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David Marburger

David Marburger

Since April 2016, I have served as the Small Grains Extension Specialist at Oklahoma State University. My research and extension efforts focus on delivering science-based recommendations in order to increase small grains production and profitability for stakeholders throughout Oklahoma and the southern Great Plains.

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Wheat Disease Update – March 25, 2017

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

Department of Entomology & Plant Pathology

Oklahoma State University – 127 Noble Research Center

405-744-9958

 

Foliar wheat diseases remain relatively quiet in Oklahoma.  On scouting done around Stillwater and from trips taken Thursday and Friday to west and a bit northwest of Oklahoma City and northwest up to Enid, the only foliar diseases of any consequence observed were leaf spot diseases (e.g. tan spot) in no-till fields (Figure 1A).  Most of the wheat I saw was between 2 nodes readily apparent (GS 7) but ranged up to GS 8-9 (flag leaf just emerging to fully emerged).  In some fields, powdery mildew was moderately severe on lower leaves in “hot spots,” but again, not at a high frequency.  I saw no stripe and only sparse leaf rust, but Dr. David Marburger had a photo of stripe rust (Figure 1B) sent to him from a grower (Anderson Farms) that indicated the photo was taken from experimental plots at the Noble Foundation near Ardmore (Carter County).  Apparently there was not much stripe rust, but there were low to moderate levels of leaf rust (Figure 1C).

figure1afigure1bfigure1c

Figure 1. (A) Tan spot symptoms as seen on wheat foliage in no-till fields; (B) stripe rust as seen in south central OK (Carter County); (C) a photo of leaf rust to compare with stripe rust.

Final first hollow stem update for the season – Goodwell 3/21/17

First hollow stem (FHS) is the optimal time to remove cattle from wheat pasture. This occurs when there is 1.5 cm (5/8” or the diameter of dime) of stem below the developing grain head (full explanation). Last Friday (3/17), 6 of the remaining 8 wheat varieties reached FHS (Table 1). Joe and Spirit Rider were the final two, and both hit FHS yesterday (3/20). Keep in mind that several factors influence the onset of FHS. These include the wheat variety, location, temperature, available moisture, grazing, and planting date (later sown wheat will typically reach FHS later). The First Hollow Stem Advisor and the updates we provide give an indication of the FHS stem conditions in a particular area. However, because of the number of factors that can influence when FHS occurs, we cannot stress enough the importance of checking for FHS on a field-by-field basis. 

 

Table 1. First hollow stem (FHS) results by variety collected from non-grazed, irrigated plots at Goodwell on 3/9/17, 3/11/17, 3/13/17, 3/15/17, 3/17/17, and 3/20/17. Plots were sown on 10/6/16. The threshold target for FHS is 1.5 cm (5/8” or the diameter of a dime). The amount of hollow stem for each variety represents the average of ten measurements. Varieties that have reached FHS are highlighted in red.

table 1 goodwell 3.21.17

Assessing freeze injury on wheat

Temperatures in Oklahoma over the weekend (Fig. 1) and on Tuesday night into Wednesday morning (Fig. 2) dipped low enough to potentially cause some level of injury to the wheat crop. There were a number of areas that spent at least a couple hours with temperatures in the mid to lower 20s.

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Figure 1. Hours spent below freezing (32°F) last week (3/6 – 3/12). Most of these cold temperatures occurred from 3/11-3/12.

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Figure 2. Minimum air temperatures during Tuesday night (3/14) into Wednesday morning (3/15).

 

Keep in mind that the temperature recorded by the nearest weather station or at your house may not quite reflect the actual temperature that the wheat canopy experienced. Factors such as elevation and topography can influence the temperature, as well as things like large amounts of residue in a no-till situation, for example. Therefore, it is important to monitor each field.

