samples that tested positive for WSMV and HPV during each of the last three years. In 2017, which was the last year mite-transmitted virus diseases were prevalent in Oklahoma, 103 wheat samples were tested by the Plant Disease and Insect Diagnostic Lab at OSU for presence of mite-transmitted viruses. Of these 103 samples, 69 (67%) tested positive for WSMV and 22 (21%) tested positive for HPV. In 2018, only 12 of 126 (10%) samples tested positive for one or both of these viruses. In 2019, only 21 samples were submitted for testing with 7 samples (33%) testing positive for WSMV (no positives for HPV). In 2020, few samples (less than 5) tested positive for any of these viruses. This lower number of positive samples in 2019 and 2020 likely was the result of an overall later planting date of wheat in the fall of 2018 due to wet conditions and in fall 2020 due to extremely dry conditions. I believe this later planting date in conjunction with more awareness and action in limiting the green bridge helped to lower the incidence and severity of the mite-transmitted viruses in Oklahoma in both 2019 and 2020.
Finally, seed treatments and insecticides are NOT effective in controlling the mites or these mite-transmitted virus diseases. Regarding resistant varieties, there are several winter wheat varieties that have resistance to either WSM or the curl mites, but the adaptation of these varieties to Oklahoma is limited, and the resistance is not typically an absolute resistance to the disease. Hence, severe and continuous disease pressure especially at higher temperature (greater than about 75 F) can overcome the resistance. For more information on mite-transmitted virus diseases, see OSU Fact Sheet EPP-7328 (Wheat Streak Mosaic, High Plains Disease and Triticum Mosaic: Three Virus Diseases of Wheat in Oklahoma).
Aphid/barley yellow dwarf (BYD) complex: Viruses that cause BYD are transmitted by many cereal-feeding aphids (Figure 3). BYD infections that occur in the fall are the most severe because virus has a longer time to damage plants as compared to infections that occur in the spring.
Several steps can be taken to help manage BYD. First, a later planting date (after October 1 in northern Oklahoma and after October 15 in southern Oklahoma) helps reduce the opportunity for fall infection. Second, some wheat varieties tolerate BYD better than other varieties; however, be aware that no wheat variety has a high level of resistance to the aphid/BYD complex. For a listing of reaction of wheat varieties to BYD, other diseases and insect pests, and agronomic traits there are several sources available including variety comparison charts from Oklahoma State University (www.wheat.okstate.edu) and Kansas State University (https://bookstore.ksre.ksu.edu/pubs/MF991.pdf), and the annual wheat variety publication titled, “Wheat Varieties for Kansas and the Great Plains by Layton Ehmke (34 Star Publishing Inc.; firstname.lastname@example.org; https://thewheatfarmer.com; 1-844-643-0170). Third, control aphids that transmit the viruses that cause BYD. This can be done by applying contact insecticides to kill aphids, or by treating seed before planting with a systemic insecticide. Unfortunately, by the time contact insecticides are applied, aphids frequently have already transmitted the viruses that cause BYD. Systemic seed-treatment insecticides containing imidacloprid or thiamethoxam can control aphids during the fall after planting. This may be particularly beneficial if wheat is planted early to obtain forage. Be sure to thoroughly read the label before applying any chemical.
Hessian fly: Hessian fly (Figure 4) infestations can occur in the fall and spring. Fall infestations arise from over-summering pupae that emerge when climate conditions become favorable. In states north of Oklahoma, a “Hessian fly free” planting date often is used to help limit fall infestations by Hessian fly. However, such a planting date does not apply in Oklahoma because Hessian fly can emerge in Oklahoma as late as December (Figure 5).
Delayed planting (after October 1 in northern Oklahoma, and after October 15 in southern Oklahoma) can help reduce the threat of Hessian fly, but a specific “fly free date” does not exist for most of Oklahoma as it does in Kansas and more northern wheat-growing states. This is because smaller, supplementary broods of adult flies emerge throughout the fall and winter. A number of varieties are resistant to Hessian fly; for a listing of reaction of wheat varieties to Hessian fly, other diseases and insect pests, and agronomic traits there are several sources available including variety comparison charts from Oklahoma State University (www.wheat.okstate.edu) and Kansas State University (https://bookstore.ksre.ksu.edu/pubs/MF991.pdf), and the annual wheat variety publication titled, “Wheat Varieties for Kansas and the Great Plains by Layton Ehmke (34 Star Publishing Inc.; email@example.com; https://thewheatfarmer.com; 1-844-643-0170). Hessian fly infestations can be reduced somewhat by destroying volunteer wheat in and around the field at least two weeks prior to emergence of seedling wheat. Seed treatments that contain imidacloprid or thiamethoxam will also help reduce fall infestations of seedling wheat, especially if combined with delayed planting and volunteer destruction. For more information on Hessian fly, see OSU Fact Sheet: EPP-7086 (Hessian fly Management in Oklahoma Winter Wheat).
