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

Although reports of powdery mildew continue to come in from around the state, perhaps the more important news is that other foliar diseases have started to become active. On April 24, Septoria tritici blotch (Figure 1) was prevalent on lower leaves throughout the variety trial near Walters, OK. Walters is located in southwestern Oklahoma about 20 miles south of Lawton and 10 miles north of the Texas border. Although interesting, Septoria tritici blotch is not the disease of concern as in this trial there also was active leaf rust on lower leaves (Figure 2) and stripe rust on the leaves just below the flag leaf. Dr. Brett Carver (OSU Professor/Wheat Breeder) and Branden Watson (OSU PaSS Graduate Student) also reported active stripe rust at various levels in trials located near Chickasha, OK in central Oklahoma (Figure 3). The photo from Dr. Carver (the right photo in Figure 3) shows much more severe stripe rust than was seen near Walters. These observations indicate that both stripe and leaf rust are increasing through southern and central Oklahoma. This activity will increase through the coming weeks as the forecast indicates continued moisture (rains and dew) coupled with moderate temperature. Wheat in southern Oklahoma was approaching or was actively flowering, so the option of using a fungicide to protect yield potential either is at hand or may be too late. Typically foliar fungicides should be applied for wheat rust control between flag leaf emergence and complete head emergence (Feekes’ growth stages 8 to 10.5). Some fungicides (e.g., Aproach, Headline, Nexicor, Priaxor, and Twinline) are so labeled. However, some fungicides (Tilt, Quilt Xcel, and Trivapro) are labeled for a later application (Feekes 10.5.4, which is the end of flowering with the kernel watery ripe). Many others no longer have a growth stage deadline, but rather are limited by a pre-harvest restriction. That is, there must be a certain number of days that elapse between application and harvest. For some fungicides (Caramba, Folicur, Proline 480, and Prosaro) this is 30 days. For Absolute Maxx, it is 35 days, and for Aproach Prima, it is 45 days. For some fungicides it is a combination of growth stage and days between application and harvest. For specific information, please consult the label for the fungicide. Most fungicides labeled for wheat rust control must be applied by the start of flowering (Feekes’ growth stage 10.5). The only fungicides I know of that have a label allowing for a later application are Tilt, Quilt Xcel, and Trivapro, which can be applied up to Feekes’ 10.5.4 (end of flowering with the kernel watery ripe). In addition to these application deadlines, there often are required pre-harvest intervals so you must allow for a specific number of days to elapse between application of the fungicide and harvest.  For specific information, please consult the label for the fungicide. Additional information related to foliar fungicides also can be found in OSU Current Reports 7668, Foliar Fungicides and Wheat Production in Oklahoma, which is available at: http://wheat.okstate.edu/wheat-management/diseasesinsects/CR-7668web2018.pdf.

Figure 1.  Septoria leaf blotch on lower wheat leaf in the variety trial near Walters, OK.

Figure 2.  Leaf rust on lower wheat leaves near Walters, OK on April 24, 2018.

Figure 3.  Stripe rust on a wheat leaf (not a flag leaf) near Chickasha, OK on April 24, 2018 (top photo credit to Mr. Branden Watson; bottom photo credit to Dr. Brett Carver).

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

Powdery mildew (Figure 1) continued to be the primary wheat foliar disease this past week in Oklahoma. Around Stillwater, I have seen powdery mildew on the lower and mid-leaves with severities reaching 90% on the lower leaves. A preponderance of powdery mildew also was indicated by the scouting reports sent in from counties across the central tier of Oklahoma to Zack Meyer (Extension Educator; Kingfisher County) where mildew was reported in Washita and Kingfisher Counties at a light (<25%) severity on lower and mid-leaves. Across the northern tier of counties in Oklahoma, powdery mildew was reported to Josh Bushong (Area Extension Agronomy Specialist; northwest district) from Noble and Garfield Counties at light and heavy (>25%) severities on the lower and mid-leaves. Wheat in central Oklahoma was reported at growth stages 8-9 (flag leaf emerging to flag leaf fully emerged). In northern Oklahoma, wheat was extremely variable with growth stages from 2-8 (tillering to flag leaf emerging) being reported. Again, I want to thank all the educators that participated in this pilot program for reporting powdery mildew, leaf rust and stripe rust, and I would encourage more participation to facilitate the warning of these three foliar diseases of wheat.

Figure 1.  Severe powdery mildew on lower wheat leaves.

There were not any reports this last week regarding foliar diseases from Texas, and there still have not been any significant reports of stripe or leaf rust across Oklahoma. There was one report of stripe rust in south central Oklahoma from Anderson Farms located near Ardmore, OK. As you can see in Figure 2, the lower leaf shows a heavy infection of active stripe rust while the top leaf shows a heavily infected leaf that has transitioned to the dormant (telial) spore stage of stripe rust. This happens as temperature rises with both day and night temperature being important. Typically day temperature needs to consistently be above about 75-80 F and night temperature about about 65 F. This transition along with very limited reports of stripe rust in Texas and Oklahoma indicate that stripe rust should not be a major factor in wheat this year in Oklahoma. Leaf rust could still develop, but inoculum will need to increase, and to date, there has not been widespread weather (cool and moist) that favors either rust.

