This article was written by Bob Hunger, Extension Wheat Pathologist

Wheat diseases are active across Oklahoma, but perhaps the bigger story is the freeze damage that Dr. Amanda de Oliveira Silva has observed in south-central and southwestern OK.  For information on that, please see her update at: https://osuwheat.com/

Around Stillwater, wheat is mostly at or just past flowering with kernels beginning to form.  This is similar to the wheat I saw on a trip this past Friday to Chickasha (central OK), Altus and Tipton (southwestern OK).  However, Josh Anderson (Senior Research Associate, Noble Research Institute, Ardmore, OK) indicated to me in an email on 24-Apr that, “Most all local lines are well past flowering and at the soft dough stage.”

Based on that trip as well as samples received and input from others, I would offer the following to help explain the browning and death of upper leaves (flag leaf and F-1 leaf).  One factor is the freeze, which seems to cause a burning or death to the leaf tip up to the outer third of the leaf [see Dr. de Oliveira Silva’s post for photos of this (https://osuwheat.com/)].  Stripe rust is another factor involved, and has been reported to range from a low to a high incidence and severity.  Sometimes this striping has sporulation in it of urediniospores (yellowish-orange color – Figure 1-left photo), teliospores (black spore spots – Figure 1; center photo), or, just striping that is chlorotic that becomes necrotic (Figure 1; right photo).  Based on my recent trip, it appears that stripe rust across central and southern OK is moving from the urediniospore stage to the teliospore stage.  This transition indicates that temperature is becoming too warm for the stripe rust fungus to remain active, so spore production is being switched to teliospores.  Reports of leaf rust have been made [e.g., Josh Anderson (Senior Research Associate, Noble Research Institute, Ardmore, OK) in south-central OK and Gary Strickland (Extension Educator; Jackson County)], but presently the incidence and severity of leaf rust seems to be staying low.  However, an increase in leaf rust could become a reality as temperature increases and if moisture/dews remain common.

Figure 1.  Various expressions of wheat stripe rust observed across Oklahoma the week of April 20th.  Leaf with active sporulation of yellowish-orange urediniospores (left photo; Gary Strickland (Extension Educator; Jackson County), leaf with teliospores (center photo) and leaves with yellowing or dead stripes but with no or little sporulation (right photo).

Septoria leaf blotch (Figure 2) is another disease that is contributing to the upper leaf browning this year.  Typically Septoria leaf blotch on wheat in Oklahoma is restricted to the lower and mid canopy, and only rarely reaches the upper canopy.  This year is one of those exceptions.

Finally, there also appears to be some upper leaf browning and death from a cause not related to a disease/pathogen.  The freeze may be involved, but there also may be some other physiologically related cause (Figure 3).  This discoloration and leaf death has been observed at multiple locations, and although possibly related to a freeze, it does not seem that freeze alone always can account for the damage.  To date, we have not been able to identify a cause for this damage.

A fungicide application will help manage this upper leaf spotting/burning/browning IF the cause is a pathogen/disease such as stripe rust or Septoria leaf blotch.  However, such an application will only protect the remaining green tissue and not be able to reverse any dead area and will not help if the discoloration is due to an environmental factor such as freeze.  Also keep in mind that only certain fungicides can be applied through flowering while others have a required time interval between application and harvest.  Some have both types of restrictions.  Hence, be sure to consult the label to be in compliance with the requirements as described on the label of the fungicide.

Finally, I also should indicated that a few samples have tested positive for the presence of Wheat streak mosaic virus.  One such sample was submitted from Kingfisher County, and another photo of a sample came from the Texas panhandle (Figure 4).  So, some of the mite-transmitted virus diseases such as wheat streak mosaic are present in Oklahoma and I would suspect as temperature increases more fields with symptoms of these diseases likely will appear.

Amanda de Oliveira Silva, Small Grains Extension Specialist

As the weather warmed up last week the damage from the latest freeze event (on April 15) is starting to show in some areas of Oklahoma. A lot of wheat fields were at the flowering stage when the freeze came in, and the near the wheat is to flowering the more sensitive it is to freezing temperatures.

We split up in a group of people to look for potential freeze damage at different counties in South Central and South West Oklahoma last week. Most of the damage we are seeing is death of the flower parts followed by head discoloration (Figure 1). Temperatures got cold enough to kill the flower parts but not the cells in the green tissue, so it is taking a little bit longer for the rest of the plant to start showing the damage. The male parts (anthers) have a pale color and are dead inside of the glume (Figure 2), they are supposed to be green and turgid. In cases where pollination had already occurred when the freeze came in, we could see berries being formed, but those berries are shriveled and dried out (Figure 3).