Root and foot rots: These are caused by fungi and include several diseases such as dryland (Fusarium) root rot, Rhizoctonia root rot (sharp eyespot), common root rot, take-all, and eyespot (strawbreaker). Every year samples are received in the lab that are diagnosed with root rot. Typically wheat affected by seedling/root rots are either submitted in the fall when wheat is in the seedling stage or in later May and early June as plants are maturing. Germinating seeds and seedlings have small root systems that if infected impacts seed germination and seedling emergence (Figure 6). Later in the season (late May/early June), root rots again become apparent as maturing plants are unable to obtain sufficient moisture to finish grain development especially if drought conditions are present. In mature plants, white heads often indicates the presence of root rot (Figure 7).
In 2017-2018, the incidence and severity of root rots across Oklahoma dramatically increased compared to the 2016-2017 season. This increase likely resulted from weather conditions that favored the root rots along with heat and drought in May/June of 2018 that promoted white heads to develop. Dryland (Fusarium) root rot was the most common root rot observed in 2018, and caused significant damage to wheat in southwestern, western, northwestern OK as well as the panhandle. In 2018-2019, dryland (Fusarium) root rot again became prevalent across much of Oklahoma, but was not as damaging as the previous year likely because ample moisture and cool temperatures meant that water stress on plants was much less than in 2017-2018. Root rots were only sparsely observed in 2019-2020 and only at low severity.
Controlling root and foot rots is difficult. There are no resistant varieties, and fungicide seed treatments with activity toward the root and foot rots are effective in protecting germinating seed and emerging seedlings, their activity usually involves early-season control or suppression rather than control at a consistently high level throughout the season. Often, there also are different “levels” of activity related to different treatment rates, so again, CAREFULLY read the label of any seed treatment to be sure activity against the diseases and/or insects of concern are indicated, and be certain that the seed treatment(s) is being used at the rate indicated on the label for activity against those diseases and/or insects. Later planting (after October 1 in northern Oklahoma and after October 15 in southern Oklahoma) also can help reduce the incidence and severity of root rots, but planting later will not entirely eliminate the presence or effects of root rots. If you have a field with a history of severe root rot, consider planting that field as late as possible or plan to use it in a “graze-out” fashion if that is consistent with your overall plan.
For some root rots, there are specific factors that contribute to disease incidence and severity. For example, a high soil pH (>6.5) greatly favors disease development of the root rot called take-all. OSU soil test recommendations factor in this phenomenon by reducing lime recommendations when continuous wheat is the intended crop. Another practice that can help limit take-all and some of the other root rots is the elimination of residue. However, elimination of residue by tillage or burning does not seem to affect the incidence or severity of eyespot (strawbreaker).
Seed treatments: There are several excellent reasons to plant seed wheat treated with an insecticide/fungicide seed treatment. These include:
1. Control of bunts and smuts, including common bunt (also called stinking smut) and loose smut. The similarity of these names can be confusing. All affect the grain of wheat, but whereas common bunt spores carryover on seed or in the soil, loose smut carries over in the seed. Seed treatments labeled to control bunts and smuts are highly effective. If common bunt (stinking smut) was observed in a field and that field is to be planted again with wheat, then planting certified wheat seed treated with a fungicide effective against common bunt (stinking smut) is strongly recommended. If either common bunt (stinking smut) or loose smut was observed in a field, grain harvested from that field should not be used as seed the next year. However, if grain harvested from such a field must be used as seed wheat, treatment of that seed at a high rate of a systemic or a systemic + contact seed treatment effective against common bunt (stinking smut) and loose smut is strongly recommended. In 2020, loose smut in fields and common bunt in harvested grain was observed at higher incidence and severity than for several years, so I strongly recommend planting certified wheat seed that was been treated with a fungicide labeled for control of bunt and smut. For more information on common bunt (stinking smut) & loose smut, see: http://www.entoplp.okstate.edu/ddd/hosts/wheat.htm and consult the “2020 OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control (OCES publication E-832),” and/or contact your County Extension Educator.
2. Enhance seedling emergence, stand establishment, and forage production by suppressing root, crown and foot rots. This was discussed above under “Root and Foot Rots.”
3. Early season control of the aphid/BYDV complex. This can be achieved by using a seed treatment containing an insecticide. Be sure that the treatment includes an insecticide labeled for control of aphids.
4. Control fall foliar diseases including leaf rust and powdery mildew. Seed treatments are effective in controlling foliar diseases (especially leaf rust and powdery mildew) in the fall, which may reduce the inoculum level of these diseases in the spring. However, this control should be viewed as an added benefit and not necessarily as a sole reason to use a seed treatment.