 

What temperatures can damage wheat plants? This will depend on the growth stage. The susceptibility and temperature threshold to freeze injury steadily increases as we progress throughout the spring from jointing to heading and flowering. Earlier maturing varieties may be injured more from these recent freeze events than later maturing varieties because they are likely more advanced. Anecdotal evidence suggests there are varietal differences in resistance to spring freeze injury, but this is likely due to differences in plant growth stages when the freeze event occurred. Figure 3 provides a general guide, or rule of thumb, to the minimum temperature threshold and its impact on yield. It is difficult to have exact numbers because each freeze event is unique. While a field at the jointing growth stage could have spent two hours at 24° F, it is possible that the same amount of injury could occur with a 28° F temperature that was sustained for a longer period of time.

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Figure 3. Temperatures that can cause injury to winter wheat at different growth stages. Source: Kansas State University.

 

Another important thing to keep in mind is that we need to be patient before going out to assess freeze injury. The extent of a significant freeze event may not be apparent 1 or 2 days after. If warm temperatures return quickly, you should still wait about 5-7 days after the freeze event before determining the injury. If temperatures remain cool, it may take 10-14 days before the extent of the injury can be fully assessed. With our recent cold temperatures occurring earlier this week, beginning to check plants this weekend and into early next week should suffice, especially with the warmer temperatures forecasted.

 

What are some freeze injury symptoms to look for? With these recent freeze events, the most likely symptom will be leaf tip “burning” or “die-back” (Fig. 4). This is very often just cosmetic and will not hurt yield in the end. In areas that dipped into the lower 20s, damage can result in the entire leaf turning yellow to white, and the plants becoming flaccid (Fig. 5). You may even notice a “silage” smell after several days.

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Figure 4. Leaf tips which have turned brown (necrotic) due to freezing temperatures. Photo courtesy of Josh Bushong, OSU northwest area Extension agronomist.

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Figure 5. More severe freeze damage causing the leaves to turn yellow-white with plants losing their overall turgidity. Source: Kansas State University.

 

The most important plant part to check though is the growing point. To get a look at the growing point, you can slice the stem open lengthways. A healthy growing point will have a crisp, whitish-green appearance and will be turgid (Fig. 6). Often, you can lightly flick the head. If it does not break and bounces back, it is still healthy. If it is mushy, limp, and parts of it break off when you lightly flick it, then it has been compromised and will likely soon turn necrotic. Sometimes plants can appear healthy overall, but the growing point has been damaged or killed (Fig. 7-8). Another indication that the growing point has been compromised is the next emerging leaf is already necrotic.

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Figure 6. Close up of a healthy wheat head (growing point). Photo: Brenda Kennedy and Dr. Carrie Knott, University of Kentucky.

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Figure 7. Plants that appear healthy could have damaged heads. Photo: Dr. Jeff Edwards.

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Figure 8. A close up view of the damaged wheat head from Figure 7.

 

The last comment I want to make is the percent of damaged heads that you find may not translate into percent yield loss, especially with the growth stage a lot of our wheat is at. There is still opportunity for wheat even at the jointing stage (GS 6) to produce additional tillers and/or retain secondary tillers. Whether or not these tillers are able to compensate for larger tillers that were lost due to freeze will highly depend on the subsequent weather. If conditions are favorable, there is a chance for late emerging tillers to have a shot at producing grain. If the wheat is more advanced though, it will be tougher to make this type of recovery.

 

First Hollow Stem Update – Goodwell 3/16/17

First hollow stem (FHS) is the optimal time to remove cattle from wheat pasture. This occurs when there is 1.5 cm (5/8” or the diameter of dime) of stem below the developing grain head (full explanation). Since our last post on Tuesday, 8 of the 16 wheat varieties measured yesterday by Dr. Tracy Beedy and her crew have reached FHS (Table 1). Keep in mind that several factors influence the onset of FHS. These include the wheat variety, location, temperature, available moisture, grazing, and planting date (later sown wheat will typically reach FHS later). The First Hollow Stem Advisor and the updates we provide give an indication of the FHS stem conditions in a particular area. However, because of the number of factors that can influence when FHS occurs, we cannot stress enough the importance of checking for FHS on a field-by-field basis. 

