Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Wheat Pathologist
Oklahoma: Reports of stripe rust were more common from Oklahoma this past week. Yesterday (25-Apr) I and Nathalia Grachet (OSU graduate student) looked at wheat in central Oklahoma to the southwest, west, and northwest of Oklahoma City (OKC). Wheat in this area was variable, but mostly around GS 10 (boot stage) to heads just emerging. Fields where freeze damage occurred showed a wide range of tiller maturity.
Fields around Apache, OK (about 75 miles southwest of OKC) including the variety trial showed light powdery mildew and leaf spotting (tan spot/septoria/stagonospora) with stripe rust found in one field located about 10 miles west of Apache – not severe but the incidence was spread across the field (variety unknown). On our return trip to Stillwater, we found stripe rust on lower to mid leaves in the variety demo at Minco (about 25 miles southwest of OKC) with the most severe rust on Duster (photo below). Powdery mildew was severe on lower leaves of the wheat in the field surrounding the variety demo. No rust was observed at the variety trial at Kingfisher (about 30 miles northwest of OKC), and no leaf rust was found at any stop. Although not severe, stripe rust also was observed this past week around Stillwater/Perkins by Dr. Art Klatt (OSU wheat geneticist/breeder) and by Mark Gregory (OSU Southwest Extension Agronomist) in the variety trial near Chickasha (30 miles southwest of OKC). Dr. Klatt also reported severe powdery mildew in his plots near Perkins. Symptoms indicative of barley yellow dwarf are common around Stillwater and on the trip yesterday, however, freeze damage symptoms make it difficult to comfortably identify BYD without confirmation in the lab.
Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Wheat Plant Pathologist
Just a few updates from this week. First, Mark Gregory (Southwest Area Extension Agronomist) reported the first stripe rust in Oklahoma for 2013 (see photo below). He found this stripe rust near Paul’s Valley on an Endurance/Duster mix, and “had to look hard to find it.”
David Nowlin (Caddo County Extn. Educator) sent in a wheat sample (Duster) exhibiting symptoms typical of wheat streak mosaic. If confirmed, this would be the second sample of WSM received from south central and southwestern OK.
Dr. Art Klatt (OSU Wheat Geneticist) has been this last week in Castroville, TX (far southern TX) rating breeder plots/lines for leaf and stripe rust. He has reported that, “In Castroville there is heavy leaf rust. We also found a little stripe rust but not enough to take readings and it was beginning to shut down due to temps. “ In Oklahoma, Dr. Klatt has reported seeing heavy powdery mildew in plots near Perkins, OK and at Stillwater, but has not yet seen any leaf or stripe rust.
Monday, 15-Apr, I and Nathalia Grachet (OSU Graduate Student) did plot work at the variety trial near Apache, OK. Wheat there was approaching Feekes 10. Flags leaves were out, but for most tillers, heads had not yet moved into the boot. Freeze damage was the most striking symptoms in this trial and surrounding fields, but tan spot/septoria/stagonospora, powdery mildew, and barley yellow dwarf symptoms also could be found – but, no leaf or stripe rust.
Today (17-Apr) I checked my trials and nurseries here at Stillwater. Wheat was at “pre” GS 10 with flag leaves fully out but the heads not yet fully moved into the boots. Even where the boot internode was fully expanded, the head was usually just beginning to move into the boot. I found powdery mildew low, but no rusts at Stillwater. Spots indicative of BYDV have appeared.
By Tom Royer, OSU Extension Entomologist
I received some troubling news from the Texas Panhandle. Dr. Ed Bynum, Extension Entomologist from Amarillo, reported finding some greenbug populations that were shown to be resistant to chlorpyrifos, the active ingredient in Lorsban 4E, and other generic products (Govern 4E, Hatchet, Nufos, Vulcan, Warhawk, Whirlwind). You can read the full article by clicking on this url: http://texashighplainsinsects.net/april-12th-special-edition/ The bottom line: he tested some suspect greenbug populations using a diagnostic test that he developed for testing greenbugs in sorghum in the 1990’s, and found that they were resistant to chlorpyrifos at labeled rates.
This should not raise panic among growers in Oklahoma for two reasons. The first is that I have not heard of or received reports of any control failures for greenbugs in Oklahoma, in fact, generally greenbugs have been pretty scarce this winter.
The second reason is that it is late enough in the growing season to expect that the primary natural control of greenbugs, a tiny wasp called Lysiphlebus testaceipes (see picture to right), keeping greenbug numbers from becoming an outbreak.
The best course of action is to sample winter wheat fields with the Glance ‘n Go system.
Start by going to the Cereal Aphids Decision Support Tool on your computer http://entoplp.okstate.edu/gbweb/index3.htm and selecting the Greenbug Calculator.