5. Suppression of early emerged Hessian fly. Research suggests that some suppression can be achieved, but an insecticide seed treatment has little residual activity past the seedling stage and Hessian fly often infests wheat after the seedling stage.
Fall Armyworm Potential: We have seen a severe outbreak of fall armyworms infesting bermudagrass and fescues lawns this past month. The strain that is infesting these lawns is known as the “rice” strain, and it overwinters in the Florida Gulf Coast and parts of the Caribbean. We typically see the “corn” strain which typically overwinters in the Texas Gulf Coast and Mexico. The rice strain prefers rice, and grasses, and the corn strain prefers corn and sorghum and as we have seen in the past, winter wheat. This year, Oklahoma is experiencing a “double whammy” of both strains, and unfortunately, they both like wheat. So, our advice is to carefully watch your fields after they emerge and deal with any fall armyworm infestations before they take your stand.
Lanie Hale, from Wheeler Brothers sent a picture of “window paned” wheat from a field that he had scouted in 2017. He counted 3 fall armyworms per row foot from his visual count (which is treatment threshold) but when he looked closely at his photo on his computer, he saw 15 worms in an area the size of his hand (they were very tiny, and probably newly hatched). It is easy to miss some of these little worms in the field because they hide in residue and are very tiny.
Look very closely for “window paned” leaves and count all sizes of larvae. Examine plants along the field margin as well as in the interior, because they sometimes move in from road ditches and weedy areas. The suggested treatment threshold is 2-3 larvae per linear foot of row in wheat with active feeding. Numerous insecticides are registered for control, but they are much more susceptible when caterpillars are small. We won’t get relief from fall armyworms until we get a killing frost, so keep vigilant!
Consult the newly updated OSU Fact Sheets CR-7194 Management of Insect and Mite Pests of Small Grains for control suggestions.
This article was written by Bob Hunger, Extension Wheat Pathologist
During the last two days there have been a couple reports of dark wheat heads being observed in fields. This is a condition call sooty mold (aka black head mold) (Figure 1). These dark heads are the result of saprophytic (living of dead tissue) or weakly pathogenic fungi growing on the dead tissue in wheat heads. Reports of this have come from Greg Highfill (Alfalfa County Extension Educator in north central OK) and from Brad Secraw (Cleveland County Extension Educator in central OK). Additionally, I have observed severe sooty mold in some of the trials around Stillwater.
Sooty mold occurs when wheat has turned but cannot be harvested in a timely manner. Wet/humid conditions during a delayed harvest will then promote the fungal growth on wheat heads. Often wheat that has been subjected to a stress such as freeze, root rot, or drought shows a greater severity of sooty mold than if the wheat had been healthy and not stressed. This is the case in the top photo in Figure 1. The darker strips of wheat with sooty mold are the variety Pete, which was hit hard by the late freeze in April. The lighter, more golden colored heads with much less sooty mold are lines in one of Dr. Carver’s nurseries. These breeder lines were not nearly as affected by the freeze as was the Pete. Although grain yield from wheat with sooty mold often is reduced, the sooty mold itself is not the primary cause of that reduce yield. Rather, it was the stress such as a freeze or root rot that was the primary cause of the reduced yield.
One additional point to be made is that grain harvested from wheat with severe sooty mold may show a condition known as black point (Figure 2). Black point is a discoloration of the seed (typically the germ end of the seed) resulting either from infection by various fungi that typically are saprophytic but can occasionally parasitize living tissue, or from a combination of abiotic (environmental) conditions that promote the discoloration without the presence of an organism. Like sooty mold, black point often is observed when freeze damage has occurred or when harvest was delayed and dead tissue in the heads was heavily colonized by fungi that resulted in sooty mold. Black point in wheat grain can be a grading factor as the discoloration can result in black flecks in flour milled from such grain. Additionally, if used as seed wheat, kernels with black point can have reduce germination resulting in lower seedling emergence. Hence, if wheat showing black point is to be used as seed wheat, it is imperative to check the germination of that seed and to use a seed treatment that controls seed and seedling rots.
FINALLY – This likely will be my (Dr. Bob Hunger) last Wheat Disease update as my last day of work is July 9th. It has truly been a pleasure to send these updates!! I hope all of you have a great harvest this year and even better ones in the future!!!
This article was written by Bob Hunger, Extension Wheat Pathologist
Wheat tours over the last ten days included Kingfisher (Kingfisher County; south central OK), Cherokee (Alfalfa County; north central OK), Alva (Woods County; northwestern OK), Lahoma (Garfield County; north central OK), Morris (Okmulgee County; eastern OK), El Reno (Canadian County; central OK); and Buffalo (Harper County; northwestern OK). Wheat in these areas is pretty much done with flowering and kernels ranged from just forming to fully formed. Some varieties in some areas were in the milk stage with some approaching soft dough.