 

Table 1. First hollow stem (FHS) results by variety collected from non-grazed, irrigated plots at Goodwell on 3/9/17, 3/11/17, 3/13/17, and 3/15/17. Plots were sown on 10/6/16. The threshold target for FHS is 1.5 cm (5/8” or the diameter of a dime). The amount of hollow stem for each variety represents the average of ten measurements. Varieties that have reached FHS are highlighted in red.

table 1 3.16 goodwell

Wheat Disease Update – March 15, 2017

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

Department of Entomology & Plant Pathology

Oklahoma State University – 127 Noble Research Center

405-744-9958

        I have not sent out an update since February 21 for several reasons, but the primary one is that wheat diseases in Oklahoma have been sparse. Wheat around Stillwater (STW) is at growth stage 6 or 7 and is showing good growth. On March 9, I looked at wheat in a loop from STW west to Marshall (30 miles west of STW) and Hennessy (50 miles west of STW), then south to Kingfisher and finally to El Reno (25 miles west of OKC). At that time and in the 8-10 fields/variety trials I stopped at, the only disease I saw was powdery mildew at a low incidence. On March 15, Patrick Rydzack (PLP graduate student), Branden Watson (Plant & Soil Science graduate student), and I visited trials at Chickasha (30 miles southwest of OKC), Apache (35 miles southwest of Chickasha), and again in El Reno. We found a few small pustules of leaf rust at Chickasha and some light to moderate powdery mildew (especially at Apache), but overall the foliage was green. Wheat at Apache was GS 7 (2 nodes at base of stem). These observations and other input from around the state indicate that leaf rust is present only in trace amounts, and stripe rust has yet to be observed in Oklahoma.

        In contrast, I have seen tan spot and have heard several reports of leaf spot diseases in no-till fields. On March 13, Josh Bushong (NW Area Extn Agron Speclt), Corbin DeWitt (Extn Edctr-Kay County), and I visited a field near Ponca City that had severe tan spot on the lower leaves; in my estimation, sufficiently severe to merit a fungicide application. I also have had similar situations described to me from fields in Kiowa County (southwest OK), but have not directly seen them. Typically tan spot/Septoria/Stagonospora (the leaf spot foliar diseases) are more severe in no-till fields where wheat residue has been retained on the soil surface. The leaf spots on lower leaves (Figure 1) can be severe and will continue to move up the foliage as long as moisture and temperature are favorable.

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fig1b

Figure 1. (top picture) Severe tan spot on lower wheat leaves. Note wheat residue on soil surface in the background.  (bottom picture) A close up of wheat residue with ‘pseudothecia’ (i.e., spore containing structures) of the fungus that causes tan spot.  Spores are released from these structures that infect the lower wheat leaves.

        If infection is severe on lower leaves, spraying with a fungicide (this early in the season I would go with the lesser expensive generic fungicide) will help limit spread of these leaf spot diseases to older foliage. For additional information regarding early season foliar wheat diseases and possible control with an early fungicide application, see our fact sheet (PSS-2138) that discusses split application of fungicides by clicking here or visiting the wheat.okstate.edu website.

Reports/excerpts of reports from other statesI received a report of leaf rust in south Texas from Dr. Gary Odvody (Texas A&M AgriLife Research & Extension Center, Corpus Christi, TX). However, this report indicated only that leaf rust was observed and mostly focused on severe oat crown rust, which will not infect wheat.

Army Cutworms Reported in Some Wheat Fields (and Check Your Canola)

This article was written by Dr. Tom A. Royer, Extension Entomologist.

 

I have received two reports of army cutworms infesting wheat and one report of them infesting canola.

