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

The disease situation in Oklahoma has not changed significantly since my last update on May 15^th.  The weather in Oklahoma has remained atypically cool and mostly dry; however, off-and-on rain is forecast over much of the state starting tonight and into next week.  This will provide excellent conditions for continued grain filling, and typically would also provide conditions favorable for foliar diseases to increase.  However, over about the southern half of the state, much of the canopy has turned or is turning and no longer will be threatened from foliar diseases.  For example, Dr. Brett Carver (OSU Wheat Geneticist & Breeder) was in Tipton (southwestern OK) earlier this week and indicated most of the wheat he saw was within about 7-10 days of being ready to harvest.  Here around Stillwater, most of the canopy is turning, and the grain is mostly around late milk to early soft dough.  On wheat such as this, foliar diseases are no longer a concern, and wheat at this stage even with green foliage is way past the time to spray.  Josh Bushong (Area Extension Agronomy Specialist) indicated diseases overall are lacking in northwestern Oklahoma and the Oklahoma panhandle primarily because of dry conditions.

On Tuesday, 19-May, I took a trip to Kildare (north of Ponca City) over to Cherokee (northern OK) and to Lahoma (just west of Enid).  On the same day, Dr. Amanda de Oliveira Silva (Ast Professor & Small Grains Extension Agronomist) started in Alva (about 15 miles west of Cherokee) and then worked her way east to Cherokee and Lamont (north-central OK about 15 miles west of I-35 at the Tonkawa exit, highway 11).  Wheat in a field near Alva and in the variety trials near Cherokee, Lamont, and Kildare were in the late milk to soft dough stage.  In a field near Alva, Dr. de Silva saw very little rust (leaf or stripe) or other foliar diseases (too dry).  At the variety trial near Cherokee, rust (both leaf and stripe) as well as powdery mildew were present but sparse.  In the variety trial near Lamont, Dr. DeSilva did observe moderately severe leaf rust but the leaves, although still having some green area, are turning/dying quickly.  The most common symptoms seen at all these variety trials was the leaf browning/spotting (Figure 1) that I’ve talked about in the last couple of updates.  This browning/yellowing/leaf spotting is curious, because there appears to be diseases involved as well as environmental/weather factors such freeze and perhaps drought and wind.  For example, in the photo on the right in figure one, you can see dead areas consistent with Septoria leaf blotch as indicated by the irregular, small blotchy areas in which pycnidia (small black spots) of the Septoria fungus can be observed.  Many of the other spots appear to be consistent with tan spot symptoms, but the tan spot fungus has been only occasionally isolated from such leaves.  I am continuing to isolate from leaves such as those in Figure 1 to try to ascertain the cause, but remain puzzled at the cause.

So in summary, the time for active foliar diseases in Oklahoma is coming to an end as the crop is maturing and moving toward senescence.  It is possible that foliar diseases such as leaf rust could still appear in northwestern OK and the OK panhandle, but dry conditions have prevented that to this point in time, and the wheat crop in Oklahoma is moving steadily toward harvest.

This article was written by Bob Hunger, Extension Wheat Pathologist

This past week was quite cool across Oklahoma (mid 60s F in Stillwater today) and central Oklahoma received needed rain.  Unfortunately, western OK and the panhandle were not as fortunate and remain dry.  Wheat around Stillwater this week was as far along as the milk to soft dough stage; wheat today at Lahoma in north-central OK ranged considerably but was as far along as approaching full kernel but watery.

Stripe rust has fairly well shut down across the state as indicated by turning to the telial spore stage (see 27-April update at http://entoplp.okstate.edu/pddl/2020/PA%2019-15.pdf), and although this cool weather with rainfall and dew in central OK will favor stripe rust, I doubt if we will stay sufficiently cool and wet for stripe rust to pick up again across Oklahoma (plus, northwestern OK and the panhandle remain dry).  However, leaf rust has started to become more prevalent over this past week and likely will continue to increase in areas where there is rainfall or dew formation.  This past week around Stillwater, both Dr. Brett Carver and I were able to rate plots and trials around Stillwater for leaf rust incidence and severity.  Additionally this past week here at Stillwater for the first time in about a decade, I found wheat stem rust.  Recall there are three wheat rusts including stripe rust, leaf rust, and stem rust.  Stem rust is rare in Oklahoma and is only observed in years when we have an extended cool spring (much like this one).  Figure 1 shows photos comparing these three rusts on wheat leaves.  In contrast to the implication of its name, stem rust can occur on leaves as well as on stems, whereas pustules of stripe rust and leaf rust are more typically found on leaves.