By answering a few simple questions, you can determine an economic threshold for controlling greenbugs. This threshold is based on the estimated cost of treating the field and the estimated price of wheat. Once a threshold is calculated, you can print a Glance ‘n Go scouting form, take it to a field and record your sampling results. The form will help you to decide if the field needs to be treatment for greenbugs. There are several things that make Glance ‘n Go a good way to make such a decision. You only have to “Glance” at a tiller to see if it has greenbugs (no counting greenbug numbers). You can make a decision to treat “on the Go” because you stop sampling once a decision is reached (no set number of samples). Finally, you can account for the activity of the greenbug’s most important natural enemy, Lysiphlebus testaceipes.
Aphid Mummies (below)
When scouting with the Glance ‘n Go system, keep a running count of tillers that have aphid mummies and a running count of tillers that are infested with one or more greenbugs. After each set of 5 stops, the Glance ‘n Go form directs you to look at your total number of infested tillers and tillers with mummies. If there is enough parasitoid (mummy) activity, you will be directed to stop sampling and DON’T TREAT, even if you have exceeded the treatment threshold for greenbugs! Why? Because research showed that at that level of parasitism, almost all of the healthy-looking greenbugs have been “sentenced to death” and will be ghosts within 3-5 days. If they have received their “sentence” you can save the cost of an unnecessary insecticide application.
Treatment thresholds will probably fall around 2-4 greenbugs per tiller, but make sure you are using the Spring (January-May) form, not the Fall (Sept-December) form. If a field needs to be treated, check with Current Report CR-7194, “Management of Insect and Mite Pests in Small Grains”. If you treat for greenbugs and have a failure, please contact our Department and we will investigate further to determine
By Dr. Bob Hunger, OSU Extension Plant Pathologist
My counterpart in Arkansas, Dr. Gene Milus (University of Arkansas), just sent out an important message about being careful to not exceed the maximum amount of a fungicide applied to a crop in a single year. Such a consideration could be an issue where more than one fungicide application is made. This is especially true if a generic of tebuconazole is applied, as this chemical also is in Prosaro; however, multiple applications of the same fungicide may also exceed the maximum amount of chemical that can be applied in a single year. Reading the label is the best place to determine the maximum amount of a chemical that can be applied in a single season and the exact amount of a chemical(s) that is in a fungicide. A quick reference to what chemicals are in the typical fungicides used on wheat in Oklahoma is the OSU Current Report (CR-7668) that Dr. Jeff Edwards and I recently updated (available at www.wheat.okstate.edu)
In Arkansas and many states through the mid-western region of the U.S., two or even three fungicide applications on wheat are more common, with the last application typically targeted toward Fusarium head blight (scab). In Oklahoma, where scab usually is not a concern, deciding to apply one fungicide application typically has been the only consideration. However, in recent years making two fungicide applications have become more common because of higher wheat prices, lower cost of fungicides, and increased no-till wheat acres that favor early diseases such as powdery mildew, tan spot, and septoria/stagonospora. It is in these situations that care must be taken to insure label compliance. The following message from Dr. Milus addresses this point. The report below is also at http://www.arkansas-crops.com/ along with other ag news from Arkansas.
Given the early onset of stripe rust and the cool rainy weather in recent weeks, some wheat growers are considering two or three fungicide applications. This is a new phenomenon for Arkansas growers and requires some planning to stay within the legal limits for total amounts of particular fungicides that can be applied to wheat fields. The fungicide label lists to total amount of each active ingredient that can be applied per acre per year. These amounts usually are given in pounds of active ingredient (lb ai) which require some math to translate into fluid ounces (fl oz) of particular products. The total amounts and usual application rates below are for fungicides most likely to be used in Arkansas.
Tebuconazole: total amount = 0.11 lb ai/A = 4 fl oz/A.
Products containing only tebuconazole include Folicur (no longer being sold), Orius, Tebucon, Tebustar, Tebuzol, Tegrol, and Toledo.
Prosaro: total amount = 8.2 fl oz /A = 0.11 lb ai each of prothioconazole and tebuconazole.
(Note that no Prosaro can be applied if 4 fl oz of a tebuconazole product was applied earlier because Prosaro is half tebuconazole.)
Propiconazole: total amount = 0.22 lb ai/A = 8 fl oz/A.
4 fl oz/A = 0.11 lb propiconazole.
Products containing propiconazole include Tilt, Bumper, Fitness, Propiconazole E-AG, and PropiMax
Quilt Excel: total amount = 28 fl oz/A = 0.22 lb propiconazole + 0.26 lb azoxystrobin
14 fl oz Quilt Excel = 0.11 lb propiconazole + 0.13 lb azoxystrobin.
(Therefore 4 fl oz of a propiconazole product + 14 fl oz of Quilt Excel can be legally applied.)
Pyraclostrobin (Headline): total amount = 18 fl oz/A = 0.29 lb ai/A
9 fl oz = 0.147 lb Pyraclostrobin (Note the slight discrepancy between the total amounts expressed as fl oz/A and lb ai/A.)