 

Army cutworms overwinter in Oklahoma. They tolerate cold temperatures and feed throughout the winter months. Adult army cutworm moths migrate to Oklahoma each fall from their summer residence in the Rocky Mountains. They usually seek bare or sparsely vegetated fields (like a newly prepared field ready for sowing wheat, or a field that was “dusted in” but not yet emerged). Just because army cutworm moths prefer to deposit eggs in bare soil, it doesn’t mean that no-till fields with residue are safe from infestations; so ALL wheat fields need to be scouted.

 

Moths lay eggs from August through October that hatch soon after being deposited. This explains why a producer often sees different sizes of larvae in a field. Army cutworms feed throughout the winter and molt seven times before they turn into pupae in the soil. Most larvae will be gone by late March, and adult moths begin emerging in April and fly back to the Rocky Mountains to spend the summer.

fig 1 - wheat field

Army cutworm damage in a wheat field

 

Army cutworms can cause severe stand loss of wheat and canola if not controlled. Cutworm damage often goes unnoticed through the winter because the caterpillars grow slowly and don’t get big enough to cause noticeable damage until temperatures warm in the spring. Unfortunately, if wheat is not growing rapidly because poor growing conditions due to drought (which cutworms also like), they can be even more devastating.

 

So it becomes important to check the fields for cutworms. If you notice a field at this time of year with a numbers of starlings or black birds feeding in a concentrated area of your wheat field, they are likely feasting on army cutworms!

fig 2 - cutworm on canola

Close up of an army cutworm larva

All wheat fields need to be scouted NOW. Sample a field by stirring or digging the soil to a depth of two inches at five or more locations. Also, turn over those dried up cow patties, as they are a favorite hiding place for army cutworms. The cutworms will be “greenish grey”, and will probably curl up into a tight “C” when disturbed. A suggested treatment threshold is 2-3 caterpillars per foot of row when conditions are dry or 4-5 caterpillars per row-foot in fields with adequate moisture.

 

Control suggestions are listed in Current Report-7194 Management of Insect and Mite Pests in Small Grains or E-832, the 2015 OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control.

 

Army cutworms are also a potential pest of canola. Scout fields just as you would in wheat. The suggested treatment threshold for cutworms in canola is 1-2 per row-foot. Current recommendations for control of army cutworms in canola are listed in CR-7667, Management of Insect and Mite Pests in Canola:  http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-3045/CR-7667web2017.pdf.

First Hollow Stem Update – Goodwell 3/14/17

First hollow stem (FHS) is the optimal time to remove cattle from wheat pasture. This occurs when there is 1.5 cm (5/8” or the diameter of dime) of stem below the developing grain head (full explanation). Dr. Tracy Beedy, area Extension agronomist for the panhandle region, and her crew recently started collecting FHS measurements for 30 wheat varieties planted at the Oklahoma Panhandle Research & Extension Center. Results, including measurements taken yesterday (3/13), are listed below in Table 1. Keep in mind that several factors influence the onset of FHS. These include the wheat variety, location, temperature, available moisture, grazing, and planting date (later sown wheat will typically reach FHS later). The First Hollow Stem Advisor and the updates we provide give an indication of the FHS stem conditions in a particular area. However, because of the number of factors that can influence when FHS occurs, we cannot stress enough the importance of checking for FHS on a field-by-field basis. 

 

Table 1. First hollow stem (FHS) results by wheat variety collected from non-grazed plots at Goodwell on 3/9/17, 3/11/17, and 3/13/17. Plots were sown on 10/6/16. The threshold target for FHS is 1.5 cm (5/8” or the diameter of a dime). The amount of hollow stem for each variety represents the average of ten measurements. Varieties that have reached FHS are highlighted in red.

table 1 goodwell

Mites in Wheat Fields

Areas of Oklahoma with dry conditions have begun to see to some visible wheat injury due to brown wheat mite and winter grain mite. Late last week, Darrell McBee, Harper County Ag Extension Educator, indicated to me that he had been fielding quite a few phone calls in regards to mites in wheat. Other areas, including the panhandle and areas closer to the north central part of the state, have also reported some mite injury.