Leaf browning also is still apparent around much of the state and has multiple causes this year as discussed in the 27-Apr Disease update (http://entoplp.okstate.edu/pddl/2020/PA%2019-15.pdf).  These causes include leaf spot diseases, freeze, drought, necrosis (tissue death) due to a resistant reaction to stripe rust, and even a browning due to an unknown physiological factor.  Over this past week, both Dr. Brett Carver and I have observed another possible cause of leaf browning, which is bacterial streak (Figure 2).  Bacterial streak isn’t usually a significant problem in Oklahoma, but occasionally has been observed in low to moderate incidence.  It causes not only a browning of leaves (but without the “pepper spots” as with Septoria/Stagonospora), but also can cause a darkening of the heads (black chaff).  As I said, we have not confirmed bacterial streak to be present, but it is a possibility.

Finally, it is worth mentioning that a number of individuals have reported seeing a higher incidence of wheat heads with loose smut (Figure 3) this year.  This agrees with what Dr. Carver, Dr. Amanda de Oliveira Silva, and I have seen.  Spores of the loose smut fungus (Figure 3, right photo) are released in the field when wheat is flowering.  The spores infect the fertilized seed and are carried within the seed.  If this infected seed is used to plant the next year’s crop, the growing plant from that seed will be infected and produce loose smutted heads in the spring – hence, completing the disease cycle.  The ways to avoid problems from loose smut are to (1) not save seed wheat from fields in which loose smut is present, and (2) plant seed wheat treated with a fungicide labeled for use to manage loose smut.  Many such seed treatments are available.  I need to point out that if you have a field this year with loose smut and you plant treated wheat in this field next year, there may still be some loose smut-infected plants the next spring.  These infected plants would not be from the treated seed wheat, but are volunteer plants from the previously infected crop.  If you have questions about the bunts and smuts please contact your local county extension educator or me.

Amanda de Oliveira Silva, Small Grains Extension Specialist

As time goes by the impact of the Mid-April freeze event becomes more evident across wheat fields in the West, Southwest, and Southcentral regions of Oklahoma. Other regions of the state were lightly or not affected by the freeze.

Wheat should be filling kernels by now, but all we are finding are sterile and “empty” heads (Figure 1). Wheat continue to be green and sometimes look healthy from the outside but is not producing any grain.

A significant freeze-damage on wheat at Roger Mills and Washita counties on the West part of the state may lead to a reduction in harvested acres this year.

 “The April freeze has significantly impacted most Southwest wheat acres. Fields that remained for grain production estimates have some level of freeze damage and in just about all counties hay and graze-out acres have gone up” – Gary Strickland.  

Although multiple fields in the Southwest and Southcentral region were affected by the freeze, the level of freeze damage largely varies across and within fields. At Jackson County for example some fields are severely damaged (80%) and some are not (5%). The south and west sides of the county seem to have a heavier damage. Tilman County is showing an average of 40% damage. The eastern to southeastern and along the south part of the county seems to have the greatest damage (60-80%) and the other parts with as low as 10% of damage. Greer County is showing an average of 33% damage.

Cotton County shows an average of 40% freeze damage. It is possible that only about 50% or less of the wheat planted will be harvested as several fields were already cut for hay.

For more information about crop production and yield estimates across Oklahoma watch the 2020 Crop Reports Webinar here: https://youtu.be/UhxZt4KvgZY

Thanks to our field contributors

Aaron Henson – Tilman County Extension Educator

David Nowlin – Caddo county Extension Educator

Gary Strickland – Jackson and Greer County Extension Educator, SWREC Dryland Cropping System Specialist

Kyle Worthington – Canadian county Extension Educator

Melissa Koesler – Garvin County Extension Educator

Robert Calhoun – Senior Agriculturalist for the OSU Small Grains Program

Ron Wright – Custer County Extension Educator