Figure 2.  Active wheat stripe rust (lower leaf) and stripe rust that has transitioned to the telial (dormant) stage on wheat in south-central Oklahoma.  (Photo credit: Anderson Farms near Ardmore, OK).

The only other observation this week has been “spots” of barley yellow dwarf as reported last week. However, after the recent freeze events, these barley yellow dwarf “spots” are more difficult to discern because there is widespread burning of leaf tips from the freeze, which has a masking effect.

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

Powdery mildew (Figure 1) is showing up on lower leaves in fields and trials around Stillwater, and I have also had reports of powdery mildew on lower leaves from Extension Educators around the state. I also have seen ‘hot spots’ indicative of barley yellow dwarf (Figure 2) around Stillwater, but did not find any aphids associated with these spots. From Texas, Dr. Clark Neely(Small Grains and Oilseed Extension Specialist; Texas A&M AgriLife Extension) relayed to me on April 4 that, “Overall, I think we have avoided the stripe rust and what little was around is shutting down now. Leaf rust is around, but seems lighter than normal for the moment.” Across Oklahoma, wheat leaf and stripe rust still are largely absent, although a few “stripes” of stripe rust were found by Zack Meyer here at Stillwater this morning at an Extension Educators in-service training. This lack of the rusts in Oklahoma is supported by the recently implemented scouting program involving Oklahoma Extension Educators. This program asks county educators to look for and report weekly the occurrence of stripe rust, leaf rust, and/or powdery mildew they observe in commercial fields, variety trials, or variety demonstrations located in their counties. This information is reported by county educators from counties across southern Oklahoma to Heath Sanders (Area Extension Agronomy Specialist; southwest district), from counties across mid-Oklahoma to Zack Meyer (Extension Educator; Kingfisher County) and from counties across northern Oklahoma to Josh Bushong (Area Extension Agronomy Specialist; northwest district). The incidence and severity of these three wheat foliar diseases can then be more accurately summarized and disseminated to facilitate decisions related to applying a fungicide to help manage these diseases on susceptible varieties. For the week ending on April 5, observations reported from across southern and mid-Oklahoma (Jackson, Dewey, Washita, Blaine, and Kingfisher Counties) indicated no leaf or stripe rust and only one report of powdery mildew on lower and mid-leaves in Washita County. It is still a bit early for reports to come in from across northern Oklahoma. Thanks are extended to all the educators that participated in this pilot program, and I would encourage more participation to facilitate this reporting program.

Figure 1.  Powdery mildew observed April 5 on lower leaves of wheat in trials around Stillwater, OK.

Figure 2.  Likely barley yellow dwarf (BYD) “hot spot” observed on wheat in early April (top photo). As time proceeds, these hot spots will develop stronger symptoms of BYD including leaf discoloration ranging from yellowing (middle photo) to purpling in some varieties (bottom photo).

We have 32 different stops lined up to talk about a lot of good wheat varieties available to producers. As the weather can force us to change plans at the last minute, please contact your local county Extension office with any questions on the date, time, and location. The schedule listed below is also available on the home page of the wheat.okstate.edu website or by clicking here.

A special thanks goes out to the area and county Extension personnel and grower cooperators on helping get these field days scheduled!

Temperatures on Easter into Monday (Figure 1) and last night into this morning (Figure 2) dipped low enough throughout a large portion of the state to potentially cause some level of injury to the wheat crop. There were a number of areas that spent a significant amount of time with temperatures in the mid to lower 20s over the course of these two cold snaps. Areas in the northwest and into the Panhandle even got as cold as the mid to upper teens. On top of all that, we have cold temperatures forecasted again for later this week.

Figure 1. Minimum air temperatures (top graph) during Easter into Monday (April 1-2) and hours spent below freezing (32°F) over the past 48 hours during that same time frame (bottom graph).

Figure 2. Minimum air temperatures (top graph) during last night into this morning (April 3-4) and hours spent below freezing (32°F) over the past 48 hours (bottom graph).

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, especially as you increase the distance from where the temperature was recorded and the field itself. 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.

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 the wheat plants to freeze injury does steadily increase as we progress throughout the spring from jointing to heading and flowering. Figure 3 listed 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 an assessing 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. Since we still have cooler temperatures in the forecast, we will likely need to wait closer to the 10-14 days.

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. This will be important for areas of the state that have fields with plants which 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). Another indication that the growing point has been compromised is the next emerging leaf is necrotic.

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

Figure 7. Plants that appear healthy could have damaged heads. Photo taken several years ago courtesy of Dr. Jeff Edwards.

Figure 8. A close up view of the damaged wheat head from Figure 7. Photo taken several years ago.

Also, the percent of damaged heads may not translate into percent yield loss. There is still opportunity for wheat 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 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 tougher to make this type of recovery.

Do drought conditions exacerbate freeze injury?

When it comes to this question, it is really a mixed bag of results. 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. 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.