Home » disease

Category Archives: disease

Wheat Disease Update – 12 May 2022

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).

Figure 1. White, stunted, and drought-stressed plants showing symptoms of root/crown/foot rots (Cherokee county, Oklahoma; photo credit: Dr. Amanda Silva; May 12, 2022).
Figure 2. Pink discoloration indicates that root/crown/foot rot was caused by Fusarium (Cherokee county, Oklahoma; photo credit: Dr. Amanda Silva; May 12, 2022).

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.

Figure 3. Stripe rust on a susceptible OSU winter wheat breeding line in the Stillwater Agronomy Research Station, Oklahoma (the photo was taken on May 10, 2022).
Figure 4. Initial leaf rust pathogen infection on a winter wheat OSU breeding line in the Stillwater Agronomy Research Station, Oklahoma (the photo was taken on May 10, 2022).

Wheat Disease Update – 28 April 2022

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.

Figure 1. Wheat streak mosaic virus symptoms on the wheat variety ‘OK Corral’ (Grady County, April 13, 2022).
Figure 2. Symptoms of barley yellow dwarf infection on the wheat variety ‘OK Corral’ in a farmer field in Cleveland County, OK (the photo was taken by Bradley Secraw, extension educator, at Cleveland County on April 26, 2022).

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 varietyDoublestop 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.

Figure 3. Common root rot and Fusarium root, crown and foot rots in a wheat sample from the wheat variety ‘Doublestop CL Plus’ (Blaine County, April 13, 2022).

Wheat Disease Update – 21 April 2022

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.

Figure 1. The photo on the left shows Barley yellow dwarf (BYD) infection on the wheat variety OK Corral. The photo on the right shows the BYD resistant variety Uncharted which was planted next to other infected plots. The photos were taken in the hard red winter wheat variety demonstration plots in Stillwater, OK on April, 15, 2022.

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.

Figure 2. Symptoms of wheat streak mosaic virus on the leaves of the wheat variety ‘WB 4401’ (Garfield county, Oklahoma on April 13, 2022).
Figure 3. Freeze damage on the wheat variety ‘WB 4401’ (Garfield county, Oklahoma, photo by Kevin Brown on April 11, 2022).

Wheat Disease Update – 25 March 2022

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).

Figure 1. Leaf rust and stripe rust infections on the same leaf of the susceptible wheat variety Jagalene at Uvalde, TX (Photo by Dr. Bryan Simoneaux on 3 March 2022).
Figure 2. Stripe rust infections on the susceptible wheat variety Jagalene at Uvalde, TX (Photo by Dr. Bryan Simoneaux, on 3 March 2022).

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.

Figure 3. Symptoms of wheat soil-borne mosaic/wheat spindle streak mosaic virus complex on the susceptible wheat variety Vona in Stillwater, OK

Considerations to Make before Planting Wheat this Fall – Wheat Disease and Pest Update – 09/07/2021

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).

Figure 1. Wheat curl mites and symptoms of wheat streak mosaic.
Figure 2. A commercial wheat field (right) growing adjacent to a field (left) in which volunteer wheat and grassy weeds were not controlled until the spring. The commercial field begin to show WSM symptoms in late March and the disease became severe as the spring progressed.

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.;  layton@34starpublising.comhttps://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.

Figure 3. Spot in field (left) of barley yellow dwarf (BYD) as would be seen in March or April. Many types of aphids (for example, greenbug; right) transmit the viruses that cause BYD.

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.; layton@34starpublising.comhttps://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).

Figure 4. Adult Hessian fly (left) and larvae and pupae of the Hessian fly (right).
Figure 5. Emergence of Hessian fly in Oklahoma by month from 2011-2013.

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).

