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Spring freeze damage on wheat – What did this drop in temperature do to my wheat crop?

Amanda de Oliveira Silva, Small Grains Extension Specialist

Temperature has dropped low enough in the past hours throughout some areas of the state to potentially cause some level of injury to the wheat crop (Figure 1). There were several areas that spent at least a couple hours with temperatures in the mid to lower 20s (Figure 2).

Figure 1. Air temperature change (degrees F) over the past 24 hours at each Mesonet station.
Figure 2. Number of hours spent at or below freezing (32F) over the past 48 hours at each Mesonet station.

What are the temperatures that can damage the wheat plants?

This will depend on the growth stage of the plants. 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. Earlier maturing varieties are more likely to be injured from these recent freeze events than later maturing varieties because they are likely more advanced. The susceptibility of wheat plants to freeze injury steadily increases as we progress through the spring from jointing to heading and flowering. Figure 3 below is a general guide to the minimum temperature threshold and its impact on yield. These numbers are not exact but provide a decent rule of thumb. It is difficult to have exact numbers because each freeze event is unique. While a field at the jointing growth could spend two hours at 24 F, it is possible that the same amount of injury could occur with at a 28 F temperature that was sustained for a longer period of time.

Figure 3. Temperatures that can cause injury to winter wheat at different growth stages. Source: Kansas State University publication C646: Spring Freeze Injury to Kansas Wheat

How long should I wait to assess injury?

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 wait about 5-7 days before determining the injury. If temperatures remain cool after the freeze event, it may take 10-14 days before the extent of the injury can be fully assessed.

What are some freeze injury symptoms to look for?

A common freeze injury symptom is leaf tips turning yellow and necrotic (Figure 4). This is very often just cosmetic and will not hurt yield in the end. More severe damage can result in the entire leaf turning yellow to white and the plants become flaccid (Figure 5). You may even notice a “silage” smell after several days.

Figure 4. Leaf tips which have turned necrotic due to freezing temperatures. Photo taken in March 2017 courtesy of Josh Bushong, OSU northwest area Extension agronomist.
Figure 5. More severe freeze damage causing the leaves to turn yellow-white with plants losing their overall turgidity. Source: Kansas State University publication C646: Spring Freeze Injury to Kansas Wheat.

The most important plant part to check is the growing point (i.e. the developing head)! This will be important for areas of the state that have fields with plants that are at jointing or past jointing. Sometimes we can see what look like healthy plants overall, but the growing point has been damaged or killed. 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 be turgid (Figure 6). Often, you can lightly flick the head, and if it bounces back and does not break, it is still healthy. If it is mushy, limp, and breaks or parts of it break off when you lightly flick it, it has been compromised. It may also have a brown color (Figure 7).

Freezing at the boot stage may cause the head to be trapped by the sheaths of the flag leaf resulting in issues with head emergence. Freeze during the flowering stage may result in sterility via death of the anthers (male organ) and consequently poor kernel set and grain yield losses. Another indication that the growing point has been compromised is that the next emerging leaf is necrotic and the lower stems are discolored, with lesions and enlarged nodes.

Figure 6. Close up of a healthy wheat head (growing point) with bright whitish-green color and turgid. Source: Brenda Kennedy and Dr. Carrie Knott, University of Kentucky
Figure 7. Plants that appear healthy could have damaged heads.

Also, the percent of damaged heads may not translate into percent yield loss. There is still opportunity for wheat at the jointing stage 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 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, it will be more difficult to make this type of recovery.

What is the relationship between soil moisture and freeze injury?

A lot of anecdotal evidence suggests drought conditions can make freeze injury worse, and that could very well be the case in some scenarios. Water in the soil is a good buffer to resist temperature swings and can prevent the soil from cooling as quickly as the air around it. Therefore, the temperature at the soil surface of a conventionally tilled field with good soil moisture may not get as cold as a similar field with dry soil conditions for example.

