This article was written by Meriem Aoun, Small Grains Pathologist
During my visit to wheat fields in Morris (Okmulgee County) on May 16, I observed multiple fungal and bacterial diseases. Wheat crop in Morris looked good and tall compared to other locations in Oklahoma (Figure 1). Morris got substantial amount of precipitation, which favored some fungal and bacterial diseases.
Bacterial streak (on the leaf, Figure 2) and black chaff (on the head, Figure 3) were frequently observed on multiple winter wheat varieties including ‘Big country’ and ‘WB 4401’. Bacterial streak and black chaff are two phases of the same disease and are favored by humid and warm climate, which was the case in Morris.
In Morris, I also observed Septoria leaf spot and tan spot in the lower and mid canopy, but nothing much on flag leaves. Septoria leaf spot was more common and found on varieties like ‘Skydance’ and ‘Crescent AX’ (Figure 4). Both diseases were also observed in the Stillwater Agronomy Research Station on OSU winter wheat breeding lines. In addition, spot blotch and powdery mildew were found in multiple experimental plots in Stillwater on susceptible winter wheat varieties and OSU breeding lines (Figure 5).
Powdery mildew and leaf rust were observed in both Stillwater and Morris (Figure 6 and 7). As I previously reported powdery mildew was observed in multiple locations in Oklahoma since April whereas leaf rust was first observed this year in Oklahoma during the second week of May.
In addition to these foliar diseases, I observed some head diseases including sooty mold (black head mold) (Figure 8) in wheat fields in Morris, El Reno, and Stillwater. Humid conditions promote this disease 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 healthy wheat. I also observed loose smut (Figure 9) in Chickasha, Stillwater, and Morris.
This article was written by Meriem Aoun, Small Grains Pathologist
Root/crown/foot rots were observed in multiple wheat fields in April and May in Woods, Cherokee, Blaine, Cotton, and Payne counties. Dr. Amanda Silva reported severely damaged wheat plants in drought stressed fields mainly in Cherokee and Woods counties (Figure 1). Infected plants were stunted and white and had poor root systems. Although the plants were drought stressed throughout the growing season, much of the damage was not noticeable until after wheat heading. Dr. Silva observed pinkish discoloration on infected plants in Cherokee after peeling the leaf sheath in the lower stem internodes, which indicates that the infection was caused by Fusarium (Figure 2).
The rain in early May in some locations in Oklahoma provided suitable environmental conditions for the appearance of some fungal diseases including rusts. This week, stripe rust was observed in the Stillwater Agronomy Research Station on susceptible wheat varieties like ‘Pete’ and some OSU breeding lines (Figure 3). Leaf rust was found on the susceptible wheat variety ‘OK Bullet” and on some OSU breeding lines (Figure 4). Rust diseases have not been found in other locations in Oklahoma. The current pressure is low and late compared to the previous year due to drought conditions through the growing season. However, rust disease incidence can increase in coming weeks if weather conditions are favorable.
This article was written by Meriem Aoun, Small Grains Pathologist
During April, the Plant Disease and Insect Diagnostic Laboratory at OSU received multiple wheat samples showing symptoms of streaking on the leaves. Leaf streaks were greenish yellow and parallel as shown in Figure 1. Enzyme linked immunosorbent assay (ELISA) on these samples from different wheat varieties were positive for wheat streak mosaic virus (WSM). WSM infected samples were from fields in multiple counties in Oklahoma including Payne, Blaine, Cimarron, Harper, Grady, and Garfield. A couple of samples that tested positive for WSM were also positive for high plains virus (HPV) and were from Harper and Blaine counties. Both WSM and HPV are transmitted by wheat curl mite. I also observed symptoms of barley yellow dwarf virus (Figure 2) in fields in Payne, Cleveland, and Grady counties.
I also observed leaf spotting on the wheat variety ‘OK Bullet’ in the Stillwater Agronomy Research Station. Culturing from the leaves resulted in the identification of the fungi Bipolaris sorokinana which causes spot blotch and Parastagonospora nodorum which causes septoria nodorum blotch. Parastagonospora nodorum was also recovered from leaf spots on leaves of the variety OK Corral in Cleveland County.
Around mid-April, the OSU Diagnostic Lab received a wheat sample from the variety ‘Doublestop CL Plus’ from Blaine County. I examined the sample and I found that the infected plants were stunted and brown and showed weak root systems (Figure 3). Culturing from infected tissues identified Bipolaris sorokiniana which causes common root rot and Fusarium sp. which cause root, crown and foot rots. These fungi are favored by drought conditions in Oklahoma during the fall and spring. Dr. Silva and Gary Strickland also reported seeing root rot at Cotton county.