I went to Chickasha on Monday (April 20) and found a few varieties lodged and with head starting to turn colors. I went back on Friday (April 24) with the Senior Agriculturalist of our program Robert Calhoun, and wow, we got hit! Mesonet station reported that our field spent 5 hours at 30F, and that was more than enough to harm our blooming wheat. There was a visual difference between early vs. late maturing varieties in our trial (Figure 4), but we will be able to truly assess differences from freeze-damage among varieties in the coming days. Other symptoms we found on severely damaged plants was lodging followed by enlarged nodes and split stems (Figure 5).

We found a few wheat fields with up to 100% loss from freeze damage at Garvin county. According to Mesonet, the county experienced about 6 hours at 29F. Secondary tillers were also damaged.

Caddo county spent 6 hours at 27F and wheat fields are just now starting to turn, but the heads are blank with no grain being formed and anthers are dead. At some those fields, secondary tillers were less affected than the heads from the main tillers and were blooming (Figure 6).

We were not able to find freeze injury in wheat fields at Kingfisher and Canadian counties as of April 23, but we will certainly look at more fields in those areas again this week.

Gary Strickland checked about 15 fields on the west side of Jackson county and found from 40 to 70% damage. He also reported more severe damage on the east side of the county ranging from 70 to 100% in the fields checked.

Dr. Todd Baughman checked about 30-40 fields in the Tillman County area and reported an average of 50% wheat loss due to freeze damage, but some fields were at 100% loss. I have also heard from producers that some wheat fields are severely damaged at Cotton County.

Recommendations:

Injury symptoms should become more easily identifiable by the end of this week and growers can assess damage to individual fields. I recommend opening the wheat spikelet and taking a look at the flower parts to see if they continue to be viable. Low-lying areas of the field seems to show more severe damage than higher areas of the field. So, look at different areas throughout the field to help determining the % injury. If injury is extremely variable, increase sample size. 

Talk to your insurance adjuster before making any decision. If you are not so sure about the damage give it a few more days until you can have a more clear picture of possible losses due to the freeze. If you decide to cut your wheat for hay see the recommendations from our Forage Extension Specialist Alex Rocateli below.

Freeze injury seemed to be worst in a combination of fields in lower areas planted with early maturing variety that was right at the flowering period when the freeze came in. Wheat is way more advanced in growth this growing season than usual and that is why we are seen lots of damage due to freeze this year. There is no “recovery” treatment that can be done at this point. For next seasons, producers could consider planting varieties with different maturity range to spread the freeze-injury risk.

We think in a couple days we will have a more clear picture of possible damage in different areas. While it is fairly easy to determine the extent of injury on individual fields, the hit or miss nature of freeze injury this year makes it difficult to estimate the total impact on the Oklahoma wheat crop as a whole.

Considerations for haying freeze damaged wheat by Alex Rocatelli, OSU Forage Extension Specialist

Haying wheat for forage is an option for wheat that was affected by the latest freeze event. However, before making any decision, producers should evaluate the freeze loss in their entire wheat field. Therefore, a field may have severe damage and very low grain yield potential in some areas. At the same time, minor freeze injury may allow worthwhile grain yields in other areas. To date, most of the wheat fields are within late boot to flowering stages. At these growth stages, forage yield is almost maximized, while forage quality remains good (19-15% crude protein/68-66% total digestible nutrients). In other words, this is good hay! However, freeze-damaged wheat degrades very rapidly; therefore, it is recommended to cut, cure, and bale it as soon as possible. Unfortunately, we have chances of rain statewide in the next days, which may cause some rain damage during the curing process. The severity of rain damage in curing wheat hay will depend on rainfall amount and intensity. High rainfall amount pouring for a long time can cause more damage than low amounts pouring in a short time. Finally, freeze-damage kills the wheat head; however, the plant is still alive, taking up nitrate from the soil. Under this condition, wheat plants will not utilize nitrate to produce grain, accumulating it into the stems. High amounts of nitrate in forage can be toxic to livestock; therefore, nitrate forage testing is highly recommended for freeze-damage wheat hay. For more information on nitrate toxicity, follow the link: https://extension.okstate.edu/fact-sheets/nitrate-toxicity-in-livestock.html

Thanks to all field collaborators for this post:

Aaron Henson – Tilman County Extension Educator

David Nowlin – Caddo county Extension Educator

Gary Strickland – Jackson County Ext Center

Kyle Worthington – Canadian county Extension Educator

Melissa Koesler – Garvin County Extension

Robert Calhoun – Senior Agriculturalist Small Grains Program

Sumit Sharma – Assistant Extension Specialist Goodwell

Todd Baughman – Weed Scientist and Research Professor

Amanda de Oliveira Silva, Small Grains Extension Specialist

Temperatures dropped well below freezing in the past hours throughout the state and there is a high potential for freeze injury to Oklahoma wheat (Figure 1). There are several areas that spent at least a couple hours with temperatures in the mid to lower 20s (Figure 2).