In theory, the plants themselves under drought conditions should actually be able to withstand cooler temperatures than non-stressed plants as less water content in the plant cells increases the solute concentration (i.e., it takes longer for those cells to freeze). Using the conventionally tilled field example above, we cannot automatically say that a field with dry soil conditions will have worse freeze injury than a field with adequate moisture. Also, if the weather conditions during the day(s) prior to the freeze event were warm and sunny, a significant amount of heat may still be radiated from a field with dry soil conditions and provide some buffer against freeze injury.

Final thoughts

Remember that each freeze event is unique and freeze injury needs to be checked on a field by field basis. The amount of injury observed will depend on the growth stage of the plants, how low the temperature got, and how long it stayed at those cold temperatures. Other factors such as elevation, residue cover, and moisture can influence the observed temperature within the canopy as well. Because of the number of influential factors, it is important to check each field. It is possible to have variability in injury symptoms among fields and even within fields.

Resources

Contact your local county Extension office.

For additional read refer to C646 Spring Freeze injury to Kansas Wheat

Wheat Disease Update – 27 March 2020

This post was written by Bob Hunger, Extension Wheat Pathologist

Over the past 7 to 10 days, multiple photos and samples have come to the lab describing wheat fields with yellowing of older/lower wheat leaves, which occasionally has spread to a lesser extent to younger/higher leaves.  The first reaction to this is that these symptoms are the result of leaf spot diseases such as tan spot, Septoria leaf blotch, and Stagonospora nodorum blotch.  Although that often is an accurate diagnosis, in some of these cases I have not been able to isolate the pathogens that cause these diseases.  What follows is my best explanation for this.

First, yellowing and leaf death is the result of natural senescence of the older leaves that then are colonized by saprophytic fungi that lead to a mottling appearance in the leaves.  That is what I believe happened in the sample represented by photos in Figure 1 (photos from Brooke King).    Notice the leaves in the white boxes in the photo on the left.  These leaves have dead tissue that appears quite “old” and has been colonized by saprophytic fungi that add to the mottled appearance of the leaves as can be seen more clearly in the center photo.  A few of these leaves you can see in Figure 1 also have a general yellowing that indicates the leaf is dying and will be colonized by saprophytic organisms including fungi that live only on the dead tissue and do not cause leaf spotting diseases.  I have not been able to isolate pathogenic fungi from leaf samples such as this, and therefore surmise that abiotic factors such as temperature and soil wetness contributed to the leaf yellowing that then led to colonization by saprophytic fungi and other organisms.  Another factor often present in such cases is that there is no or only minimal wheat residue in these fields.  Lack of wheat residue indicates there should be no or very little inoculum of the fungi that cause the leaf spotting diseases mentioned earlier, which indicates that leaf spot diseases are not involved.

Figure 1.  Yellowing and dying of lower leaves most likely the result of leaf senescence followed by colonization of the dead tissue by saprophytic fungi. (photo credit:  Brooke King)

At the other end of the spectrum, are samples that have come in such as those in Figure 2 (Zack Meyer, FMC).  In Figure 2, the leaf in the white box in the photo on the left is typical of tan spot.  Also note the presence of heavy wheat straw residue on the ground and the presence of the black fruiting bodies of the fungus that causes tan spot on the straw residue (photo on the right).    

Figure 2.  Leaf yellowing and spotting in a wheat field that definitely fits the pattern of tan spot. 

Other photos of leaves showing primarily tan spot have been sent in by Gary Strickland (County Educator; Jackson County) in southwestern OK (Figure 3 – photo on the left).  It is likely that Septoria leaf blotch also is present in these samples represented in Figures 2 and 3, and in fact in the photos submitted by Zack Meyer, Septoria is present on one of the leaves as shown in the photo on the right in Figure 3.  Note that leaf spots such as this are not visible in the photos in Figure 1.  That’s not to say there are absolutely no leaf spots present in sample 1, just not at anywhere near the frequency as in photos in Figures 2 and 3.