This article was written by Meriem Aoun, Small Grains Pathologist
In my previous update on April 12, I reported barley yellow dwarf virus (BYD) infection on the susceptible wheat variety ‘Pete’ in the BYD nursery in the Stillwater Agronomy Research Station. Last week, Dr. Amanda De Oliveira Silva (OSU Extension small grains specialist) observed yellowing of the leaf tips in most of the hard red winter wheat variety demonstration plots in Stillwater (Figure 1). We performed Enzyme linked immunosorbent assay (ELISA) on symptomatic samples from different OSU wheat varieties including ‘OK Corral’, ‘Strad CL Plus’, ‘Guardian’, ‘Baker’s Ann’, ‘Showdown’, and ‘Breakthrough’. All samples were tested positive for BYD. In these demonstration plots, the variety ‘Uncharted’, which carries two BYD resistance genes, Bdv1 and Bdv2, was the most resistant OSU wheat variety to BYD (Figure 1). Dr. Silva indicated that the plots planted earlier in September were more infected than the plots planted later in October. This shows the importance of breaking the ‘green bridge’ to manage this virus which is transmitted by cereal aphids.
During the last couple of weeks, the Plant Disease and Insect Diagnostic Laboratory at OSU received wheat samples from Garfield, Blaine, Cleveland, and Harper counties in Oklahoma and from an unknown location in Kansas state. I examined these samples and I observed yellowing and streaking indicative of viral infection (Figure 2). I did not observe any symptoms of fungal diseases on the leaves of the received samples. For the samples received from Garfield county and Kansas state, many of the leaves were dry and brown suggesting freeze damage (Figure 3).
Some of these samples were tested using ELISA for several viruses that affect wheat in the Great Plains. The sample from Cleveland county which was collected on the wheat variety OK Corral was positive for BYD. The sample from Kansas state (from the variety ‘Zenda’) and the sample from Garfield county (from the variety ‘WB 4401’) were tested positive for wheat streak mosaic virus (WSM). ELISA for the remaining samples from Blaine and Harper counties is in progress but the symptoms suggest WSM infection.
This article was written by Dr. Meriem Aoun, Small Grains Pathologist
During the first and second week of April, some wheat diseases appeared in Oklahoma. For example, in the Stillwater Agronomy Research Station, I observed high powdery mildew infection on the susceptible wheat variety ‘OK Bullet’ (Figure 1). Similarly, Bradley Secraw (Extension educator at Cleveland county; March, 31, 2022) found little powdery mildew infection on the variety ‘OK Corral’ which is moderately resistant to this disease. In Stillwater and on April 11th, I observed initial stripe rust infection on OK Bullet (Figure 2). Also recall in my previous update of 25-March, I indicated seeing little stripe rust infection in Jackson county. Therefore, I encourage growers to start scouting their fields for these diseases, especially if they are growing susceptible varieties. We will continue to monitor these diseases as we approach flag leaf stage and provide recommendations.
In the Stillwater Agronomy Research Station, I also observed barley yellow dwarf virus (BYD) symptoms on the susceptible wheat variety ‘Pete’. The symptoms appeared as yellow, red/purple discoloration on the leaves as shown in Figure 2. This virus is transmitted from plant to plant by cereal aphids. Enzyme linked immunosorbent assay (ELISA) on a symptomatic sample from Pete was positive for two BYD strains; BYD strain 2 (BYDV-PAV) and cereal yellow dwarf (CYDV-RPV).
In Stillwater, I observed yellowing on the wheat variety ‘Lonerider’. Older leaves were completely chlorotic (Figure 3). Laboratory diagnosis of a sample using ELISA was positive for wheat streak mosaic virus (WSM) which is transmitted by wheat curl mite. This disease is an issue in our region as many wheat varieties growing in Oklahoma are susceptible to WSM.
This article was written by Meriem Aoun, Small Grains Pathologist
Based on my observations in Stillwater wheat fields and communications with multiple county educators in Oklahoma, it is relatively quiet in terms of diseases. In southwestern Texas and during the first week of March, Dr. Amir Ibrahim (Regents Professor & Small Grains Breeder/Geneticist; Texas A&M AgriLife Research) and Dr. Bryan Simoneaux (Research Associate, Texas A&M AgriLife Research) reported infections of stripe rust and leaf rust in naturally infected rust nurseries.
In Castroville, TX (29.3558° N, 98.8786° W) nursery, Drs. Ibrahim and Simoneaux observed a little bit of leaf rust in the lower canopy of the hard red winter wheat variety ‘Jagalene’. In the Uvalde, TX (29.2097° N, 99.7862° W) nursery, they observed some leaf rust on the lower canopy of the hard red winter wheat varieties Jagalene and ‘TAM 110’, however leaf rust infection did not spread uniformly throughout the nursery. They also found good stripe rust infection on Jagalene in Uvalde, TX (Figure 1 & 2).
In southwestern Oklahoma and during the first week of March, Gary Strickland (Jackson County Extn Educator) reported seeing only very little tan spot on bottom leaves but nothing major (in terms of percentage infestation). He also noted a few leaves infected with stripe rust. Gary Strickland mentioned that the major issue he observed was winter grain mites.
In the Stillwater Agronomy Research Station and on 24 March 2022, I am starting to observe symptoms of the wheat soil-borne mosaic (SB)/wheat spindle streak mosaic (SS) virus complex on the susceptible hard red winter wheat variety ‘Vona’ in the SB-SS nursery (Figure 3). However, due to the use of resistant varieties, these viral diseases are not a problem in Oklahoma and the central plains.
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 believethis 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!
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.
Active spore stage of stripe rust
Survival spore stage of stripe rust
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.”