Figure 6. A healthy plot of wheat in the fall as a result of using a seed treatment (left); a poor stand of wheat in the fall in a non-treated plot; a healthy seedling (left) compared to two seedlings (center and right) showing symptoms of common root rot. Notice the darkened sub-crown internode on the seedlings in the center and on the right as well as the reduced top growth compared to the healthy seedling on the left.
Figure 7. White heads indicative of root rot (left); darkened roots indicative of take all root rot (center); wheat killed by dryland root rot split open to show the pinkish growth of the causal fungus, Fusarium (right).

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. 

Symptoms like “window pane” in the leaves indicate feeding from fall armyworm.

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.

Wheat Disease Update – 9 June 2021

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. 

Figure 1. A field view of sooty mold (black head mold) on wheat at Stillwater, OK on June 9, 2021 (top photo). Note the darker appearance of the wheat heads in the alternating long, solid strips of wheat compared to lighter, more golden colored heads in the middle strip. The bottom two photos show wheat heads with sooty mold. [Photo credits bottom two photos – Left photo; Greg Highfill (Extn Educator, Alfalfa County); photo on right; Brad Secraw (Extn Educator, Cleveland County)].

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.

Figure 2. Wheat kernels with black point. The wheat kernels to the left and right show typical black point. The kernel in the middle is healthy. Ignore the reddish-pink color in the outer kernels as this is from an applied seed treatment.

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!!!

Wheat Disease Update – 21 May 2021

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.

Figure 1. Stripe and leaf rust both observed on wheat at Lahoma on May 13/14. A mixture of stripe and leaf rust (photo on left) compared to mostly all leaf rust (middle and right photos). [Photo credits – Dr. Amanda de Oliveira Silva; OSU Small Grains Agronomist].

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.

Figure 2. Darkened heads observed on wheat heads in a trial located in eastern Oklahoma near Morris. These darkened heads were caused by Septoria/Stagonospora, black chaff and/or freeze. [Photo credits – Dr. Amanda de Oliveira Silva; OSU Small Grains Agronomist].
Figure 3. Black chaff (bacterial streak) on wheat at Chickasha in 2013. Photo on the left is of a leaf infection; center photo shows darkening of the head and the stem just beneath the head; photo on the right shows the alternating dark and light pattern often seen on awns of wheat heads infected with the bacterium that causes black chaff. [Photo credits: Dr. Jeff Edwards; Oklahoma State University]

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.

Figure 4. Symptoms of take all root rot. White heads (photo on left) as the plants mature often indicate presence of a root rot. Lower, blackened stems and crowns of tillers with white heads resulting from take all root rot (photo on the right).

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

Wheat Disease Update – 10 May 2021

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.

Figure 1. Tan spot (photo on left) observed on May 3rd by Dr. Amanda Silva (OSU Small Grains Agronomist) in the variety trial at Homestead, OK in west-central OK. Center and photo to the right show stripe rust that has infected and is sporulating in a wheat head. The photo credit for these two photos goes to Leon Fisher and came to me via Jerry Goodson and Mike Schulz (Station Supt, Altus).

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

Wheat Disease Update – 23 April 2021

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).

Figure 1. Severe stripe rust in a susceptible variety (Pete) planted next to resistant breeder lines in a nursery at Stillwater, OK (photo on the left). The photo on the right shows the severity of stripe rust pustules on an individual leaf of Pete.

      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.

Figure 2. Severe leaf rust on Triumph 64 wheat near Perkins, OK (about 20 miles south of Stillwater, OK). Top two photos show the yellowish cast to the foliage. The middle photo shows stripe rust pustules associated with severe chlorosis (yellowing) of the foliage. The bottom photo shows Jeff’s tractor after applying a fungicide spray two days ago.

      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/

Wheat Disease Update – 14 April 2021

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.

Figure 1. Wheat leaves observed near Stillwater, OK on 4-13-2021 with powdery mildew and stripe rust (top photo) and with stripe rust (bottom photo).

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

Figure 2. Wheat streak mosaic virus has been observed in northwestern Oklahoma and western Texas.