Figure 3.  Wheat leaves showing typical symptoms of tan spot (photo on left from Gary Strickland), and a leaf showing a lesion typical of Septoria leaf blotch (photo on the right from Zack Meyer, FMC).

So, in summary, leaf spotting diseases are making their presence felt in Oklahoma, which is not surprising given the temperature and moisture we have been experiencing.  This is especially true in no-till, wheat-following-wheat fields where abundant wheat straw residue is present.  This also could be occurring in conventional tilled fields that are wheat after wheat where there still is wheat residue present.  However, in fields where wheat residue is not present, I believe the yellowing and spotting is most likely due to abiotic conditions that led to leaf senescence followed by colonization by saprophytic fungi, or other fungi that are weakly pathogenic.  Only in cases where I can isolate the causal pathogen can I say that a certain disease is involved.  In cases where I cannot isolate a pathogen, than I have to look at abiotic causes.  In fields where leaf spotting diseases are present, an early application of a fungicide should help to manage leaf spot diseases as well as powdery mildew and stripe rust, but a second application may be needed later to help manage leaf rust and possibly stripe rust.

First Hollow Stem update – 2/28/2020

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). To give you a point of reference, the average FHS date over the past 20 years at Stillwater is March 6.

The latest FHS results from our forage trials in Chickasha (Table 1) and Stillwater (Table 2) are listed below. In Chickasha, all varieties except for Doublestop CL Plus have have passed the 1.5 cm threshold. In Stillwater, all varieties have passed the 1.5 cm threshold. We have had a fairly warm winter and plenty of moisture in the soil, which may have hastened plant development and the progress of FHS as compared to the 20-yr average.

The Mesonet 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, it is extremely important to check for FHS on a field-by-field basis

Table 1. First hollow stem (FHS) results for each variety collected at Chickasha. Plots were planted on 09/19/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Table 2. First hollow stem (FHS) results for each variety collected at Stillwater. Plots were planted on 09/18/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

First Hollow Stem update – 2/26/2020

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). To give you a point of reference, the average FHS date over the past 20 years at Stillwater is March 6.

The latest FHS results from our forage trials in Chickasha (Table 1) and Stillwater (Table 2) are listed below. Almost all of the wheat varieties at Chickasha and Stillwater have passed the 1.5 cm threshold.

The Mesonet 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, it is extremely important to check for FHS on a field-by-field basis

Table 1. First hollow stem (FHS) results for each variety collected at Chickasha. Plots were planted on 09/19/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Table 2. First hollow stem (FHS) results for each variety collected at Stillwater. Plots were planted on 09/18/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

First Hollow Stem update – 2/21/2020

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). To give you a point of reference, the average FHS date over the past 20 years at Stillwater is March 6.

The latest FHS results from our forage trials in Chickasha (Table 1) and Stillwater (Table 2) are listed below. Almost all of the wheat varieties at Chickasha and Stillwater have passed the 1.5 cm threshold.

The Mesonet 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, it is extremely important to check for FHS on a field-by-field basis

Table 1. First hollow stem (FHS) results for each variety collected at Chickasha. Plots were planted on 09/19/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Table 2. First hollow stem (FHS) results for each variety collected at Stillwater. Plots were planted on 09/18/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

First Hollow Stem update – 2/18/2020

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). To give you a point of reference, the average FHS date over the past 20 years at Stillwater is March 6.

The latest FHS results from our forage trials in Chickasha (Table 1) and Stillwater (Table 2) are listed below. Most of wheat varieties at Chickasha and Stillwater have passed the 1.5 cm threshold.

The Mesonet 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, it is extremely important to check for FHS on a field-by-field basis

Table 1. First hollow stem (FHS) results for each variety collected at Chickasha. Plots were planted on 09/19/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Table 2. First hollow stem (FHS) results for each variety collected at Stillwater. Plots were planted on 09/18/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

First Hollow Stem update – 2/14/2020

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). To give you a point of reference, the average FHS date over the past 20 years at Stillwater is March 6.