Diseases at these locations varied considerably but overall, a wider range of diseases was observed. Some locations such as Cherokee, Alva and Buffalo had relatively light foliar disease incidence with some leaves indicating barley yellow dwarf and wheat streak mosaic (and/or other mite transmitted viruses). Around Stillwater and at Lahoma, although stripe rust was still prevalent leaf rust is making an appearance (Figure 1). At others such as Kingfisher, Morris and El Reno, leaf rust could be found but stripe rust seemed to still be more prevalent. Leaf spot diseases also were observed at most of these locations, but these foliar diseases were not as prevalent as the rusts.
Darkened heads were observed at several locations but were most prominent and prevalent at Morris in eastern OK (Figure 2). Darkened heads like this can result from several causes. If Septoria and/or Stagonospora are present on lower leaves, these fungi can move up onto the heads and cause a glume blotch that has this appearance. Another possibility is a bacterial disease called black chaff or bacterial streak (Figure 3). Black chaff will occur on leaves (Figure 3; photo on left), but also can move onto heads (Figure 3; center photo). Note on this center photo how the stem (peduncle) immediately beneath the head shows darkened lesions like those on the head. Finally, awns of heads infected with black chaff often show an alternating pattern of dark and white (Figure 3; photo on right). Another possible cause of these dark heads is presence of a gene that confers resistance to wheat stem rust. In this case, the result is not a disease, but rather an association with the presence of that gene. Regarding the darkened heads observed in the trial near Morris, Dr. Silva and I agree it is most likely the majority of the darkened heads observed likely resulted from freeze damage as many of these heads also were totally or partially sterile (see Dr. Silva’s blog at https://osuwheat.com/2021/05/18/freeze-damage-update/). However, Septoria/Stagonospora and black chaff also contributed as symptoms of these diseases were observed in the field.
A final disease observed this past week was indicated by the sporadic occurrence of white heads in some parts of the field. Examination of plants/tillers associated with these white/yellowing heads revealed symptoms typical of take all root rot (Figure 4). However, I am not yet certain that these tillers had take all as symptoms of other root rots also were present. Hence, samples were brought back to the lab for isolation and identification. Look for an update on this in my next report but be aware there likely will be root rot showing up in some areas of the state.
This next week marks the end of the wheat field days in Oklahoma with four coming next Thursday and Friday in the Oklahoma panhandle. A complete schedule of the remaining field days can be viewed at: http://wheat.okstate.edu/virtual-plot-tour/2021OSUWheatFieldTours.pdf
This article was written by Bob Hunger, Extension Wheat Pathologist
Wheat tours last week included Homestead (Blaine County; west-central OK), Afton (Ottawa County; northeastern OK), Sentinel and Tipton (Washita and Tillman Counties, respectively; southwestern OK) and Kildare and Lamont (Kay and Grant Counties, respectively; north-central OK). At Homestead, Dr. Amanda Silva (OSU Small Grains Agronomist) saw primarily tan spot (Figure 1) as this trial was planted in a field of wheat after wheat. Sentinel was fairly free of foliar diseases, but the trial at Tipton was severely infected with stripe rust. The incidence and severity of stripe rust at Tipton also was observed by Dr. Brett Carver (OSU Wheat Breeder/Geneticist) who indicated that he saw severe stripe rust in his trials at Tipton as well. Near Chattanooga OK, also in SW OK, there was a report of stripe rust occurring in wheat heads (Figure 1, center photo and photo to the right). Over the years, I have occasionally observed this in Oklahoma, and it typically is a signal that stripe rust has been severe. As far as I know, the grain is not infected, but rather it is the plant tissue surrounding the grain. These reports of severe stripe rust contrast with what Dr. Silva and I observed at Afton, Kildare, and Lamont where little foliar disease of any type was observed. We did however see symptoms indicative of barley yellow dwarf at all locations and some indicative of the mite-transmitted virus diseases such as wheat streak mosaic and high plains disease.
This week will be spent at wheat field days in central, north central, and northwestern OK including trials near Cherokee, Kingfisher, Thomas, Alva, and Lahoma. A complete schedule of the remaining field days can be viewed at: http://wheat.okstate.edu/virtual-plot-tour/2021OSUWheatFieldTours.pdf
This article was written by Dr. Bob Hunger, Extension Wheat Pathologist
Just a brief update to relay that stripe rust continues to increase across Oklahoma. Bryan Vincent (Crop Consultant; north-central OK) reported severe stripe rust in “hot spots” on an unknown variety just north of Lamont, OK (Grant County) close to the Kansas border (Figure 1; left photo). In Major County, which is immediately south of Grant County, Josh Coltrain (Winter Wheat Technical Development Lead, Syngenta) reported he had, “found quite high incidence of stripe rust” in Syngenta’s plots near Carrier, OK. Here around Stillwater, I have observed severe stripe rust in spreader strips of the susceptible variety Pete. These infections stood out because of resistant breeder lines planted immediately adjacent to the strips of Pete (Figure 1; center photo and photo to the right).