Metconazole (Caramba): total amount = 34 fl oz/A = 0.20 lb ai/A
17 fl oz = 0.10 lb Metconazole
Twinline: total amount = 18 fl oz/A = 0.10 lb metconazole + 0.15 lb pyraclostrobin
9 fl oz/A = 0.05 lb metconazole + 0.076 lb pyraclostrobin
(Note the discrepancies for total amounts metconazole and pyraclostrobin depending on which products are used.)
Wheat disease updates are written by OSU Extension Plant Pathologist Dr. Bob Hunger and posted on the World of Wheat blog.
Oklahoma: A couple of “firsts” occurred this week. Mark Gregory (Southwest Area Extension Agronomist) reported the first leaf rust in Oklahoma for 2013 (see photo below). It was on Overley near Devol, OK, which is near the Red River north of Wichita Falls, TX. The wheat was at GS 10-10.1 (boot to heads just emerging), and was in a field with quite a bit of damage from the freeze. From Mark’s description, the prevalence was fairly low as he indicated he had to look around quite a bit to find rust pustules. Along these lines, Dr. Jeff Edwards and I recently updated “Foliar Fungicides and Wheat Production in Oklahoma – April, 2013” (OSU Current Report CR-7668). This publication provides answers to many of the common questions typically asked about wheat foliar fungicides and provides a table listing the most common fungicides available to control wheat foliar diseases. You can find the publication at http://www.wheat.okstate.edu or by clicking here.
We also had our first confirmed sample of wheat streak mosaic in Fuller wheat from southwestern OK (near Sentinel) in Washita County. This was in a field that had been sprayed for volunteer wheat last fall. Perhaps sufficient time was not allowed to elapse between the spraying of the volunteer and the planting of the wheat in the fall – remember, two weeks should be allowed between the complete death (not the spraying) of volunteer wheat and the emergence of seedling wheat in order to kill the wheat curl mites that transmit Wheat streak mosaic virus.
Next week I’ll be looking around more of the state for diseases – thankfully it is too rainy today!!!!
Large amounts of freezing rain, sleet, hail, etc. hit the Oklahoma wheat belt on April 10, 2013 and temperatures are expected to drop to the mid to upper 20’s this evening (I posted a couple of pictures below). Wheat development ranges from early heading in southern Oklahoma to just past jointing in northern Oklahoma and the Oklahoma Panhandle. If forecasts are correct, wheat tillers in southwest Oklahoma that escaped the first freeze have a good chance of being taken out by this freeze. Central and northcentral Oklahoma has quite a bit of ice-covered wheat. Ice-covered wheat will remain at approximately 32F and this might be just warm enough to escape severe injury. If the ice melts, however, and temps drop into the 20’s even wheat that is just past the jointing stage can be injured. At this stage it is certain that we will have some freeze injury to the majority of the Oklahoma wheat crop, but it will be a good 7 – 10 days before we can accurately assess the level of injury.
I have been asked if there is a 1:1 relationship between % freeze injury and % grain yield loss. Generally, the answer is no. DISCLAIMER — the values I am about to discuss are approximations and have huge margins of error — An otherwise healthy wheat crop that sustains 10% freeze injury prior to boot would probably suffer yield losses in the order of 0 to 5%. This is because the plant will divert resources to the remaining wheat heads. If damage is sufficient to reduce the final head count below a critical mass (around 400 heads per square yard) the relationship between % freeze injury and % yield loss will be much closer. So a 60% freeze injury might result in a 40 – 50% loss in grain yield. Again, these numbers are rough estimations and environmental conditions following the freeze will greatly impact the plant’s ability to compensate after freeze.
On April 4th I toured southwest Oklahoma and surveyed freeze injury to wheat. In my experience, most freeze events are overhyped; however, this one was the real deal Holyfield. I traveled a route from Faxon to Chattanooga to Altus to Blair and ended up at Apache. Damage was similar at all sites, with injury ranging from 50 to 80%.
The best looking wheat was the hardest hit. Particularly troubling are some fields in the Altus area that easily had 80 bushel potential prior to the freeze. In most of these fields we are too far past the tillering stage to have yield compensation from secondary tillers. Late-emerging fields that were jointing or smaller escaped the freeze with little injury. Fields that had been heavily grazed and/or under-fertilized also escaped with relatively minor injury. Conditions improved slightly when I checked wheat in the Chickasha area and injury was more in the 10 – 30% range.
I am frequently asked if the injured wheat head will go ahead and “push through” as the season progresses, and the answer is no. So, if you see heads emerging out of the boot in a few weeks, they are likely not damaged and a head count at this stage will be a reasonable estimate of fertile heads. Since there will not be additional stem elongation in freeze injured wheat, it will not accumulate as much tonnage as in a ‘normal’ year.
I have posted a few pictures below showing freeze injury symptoms. Freeze injury can vary greatly among fields and even within a field. So, it is important to check several sites within a field and split several stems when determining the percent injury. Check early maturing varieties such as Jackpot, Billings, and Everest first, as they are most likely to have injury.