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The stripe rust epidemic of 2015 is still fresh on the minds of many wheat farmers. Reports of active stripe rust on wheat in southern Oklahoma has producers now wondering if we are in for a repeat in 2016. While it is too early to tell if environmental conditions will favor a stripe rust outbreak in 2016, having active rust on wheat in the area satisfies at least one of the requirements for an epidemic. Most Oklahoma wheat producers will still be best advised to monitor the situation and make the fungicide decision based on yield potential and likelihood of infection when the flag leaf is emerging. Those with fields already showing heavy infection of foliar disease, however, might also benefit from a two-pass fungicide system. A few talking points and items to consider for those considering a two-pass system are posted below. A fact sheet on the topic of split application of fungicides can be found at www.wheat.okstate.edu
When to apply – The first pass in a two-pass fungicide system should be applied just after jointing. Please note that this is well after topdress nitrogen should be applied. For this and other reasons (see Dr. Arnall’s blog), tank mixing fungicides with nitrogen is generally not a good practice. Remember that the purpose of the early fungicide application is to keep disease in check until you come back with a flag leaf application in April. Going too early can result in too large of a gap between applications and enough time for disease to re-establish. Going too late can reduce the return on investment. Timing is everything with fungicides.
How much to apply – Back in the day, the discussion around split fungicide application centered on half rates for the first application. This recommendation was because of cost savings rather than disease management. The availability of low-cost, generic fungicides, though, has changed our philosophy, and a full rate of a low cost fungicide is the standard for split applications.
Which product to choose – Product choice is at the discretion of the consumer. If you are considering how to best spend your season-long fungicide budget, however, I would strongly recommend saving your “best” and perhaps most expensive product for the flag leaf application.
Watch season-long restrictions – As always, please read labels carefully and make note of season-long application restrictions. You don’t want an early fungicide application to remove the ability to apply your preferred product at flag leaf.
Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Plant Pathologist
Oklahoma: As you move from southern to northern/northwestern Oklahoma, wheat ranges from past flowering to flowering to heads emerging/approaching flowering (depending on variety and planting/emergence date). It is highly variable. Weather continues to be cool and moist except in southern, western and northwestern Oklahoma where it continues to be dry.
There were no reports of leaf rust over the last week in Oklahoma, but reports of stripe rust increased. However, none of these stripe rust sightings indicated a widespread, severe outbreak anywhere in the state. Bryan Vincent (Crop Consultant – northern Oklahoma) reported seeing strip rust on Everest wheat just west of Orienta, OK (about 40 miles west of Enid). Dr. Brett Carver (OSU Wheat Breeder) reported in his visit early in the week to his breeder nursery at Lahoma (north central OK – 10 miles west of Enid), that, “35 swipes, just one 1″ sporulating Yr lesion at Lahoma, f-3. Otherwise nothing. Will keep looking.” On Friday (10-May) at the Lahoma field day in the variety trial, I found an occasional active stripe in expected varieties (Pete, Garrison, Iba, Everest, and Armour), but these stripes were rare – only one or two per plot. However, our weather over the last couple weeks and the near forecast is for weather favorable for stripe and leaf rust, so we may begin to see more of these rust diseases. Powdery mildew continues to be found but I have not seen it yet on flag leaves or even F-1 leaves. Leaf spotting diseases (tan spot, septoria, stagonospora) are more severe in Oklahoma than typical but vary by location. For example, Dr. Jeff Edwards (OSU Smalll Grains Extension Agronomist) indicated that at his variety trial at Chickasha, leaf spotting was widespread, severe, and was on the flag leaves of specific varieties in the trial.
Regarding viruses – barley yellow dwarf is commonly observed at all locations, but is not associated with severe stunting indicating a later infection. Numerous reports have been received and many samples have now tested positive for wheat streak mosaic virus from many areas of Oklahoma. None of these samples have tested positive for either High plains virus or Triticum mosaic virus.