However, the most striking example I have seen of stripe rust in some time was observed by Jeff Wright (Coordinator of Production Operations; OFSS; Oklahoma State University) in an increase field of the old variety Triumph 64 near Perkins (about 15 miles south of Stillwater). As you can see in Figure 2 (two top photos), much of the entire field (9 acres) appears yellowish. Examination of leaves reveals severe stripe rust infection associated with yellowing of the leaf (middle photo). The bottom photo in Figure 2 is of Jeff’s tractor after applying a fungicide. Although the fungicide should protect the green leaves remaining in the field, much of the leaf tissue will be killed from the stripe rust infection. This is a good example of the importance of applying a fungicide to a susceptible variety sufficiently early to prevent such widespread infection. What and how such a big and uniform infection occurred is puzzling to me, but I suspect that is related to overwintering of the stripe rust fungus in the field.
In other wheat around Stillwater, there continues to be a high incidence and severity of powdery mildew. Barley yellow dwarf also is easily found in many trials and varieties. Dr. Tom Royer has sent out an alert about finding English grain aphids around the state. These aphids also were observed by Bryan Vincent in north-central OK and by me here around Stillwater. Finally, the wheat field days start next week, so observations from those locations will start to appear in subsequent updates. A complete schedule of those field days can be viewed at: http://wheat.okstate.edu/Home/plot-tours/
This article was written by Bob Hunger, Extension Wheat Pathologist
Last week (08-April) in southwestern OK, Gary Strickland (Jackson County Extn Educator) reported seeing only very little stripe rust, but that tan spot was still present in the lower canopy. Southwestern OK has been hot and dry, so conditions in that part of Oklahoma have not been at all favorable for foliar disease development. Also last week, Bryan Vincent (Crop Consultant; north-central OK) reported seeing mostly tan spot and powdery mildew across northern OK, with little to no rust, but some barley yellow dwarf starting to appear as well as aphids. Greg Highfill (Woods County Extn Educator) also reported heavy tan spot in a no-till, wheat- after-wheat field.
More recently, although still scattered and light, both leaf and stripe rust (more so stripe) appears to be increasing across Oklahoma. Lanie Hale (Manager, Wheeler Brothers) sent out the following report. “Yesterday, Will Bedwell and I scouted 22 wheat fields in southern Major, northeastern Dewey, and northwestern Blaine Counties. The wheat variety was unknown to us in most of the fields. In seven of the fields, we found leaf and/or stripe rust in isolated areas, certainly not widespread across the fields. We found powdery mildew in a few fields, but only where the canopy was heavy and dense. Septoria and/or Tan Spot was noted on lower leaves in most fields. Many of the fields had infestations of Bird Cherry Oat Aphids ranging from light to moderately heavy. We saw several Lady Beetles and larvae in fields; I only saw one mummified aphid indicating not many parasitic wasps are present. One of the fields had been sprayed over the weekend. The flag leaf is emerging in most fields we scouted. We did not scout any field with 100% emerged flag leaves.”
Yesterday around Stillwater, I saw wheat that ranged from flag leaves emerging to wheat headed, although by far and away most of the wheat was at the boot/pre-boot stage (GS 9 or so). By far, the most prevalent disease I observed was powdery mildew (Figure 1). Stripe rust also was present, but at a low incidence. I also noted quite a few spots or patches of barley yellow dwarf in various trials, but in contrast to other reports, I saw very few aphids.
Given these reports and observations, it is advisable for producers to start watching their fields closely and prepare for applying a fungicide, especially if growing a variety susceptible to either rust and/or the other foliar diseases. For more information on fungicides and their use, see OSU CR-7668, which can be found at: https://extension.okstate.edu/fact-sheets/foliar-fungicides-and-wheat-production-in-oklahoma-march-2016.html.
Perhaps the most striking observation is that wheat streak mosaic (Figure 2) is starting to be reported across both Oklahoma and Texas as a couple of samples have tested positive for Wheat streak mosaic virus in the last week. For more information on this and other mite-transmitted viruses, please see OSU EPP 7328 that can be accessed at: https://extension.okstate.edu/fact-sheets/wheat-streak-mosaic-high-plains-disease-and-triticum-mosaic-three-virus-diseases-of-wheat-in-oklahoma.html
This article was written by Bob Hunger, Extension Wheat Pathologist
Reports of foliar diseases, especially stripe and leaf rust, are starting to increase in southern Texas and around Stillwater. First, here is an update sent out on 24-March by Dr. Amir Ibrahim (Regents Professor & Small Grains Breeder/Geneticist; Texas A&M AgriLife Research). Dr. Ibrahim is finding both stripe rust and leaf rust increasing across southern Texas.