The latest FHS results from our forage trials in Chickasha (Table 1) and Stillwater (Table 2) are listed below. Few wheat varieties at Chickasha and Stillwater have reached or passed the 1.5 cm threshold.

The Mesonet 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, it is extremely important to check for FHS on a field-by-field basis

Table 1. First hollow stem (FHS) results for each variety collected at Chickasha. Plots were planted on 09/19/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Table 2. First hollow stem (FHS) results for each variety collected at Stillwater. Plots were planted on 09/18/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Army Cutworms Reported in Some Wheat and Alfalfa Fields

This article was written by Tom Royer, Extension Entomologist and IPM Coordinator and Kelly Seuhs, Associate Extension Specialist.

Several people, including Lanie Hale, Rob Anderson, and Mike Rosen of Wheeler Brothers and Area Extension Agronomist Heath Sanders have reported possible army cutworm activity. These reports are based on direct observations and noticeable crow and blackbird “gatherings” in some wheat and alfalfa fields in areas of western Oklahoma. Infestation levels were at the “caution” stage at this time and caterpillars measured ¼ to ½ inches.

Army cutworms tolerate cold and feed throughout the winter months. Adult army cutworm moths migrate to Oklahoma each fall (August through October) from their grounds in the Rocky Mountains.  They seek bare or sparsely vegetated fields (like a newly prepared field ready for wheat planting, or a field that was “dusted in” and had not yet or just emerged, or a newly planted alfalfa stand). The eggs hatch soon after deposition.  A producer might see different sizes of larvae in a field due to the long migration period. Army cutworms feed throughout the winter and molt seven times before they turn into pupae in the soil.  Most larvae will have pupated by mid-late March. Adult moths begin emerging in April to fly back to the Rocky Mountains to spend the summer.

Army cutworms can severely damage wheat, canola, and newly planted stands of alfalfa if not controlled. Cutworm damage often goes unnoticed through much of the winter because the caterpillars grow slowly and don’t get big enough to cause noticeable damage until temperatures warm in the spring.  One early indication cutworm presence in a field is the gathering of blackbirds and or crows that seem to be actively feeding. It becomes important to check the fields for cutworms before they cause damage and stand loss.

Figure 1. Wheat stand loss from army cutworm. Figure 2. Cutworm damage to canola

Sample a field by stirring or digging the soil to a depth of two inches at five or more locations.  The cutworms will be “greenish grey”, and will probably curl up into a tight “C” when disturbed. 

It is better to control army cutworms when they are small (½ inch long or less). Army cutworms are very susceptible to pyrethroid insecticides. At this time of year, an insecticide application can be combined with a late winter top-dress nitrogen application.  Suggested treatment thresholds for army cutworms in wheat are 2-3 worms per row foot when conditions are dry and 4-5 per row foot if moisture is adequate. Current recommendations for army cutworm control in small grains are listed in CR-7194, Management of Insect and Mite Pests in Small Grains

It is better to control army cutworms when they are small (½ inch long or less). Army cutworms are very susceptible to pyrethroid insecticides. At this time of year, an insecticide application can be combined with a late winter top-dress nitrogen application.  Suggested treatment thresholds for army cutworms in wheat are 2-3 worms per row foot when conditions are dry and 4-5 per row foot if moisture is adequate. Current recommendations for army cutworm control in small grains are listed in CR-7194, Management of Insect and Mite Pests in Small Grains

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.

In newly seeded alfalfa, the threshold is 1-2 larvae per square foot. In established alfalfa fields, the threshold is 2-4 larvae per square foot and should be adjusted based on the size of the caterpillars (2-3 per square foot if caterpillars are more than ½ inches, 3-4 per square foot if less than ½ inches). Current recommendations for control of army cutworms in alfalfa are listed in CR-7150, Alfalfa Forage Insect Control.