 

The remainder of this article was written by Dr. Tom Royer, Extension Entomologist. He has provided comments below on how to identify these two different mite species, what symptoms to look for, and how to control them:

 

There are two common mites that can injure wheat, the brown wheat mite and the winter grain mite. Producers need to remain alert so that they don’t mistake damaged wheat from small grains mites for drought or virus disease.

brown mite

Brown wheat mite with oversummering egg

 

Brown wheat mite is small (about the size of this period.) with a metallic brown to black body and four pair of yellowish legs. The forelegs are distinctly longer that the other three pair. Brown wheat mites can complete a cycle in as little as 10-14 days. Oklahoma experiences multiple generations of brown wheat mite that usually peak in spring and the last generation occurs in April. At that time, females produce a whitish egg that will over summer.

winter grain mite

Winter grain mite

winter grain mite egg

Winter grain mite egg

 

Winter grain mite is small (about the 1 mm long) with a dark blue to black body and four pair of orange-red legs and a small reddish spot on the top of its abdomen that can be seen under magnification. WGM eggs are kidney-shaped, and change from clear, to yellow to reddish-orange after several days. They are laid on leaf blades and stems or the roots near the crown. Besides wheat, many grasses serve as host plants, including barley, oats, ryegrass, and fescue. We typically experience two generations each year a fall generation and a winter generation that cycles out in March.

brown mite feeding

Leaf stippling from brown wheat mite feeding

field infested with winter grain mite

Field infested with winter grain mite

 

Both mites feed by piercing plant cells in the leaf, which results in “stippling”. The leaves take on a characteristic brown-grayish or cast and could be mistaken for injury due to herbicide. These mites are more likely to cause injury in wheat that is stressed from lack of moisture or nutrients.

winter grain mite hiding in residue.jpg

Winter grain mite hiding in residue

 

Brown wheat mites are not light sensitive, but are vulnerable to driving rains of more than 0.25 inches, which tend to reduce populations. Winter grain mites are more tolerant of rainfall, but are very light sensitive and tend to avoid bright, sunny days and windy days, so adjust your scouting accordingly. It is best to scout for winter grain mite on still, cloudy days or early morning/late evening. On sunny or windy days, they hide under the soil surface (up to a couple of inches) or congregate under dirt clods. Both mites are associated with continuous wheat production. Research suggests that brown wheat mite can be economically treated when there are 25-50 mites per leaf in wheat that is 6-9 inches tall. An alternative estimation is “several hundred” per foot of row. The best recommendation for winter grain mite is to treat when plants show visible injury and there are still mites present.

 

Only a few insecticides include either mite species on their label. Work conducted by Dr. Gerald Wilde at Kansas State evaluated several insecticides for control of winter grain mites. Of those actually registered for winter grain mite, the insecticides dimethoate (Dimethoate and other generics) and chlorpyrifos (Lorsban and other generics) were effective. Other insecticides, lambda cyhalothrin (Karate and its generics) and beta cyfluthrin (Baythroid and its generics) were also effective at the high registered rate, even if they are not specifically listed on the label.

 

For more information on these mites consult fact sheet EPP-7093 Mites in Small Grains by clicking here. If you find active mite infestations in your field, consult fact sheet CR-7194 Management of Insect and Mite Pests in Small Grains for registered insecticides, application rates, and grazing/harvest waiting periods by clicking here.  Both fact sheets can also be obtained from any County Extension Office, or found at the OSU Extra Website at http://pods.dasnr.okstate.edu.