“I visited our small grains trials at McGregor (18 miles southwest of Waco, TX) on March 18, 2021. Stripe rust (caused by P. striiformis Westend. f. sp. tritici Eriks.) continues to be active (Figure 1; photo on the left). Leaf rust (caused by Puccinia triticina Erikss.) is beginning to move to the middle canopy (Figure 1 – photo on the right).”
“We visited the naturally inoculated Rust Evaluation Nursery at Castroville, TX today. The nursery is about 196 miles from Texas A&M University’s main campus in College Station, where we are based. We also visited our trials at Uvalde, TX today. Stripe rust is not very active at both Castroville and Uvalde. Leaf rust is now picking up, especially at the Rust Evaluation Nursery at Castroville. Stripe rust is very actively spreading at the Agronomy Farm near our main campus in College Station as of our last visit on March 23, 2021. Stripe rust is also active in our trials in Greenville (50 miles northeast of Dallas). No reports yet of leaf or stripe rusts in the Texas High Plains. Leaf rust is also developing in our trials at Wharton (60 miles southwest of Houston).”
In Oklahoma, both stripe and leaf rust (Figure 2) have been observed in trials around Stillwater and near Perkins (about 15 miles south of Stillwater). Also recall in my update of 15-March, I indicated seeing powdery mildew, Septoria/Stagonospora (Figure 3) on lower leaves in many trials. These diseases also are present, and with the relatively cool and windy weather in the forecast, I expect the incidence and severity of all these diseases to increase.
This article was written by Dr. Bob Hunger, Extension Wheat Pathologist
Although relatively quiet, some wheat diseases have started to appear across Oklahoma over the last week. For example, around Stillwater I am starting to observe patches of wheat showing symptoms of the wheat soil-borne mosaic (SB)/wheat spindle streak mosaic (SS) complex. So far, I have observed these symptoms only in susceptible varieties in Dr. de Silva’s variety demo and in my SB-SS nursery. These virus diseases are not a problem in Oklahoma or the central plains due to effective and durable genetic resistance in nearly all wheat varieties planted in Oklahoma for the last four decades. However, planting a variety susceptible to either or both of these virus diseases could be an invitation to having an occurrence of these diseases. It seems as though only far northwest Oklahoma and the panhandle have environments that limit the occurrence of these two virus diseases.
In trials around Stillwater towards the end of last week, I found sparse powdery mildew and fairly abundant Septoria/Stagonospora leaf spot on leaves of ‘Ruby Lee’ (Figure 2). This was in Dr. Brett Carver’s dual purpose observation nursery, which is an early planted nursery. In no trials did I find either leaf or stripe rust, although Dr. Amanda de Oliveira Silva had found both leaf rust and powdery mildew in her demonstration trial in later January before the hard freeze and snow occurred in early to mid-February.
Figure 1. Wheat showing reaction to the wheat soil-borne mosaic (WSBM)/wheat spindle streak mosaic (WSSM) complex. Left photo: Wheat breeder line susceptible (left) and resistant (right) to WSBM. Center photo: Symptoms typical of WSBM. Right photo: Symptoms typical of WSSM.
Figure 2. Upper photo are symptoms on a wheat leaf indicative of Septoria or Stagonospora leaf blotch found near Stillwater on 13-Mar-2021. The lower photo is of a wheat leaf with symptoms indicative of tan spot observed in southwest Oklahoma by Gary Strickland, (County Educator; Jackson County), 3-11-2021.
Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Plant Pathologist
Oklahoma: I had limited trips outside of Stillwater this past week, and only was able to contact one County Educator before writing this today. Wheat around Stillwater is mostly at various stages of head emergence. I did see a few anthers on scattered heads, but not many. By contrast, Aaron Henson (County Educator; Tillman County in south-central OK) indicated wheat in his area is mostly at flowering.
During this past week, I had several calls about spraying wheat with a fungicide. Although rust (stripe and leaf rust) didn’t appear to increase this past week, conditions reverted to being more favorable for stripe rust development with rainfall, increased dews, and favorable temperature. With more rains and cool temps in the forecast, stripe rust could “reactivate” again, and leaf rust will start to come into the picture. Wheat is now at the point where it will quickly move past the stage (the start of flowering) where it can be sprayed with most fungicides. As far as I know, all wheat foliar fungicides (with the exception of Prosaro) must be applied prior to the start of flowering (Feekes’ growth stage 10.5). Prosaro can be applied through growth stage 10.5.1, which is when flowering is just starting (anthers emerged mostly from the middle of heads). Be sure to read all labels regarding a fungicides use on wheat. There also are varying pre-harvest intervals (PHIs) required for the various fungicides, and often the length of time from heading to harvest can be short in Oklahoma. So, be aware of these PHIs, and spray accordingly.