Wheat Disease Update – 13/02/2020

This article was written by Bob Hunger, Extension Wheat Pathologist

This is an early season update to summarize a few items that have come up during this week. To start however, I need to repeat that this past fall and winter have been amazingly lacking in diseases. The Diagnostic Lab only received a few wheat samples during the fall, none of which were found to be associated with a pathogen/disease. Causes included low pH, nutrition, and/or environment. This lack of disease still seems to be the predominate scenario. Around Stillwater, I was not able to find any rust or powdery mildew in any of the trials I examined this week. Additionally, it appears as though foliar disease is absent in south Texas as well as indicated by Dr. Amir Ibrahim (Regents Professor, Small Grains Breeder/Geneticist, Texas A&M University, College Station, TX) who indicated to me that, “It has been really quiet here. We have not seen stripe or leaf rust so far. I doubt the former will be an issue this year since it has not established yet and it is already getting warmer. However, I expect to see heavier leaf rust in mid-April if it continues to be this warm.”

Hence, it appears that early season stripe rust and leaf rust should not be a major concern in Oklahoma. In contrast, leaf spot diseases (especially tan spot) should be watched for if you have wheat planted into wheat residue. Josh Anderson (Senior Research Associate, Noble Research Institute, Ardmore, OK) found tan spot in no-till wheat plots planted into wheat residue near Burneyville in far south-central OK (Figure 1). Tan spot can be damaging to seedling wheat especially when it occurs in emerging spring wheat in northern states. However, tan spot also can be damaging to winter wheat if infection is severe in the spring as plants are coming out of winter dormancy. Often an early season fungicide application is used to control not only tan spot but also early season stripe rust and powdery mildew. Such an early season application (late February/March) will not provide protection from leaf rust later in the season (April/early May). If you do have wheat planted into wheat residue, I highly recommend scouting for the presence of not only tan spot, but other early season foliar diseases such as Septoria and Stagonospora leaf spots, powdery mildew, and early season stripe rust. If any of these diseases are seen as severe in late February or March, applying an early application of a fungicide may be beneficial. Keep in mind however, that the timing for an early season fungicide application does not coincide with the optimum timing for top-dressing with fertilizer. If it is likely that two applications will be used, I recommend making the first application with a lower cost generic and reserve the second application for a higher priced premium fungicide. For a photo guide to wheat diseases, go to: http://dasnr22.dasnr.okstate.edu/docushare/dsweb/Get/Document-11682/E1024%20Wheat%20Disease%20Identification.pdf

Figure 1. Leaf spotting of wheat due to tan spot on wheat growing in a no-till field near Burneyville, OK. Notice the small, tan spot present in many of the lesions as indicated by the arrows. [Photo credit: Josh Anderson, Noble Research Institute, Ardmore, OK]

For more information on fungicide applications, see: CR-7668 (Foliar Fungicides and Wheat Production in Oklahoma) and PSS-2138 (Split versus Single applications of Fungicide to Control Foliar Wheat Diseases)

First Hollow Stem update – 2/5/2020

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). To give you a point of reference, the average FHS date over the past 20 years at Stillwater is March 6.

The latest FHS results from our forage trials in Chickasha (Table 1) and Stillwater (Table 2) are listed below. Few wheat varieties at Chickasha and Stillwater have reached or passed the 1.5 cm threshold.

The Mesonet 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, it is extremely important to check for FHS on a field-by-field basis

Table 1. First hollow stem (FHS) results for each variety collected at Chickasha. Plots were planted on 09/19/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.

Table 2. First hollow stem (FHS) results for each variety collected at Stillwater. Plots were planted on 09/18/19. The threshold target for FHS is 1.5 cm (5/8″ or the diameter of a dime). The value of hollow stem for each variety represents the average of ten measurements from non-grazed plots. Varieties that have reached FHS are highlighted in red.