 

 

Greenbugs and Bird Cherry-Oat Aphids in Wheat: Decisions…

This article was written by Dr. Tom A. Royer, Extension Entomologist

 

Heath Sanders, OSU southwest area agronomist, reports of some wheat fields infested with bird cherry-oat aphids. I have seen low levels of greenbugs in some of our demonstration plots as well. The decision to control aphids is especially important right now so a producer can decide to add an insecticide with their top-dress fertilizer. Greenbug infestations results in visible injury to the plants, but bird cherry-oat aphid infestations do not produce visible damage and may go unnoticed.

fig1

My suggestion is to scout the field beforehand to determine if there are GROWING numbers of either aphid that could be of concern. While scouting, keep track of Lysiphlebus mummies. Glance n’ Go accounts for aphid parasitism from Lysiphlebus wasps. If 5-10% of bird-cherry oat aphids are mummies, more than 90% of the rest are also parasitized, and control is probably not warranted.

fig2

If greenbugs are present, use Glance n’ Go to scout. At current prices of $3.00 or $4.00 per bushel, and control costs of $4.00 to $10.00 per acre, you should select the spring Glance n’ Go forms on this link: http://entoplp.okstate.edu/gbweb/spring%20glance%20n%20go3.htm using the following guidelines:

pictable2

If aphids are mostly bird cherry-oat aphids, count the number of aphids on each of 25 randomly selected tillers across a zigzag transect of the field and note mummy activity. Unpublished research provided by Dr. Kris Giles (OSU) and Dr. Norm Elliott (USDA-ARS) combined with studies on spring wheat from the Dakotas and Minnesota indicate that 20-40 BCOA per tiller causes 5-9% yield loss before wheat reaches the boot stage. My suggestions: if BCOA numbers average 10-20 per tiller, figure on a 5% loss, if 20-40 per tiller, figure a 7% loss, and if BCOA aphids are more than 40 per tiller, figure a 9% loss.

 

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Here is a Table of Preventable Loss estimates for bird cherry-oat aphids for expected yields of 30 to 50 bushels per acre, expected wheat prices of $3.00, $3.50, and $4.00 per bushel, and bird cherry-oat aphid numbers of 10-20, 20 to 40, and over 40 per tiller.

picture-table1

Check  CR-7194, “Management of Insect and Mite Pests in Small Grains” for registered insecticides, application rates, and grazing/harvest waiting periods. It can be obtained from any Oklahoma County Extension Office, or found at the OSU Extra Website at http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2601/CR-7194web2008.pdf

First hollow stem update – Chickasha 3/4/17

First hollow stem (FHS) is the optimal time to remove cattle from wheat pasture. This occurs when there is 1.5 cm (5/8” or the diameter of dime) of stem below the developing grain head (full explanation). Because of the progress the plants have now made at Chickasha, this update will be our final one for this location. Thirty-one of the 36 wheat varieties have reached or significantly surpassed the 1.5 cm threshold (Table 1). Given the measurements of the five remaining wheat varieties that have not yet reached FHS as of yesterday, I suspect they will reach the threshold by the time we collect our measurements on Monday. As a reminder, these measurements were collected from plots under simulated grazing. Grazing delays FHS, which is why we recommend checking plants from a non-grazed area of the field (e.g., just outside the hotwire). This helps provide time for finalizing plans to remove the cattle as the grazed area reaches FHS. Keep in mind that several factors in addition to grazing influence the onset of FHS. These include the wheat variety, location, temperature, available moisture, and planting date (later sown wheat will typically reach FHS later). The First Hollow Stem Advisor and the updates we provide give an indication of the FHS stem conditions in a particular area. However, because of the number of factors that can influence when FHS occurs, we cannot stress enough the importance of checking for FHS on a field-by-field basis. 

 

Table 1. First hollow stem (FHS) results by variety collected from simulated grazed plots at Chickasha on 2/16/17, 2/22/17, 2/28/17, and 3/3/17. Plots were sown on 9/15/16. The threshold target for FHS is 1.5 cm (5/8” or the diameter of a dime). The amount of hollow stem for each variety represents the average of ten measurements. Varieties that have reached FHS are highlighted in red.

table-1-chickasha-3-3-17

*triticale variety