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. Keep in mind these temperature thresholds are not exact but provide a decent rule of thumb, and temperatures closer to the soil surface might be higher than those reported by weather stations one meter above the soil surface, especially if moisture is present. 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.

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.

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, right). 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.

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 (Figure 8). The whitish tips of the awns indicate that it was exposed to freezing temperatures and that the flower parts could have been compromised. 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 (Figure 9).

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.

A few points to consider:

Every freeze event is unique and freeze injury needs to be checked on a field by field basis – the temperatures and time durations we use regarding freeze injury are rules of thumb and are not exact. I have seen instances where conventional wisdom would indicate complete crop loss and we skate through with minimal damage.

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.

It will take a few days to see how bad things are – Symptoms may start to appear early next week and will likely be clearly identifiable by the end of next week. Healthy wheat heads will remain turgid with a green color. Damaged wheat heads will be bleached, yellow, or brown and will easily break when pushed against.

Resources

Contact your local county Extension office.

K-State has a good publication about freeze damage on wheat: https://bookstore.ksre.ksu.edu/pubs/C646.pdf

This post was written by Bob Hunger, Extension Wheat Pathologist

Reports from this last week have indicated wheat across much of Oklahoma is at the boot (Feekes growth stage 10) stage with heads just starting to emerge.  That is true around Stillwater, at Chickasha (per Dr. Brett Carver; OSU Professor/Wheat Breeder) and across southwestern OK (per Gary Strickland; County Educator; Jackson County).  Of course, wheat in northwestern OK and the panhandle is not this far along, but also has made considerable progress.

With the mostly cool and wet weather over the last couple of weeks, foliar diseases also have been observed and reported with increasing frequency.  Most prominent among these have been the leaf spotting diseases, namely, Septoria leaf blotch (Figure 1) and tan spot.  Stagonospora nodorum blotch also likely is there, but to date, I have only isolated the fungus that causes Septoria leaf blotch from samples.  These leaf spotting diseases also cause significant yellowing of wheat foliage as more thoroughly discussed in the Pest E-alert distributed on 27-March (http://entoplp.okstate.edu/pddl/2020/PA%2019-11.pdf).  However, what has taken me by surprise and confirmed by multiple sources (Carver at Chickasha and Strickland in southwestern OK) is how high these leaf spotting diseases have moved up the canopy.  Around Stillwater, I have seen symptoms of leaf blotch in the mid-canopy with some symptoms on the leaf just below the flag leaf (F-1 leaf).  Typically these diseases do not move up the canopy to the flag or the F-1 leaf.  If these leaf spotting diseases are up this high in the canopy, a fungicide spray definitely is recommended to protect the flag and F-1 leaves.

In addition to the leaf spot diseases, an increase in stripe rust also has been observed around Stillwater, at Chickasha (Carver), and in southwestern OK (Strickland).  Mostly the increase in stripe rust has been observed as small to large “hot spots” as presented in Figure 2 from Stillwater, but this indicates that stripe rust is present and active in Oklahoma.  Powdery mildew also has increased on susceptible varieties and around Stillwater has moved up into the mid canopy on a number of varieties.

Often what occurs in Texas is what we will eventually see in Oklahoma when it comes to rusts.  Hence, also note what Dr. Amir Ibrahim (Regents Professor & Small Grains Breeder/Geneticist; Texan A&M University) indicated in an update he sent out on 7-Apr.  In this update he indicated that,

“Leaf rust is uniform across the naturally inoculated evaluation nursery at Castroville, TX. (Castroville, TX is about 25 miles west of San Antonio, TX).  Leaf rust is uniform across the spring wheat. Flag leaves of susceptible spring wheats are covered with leaf rust. Leaf rust was rated 90S on the flag leaf of very susceptible spring wheat genotypes.  As of April 6, 2020, ‘TAM 110’ was rated 70S for leaf rust in the upper canopy.”

      In summary, multiple wheat foliar diseases are active in Oklahoma with the leaf spotting diseases, stripe rust and powdery mildew all being present to varying levels.  The forecast for the next 7-10 days is cool to cold with some moisture likely.  These are conditions that will be favorable for all these diseases to continue their activity and enhance their spread.  Leaf rust, although not yet a problem in Oklahoma has become severe in south Texas, which typically means we will be seeing its arrival in Oklahoma within the next couple of weeks.  Wheat across much of the state is at the boot stage with heads starting to emerge, so if a field has good yield potential now would be the time to consider applying a fungicide to protect against multiple foliar diseases.  This is especially true if a susceptible variety has been planted.

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).

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.

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.

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.

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