Active sporulation of stripe rust still can be found around Stillwater and the surrounding area. Stan Fimple (County Educator, Pawnee County just to the northeast of Stillwater) sent me the following photos showing active stripe rust. The photo on the top shows an actively sporulating “stripe” of strip rust (yellowish-orange in color), whereas in the photo on the bottom in the “stripes” you can see dark, blackish specks (teliospores) starting to appear.
Other than this, I have seen scattered leaf rust pustules on lower leaves around Stillwater, and powdery mildew also has become more apparent around Stillwater and at Lahoma as reported by Dr. Brett Carver (OSU Wheat Breeder). However, both of these diseases are at low levels on lower leaves but with coming rain and cool temperatures both (especially leaf rust) could continue to increase on the upper canopy. Around Stillwater, barley yellow dwarf spots continue to be observed but the aphids are now gone or at least in much lower in frequency. If heavy rains come over the next 3 or so days, I would imagine aphid populations will be mostly eliminated.
Finally, I want to raise awareness once again to Fusarium head blight (scab) of wheat. When wheat flowers it is susceptible to infection by the Fusarium fungus that causes scab. That time is quickly approaching. Occasionally Oklahoma has problems with this disease, typically more so in eastern/northeastern Oklahoma than through the central and western parts of the state. However, scab was severe across the state for a couple years around 2010 and there also was some reported last year. For more information on scab, please see PSS-2145 (Fusarium Head Blight (Head Scab) of Wheat: Questions & Answers) and PSS-2136 (Considerations when Rotating Wheat Behind Corn) that can be found at: wheat.okstate.edu. An additional resource is the Fusarium Head Blight Prediction Center at http://www.wheatscab.psu.edu/. At this site you can read commentaries submitted by specialists from each state but more importantly see if weather conditions in your area have been conducive to development of FHB. The site is easy to use and especially may be beneficial in helping make fungicide application decisions.
Reports/excerpts of reports from other states:
Louisiana: Dr. Stephen Harrison, Wheat & Oat Breeder, Louisiana State University, Apr 15, 2016: My research associate (Kelly Arceneaux) is at the Rice Research Station in Crowley (Southwest) Louisiana rating plots today. We plant a double-headrow set of a number of nurseries every year for disease screening at this location in collaboration with Don Growth (rice pathologist). This site is inoculated with scabby corn but is not misted due to the abundance of humidity and free moisture at this site. Nurseries include: Statewide Variety Trial, Uniform Southern Soft Red Winter Wheat Nursery, Uniform Southern Scab Nursery, Sunwheat, GAWN. Kelly reports that stem rust is heavy and widespread at this site. Leaf rust is moderate and scab is at an intermediate level, which is good for distinguishing lines. The earliest plots are starting to mature, probably just past soft dough, while the latest lines are just past heading or not vernalized and not going to head. We only received about 50% of our normal vernalization hours this winter and quite a few lines in the statewide variety trials will not be harvested due to vernalization issues.
Nebraska: Dr. Stephen Wegulo, Extn Plant Pathologist, University of Nebraska, April 14, 2016: “On Friday April 8, Jenny Rees, UNL Extension Educator, found trace amounts of stripe rust in a wheat field in Nuckolls County in south central Nebraska. Earlier this week, samples from several wheat fields in Banner County submitted to the lab of Dr. Bob Harveson (Extension Plant Pathologist) at UNL’s Panhandle Research and Extension Center in Scottsbluff were positive for stripe rust and leaf rust. This week on April 12 and 13 I surveyed wheat fields in the southernmost tier of counties in southeast, south central, and west central Nebraska. Dry weather which has prevailed over the last two weeks or so stopped rust development. I did not find rust in any of the fields I visited in the southernmost tier of counties. Several fields showed symptoms of stress from lack of moisture. Today I looked at research plots at Havelock Farm here in Lincoln (Lancaster County) and at the Agricultural Research and Development Center (ARDC) near Mead (Saunders County, about 35 miles north of Lincoln). I found a few hot spots of stripe rust at Mead (see first attachment), mostly on the lower leaves. I also found trace levels of leaf rust at Mead (second attachment). Powdery mildew was the predominant disease at Lincoln and Mead, but I also saw significant levels of Septoria tritici blotch in one research field at Mead. Wheat growth stage across the state ranges from Feekes 5 and 6 (most fields) to Feekes 7 in some irrigated fields.”
South Dakota: Dr. Emmanuel Byamukama, Extension Plant Pathologist, South Dakota State University; Apr 13, 2016: “Several winter wheat fields in central South Dakota were scouted yesterday for stripe rust. One field originally found with stripe rust last week was the only one we found with stripe rust. Stripe rust was found on old/dying leaves and some of the leaves had teliospores, indicating the source of this rust would have been from overwintered stripe rust in South Dakota.”
Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Plant Pathologist
Wheat has advance in maturity across OK this past week flag leaves definitely are emerging around Stillwater. From reports I’ve received I believe across the state wheat ranges from flag leaves emerging to heads starting to emerge (although wheat in far northwest OK and the panhandle may not be quite as far along). I didn’t hear specifics but was told that freeze damage has been observed around Kingfisher in central OK. With frost/freezing temps again last night, additional damage is possible. Drought, although not as bad as last year, also is creeping back into the picture. One producer from southwestern OK indicated to me that “leaves are rolling-up at 2 o’clock in the afternoon.” I didn’t see any wheat that looked stressed, but in several locations had to dig 4” or more to find moist soil.
In my trips this past week to central OK (Watonga) and to more north-central OK (Blackwell), I could find stripe rust, but it doesn’t appear to me that it had advanced (become more severe). In fact, Zack Meyer (Extn Educator; Kingfisher Cnty) sent me the following photo that shows the telial spore stage of the stripe rust fungus forming on wheat leaves. Look closely at the photo and you can see minute yellowish-orange pustules of stripe rust also present on the leaves (especially the greener leaf). The telial stage is considered more of a survival spore stage and indicates that stripe rust is encountering unfavorable conditions and starting to shut down. Although this is good news, stripe rust can quickly “reactivate” if favorable temperature and moisture are resumed.
Unfortunately there also is a lot of active stripe rust still in the state as I have had numerous calls from across OK to discuss spraying options, and Greg Highfill (Extn Educator; Woods Cnty) sent me the following photo showing moderate/severe and active stripe rust on wheat in northern-central OK.
Reports/excerpts of reports from other states: No reports from Texas, but did hear the following from Kansas and Nebraska.
Kansas: Dr. Erick DeWolf; Extn Plant Pathologist; Kansas State University; Manhattan, KS; Apr 1, 2016: “The Kansas wheat crop is progressing rapidly through the jointing stages of development in much of the state. Wheat in the Southeast portion of the state is at or fast approaching flag leaf emergence. The crop is generally considered to be two or three weeks ahead of schedule.
Scouting reports indicate that stripe rust is becoming established in the 2016 wheat crop. This past week, stripe rust was reported in many counties in central and eastern Kansas. The disease is still at low levels in most fields with a few exceptions in Southeast Kansas. This early establishment of stripe rust increases the risk of severe yield loss and growers should continue to monitor the situation carefully. If weather conditions become favorable, the disease could spread rapidly from the lower leaves, where it is now established, to the upper leaves that are critical for grain development. Leaf rust is still active in the western tier of counties bordering CO but remains a low levels in most fields. Powdery mildew is severe in some fields in central and eastern Kansas.”
Nebraska: Dr. Stephen Wegulo; Professor/Extn Plant Pathologist; University of Nebraska-Lincoln; Lincoln, NE, KS; Mar 31, 2016: “Yesterday March 30, 2016: Jennifer Rees, UNL Extension Educator, found trace levels of actively sporulating leaf rust in wheat fields in Nuckolls County in south central Nebraska. Nuckolls County is in the southernmost tier of counties that border Kansas. She did not find actively sporulating stripe rust; however, in one field there was evidence of stripe rust that was active last fall.”
Colorado: Dr. Kirk Broders; Ast Professor; Colorado State University; Ft. Collins, CO; Mar 29, 2016: “As I mentioned last week stripe rust is now present in eastern Colorado with a confirmed report of stripe rust in the Prospect Valley region northeast of Denver. We have received several reports of stripe rust from that same region. This past week was windy with some precipitation in this area of Colorado, so spores were spread but there was limited moisture to promote additional infection. There is rain in the forecast for this coming week and the rain is certainly needed for the wheat, but also will provide a favorable environment for stripe rust to increase because temperature is supposed to be staying in the 50s-70s for the days and 20s-40s at night. If you already have noticeable levels of rust in your field you may want to consider including a fungicide at tillering (GS 4) or when you make your herbicide application. If you do not currently have rust in your fields or in your region, I would recommend waiting until closer to flag leaf and monitor the spread of stripe rust in the state. CSU Extension specialist Wilma Trujillo was able to examine wheat in the southwest part of the state near Lamar, where stripe was present last fall. We examined these leaves and found no evidence stripe rust was able to overwinter in this region of the state. It is still early in the season, but there is certainly the possibility for stripe rust to become a serious problem in the state again this year. There are also the threat of leaf rust we should not forget about. Leaf rust has been present in western Kansas for the last 2 weeks and has likely moved into eastern parts of the state. I have not received in specific reports, but would appreciate you feedback if you have observed either stripe rust of leaf rust in you fields.