Reports/excerpts of reports from other states:
Arkansas – Dr. Gene Milus, (Professor/Small Grains Pathologist, Univ of Arkansas, Fayetteville, AR) 10-May-2013: Most wheat has flowered or is flowering now. I found a low level of leaf rust at Rohwer in the southeast corner, and this is the first confirmed leaf rust here. Stripe rust is still active. Bacterial streak showed up on several varieties in the variety test. Leaf blotch is moving up the plants. There is abundant soil moisture across most of the state.
Kansas – Dr. Erick De Wolf (Professor/Small Grains Extension Pathologist, Kansas State Univ, Manhattan, KS) 11-May-2013: The wheat is heading and flowering in southeastern and south central Kansas this week. Wheat in central Kansas is mostly in the boot stages of development with the most advanced fields beginning to head this weekend. The wheat in southwestern KS continues to struggle with drought and freeze damage and growth stage varies widely among fields.
Disease scouting this week suggests the risk of severe rust epidemics remains low in Kansas this year. I had a few more reports of trace levels of stripe rust in southeastern KS where the wheat is flowering. The levels of stripe rust are very low at this time. Temperatures are forecast to reach the upper 80’s early next week with low temps at or above 60 F. Temperatures in this range often slow the development of stripe rust but farmers in this areas should monitor the disease carefully.
I was able to find a single pustule of leaf rust in Stafford county Kansas this week, but the wheat at this location was thin from recent dry conditions. I suspect the leaf rust will not increase rapidly at this location. Other fields that I checked in southwest KS (Finney and Kiowa counties) show significant drought stress and no sign of rust. In south central and central Kansas (Pratt, Reno, and McPherson counties), the wheat is in better condition with thick canopies and good moisture recently. I found no leaf rust, stripe rust or stem rust in these areas; however, several fields had moderate levels of powdery mildew. Symptoms of barley yellow dwarf remain low or absent in all fields I have check to date.
Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Wheat Pathologist
Oklahoma: Over the last week I, Nathalia Grachet (OSU graduate student), and Brian Olson (OSU A&P) extensively looked at wheat around Stillwater, and in central (Minco, Apache), southwestern (Granite), and north central (Lahoma) Oklahoma. Additionally, OSU Wheat Breeder Brett Carver examined his plots at numerous locations in central, southwestern OK as well as at Stillwater, and Dr. Art Klatt (OSU Wheat Geneticist) examined trials around Stillwater and Perkins. Wheat in southwestern Oklahoma was damaged from drought and freeze, and little disease was discernible. Wheat in southwestern Oklahoma (where it was possible) appeared to be at various states of head emergence, and in a few cases was just starting to flower. In central OK, wheat was in various stages of heading. Around Stillwater, wheat was just starting head emergence, while at Lahoma wheat was mostly just approaching GS 10 (boot stage). However, there seems to be quite a bit of variability regarding stage of maturity depending on variety and planting date.
No leaf rust was observed at any location. The “hot spot” of stripe rust we found at Minco is still active, but my previous update incorrectly identified ‘Duster’ as the variety with the most severe stripe rust. Actually that was ‘Garrison’. Stripe rust also was on Duster but not to the same severity nor was significant stripe rust on the flag leaves of Duster. This is the only location where we observed significant stripe rust. We also observed powdery mildew and leaf spotting on lower to mid leaves at many locations. Primarily the leaf spotting appeared to be septoria/stagonospora with some tan spot mixed in, and as expected is usually somewhat more severe in no- or low-till fields.
Barley yellow dwarf was commonly observed at many locations with variable severity, but it was often difficult to differentiate damage between BYD, freeze, and drought. Wheat streak mosaic also was observed across the areas examined (see photo below). An increasing number of samples from around western OK have kept Jen Olson (Plant Disease Diagnostician) busy. Most of these samples have tested positive for BYDV, WSMV, or both, but no high plains virus or Triticum mosaic virus has been detected. This includes the Stillwater area where I have never before (since 1982) observed WSM. Another interesting find here at Stillwater included occurrence of Russian Wheat Aphid, which was confirmed by Dr. Rick Grantham (Director of the Plant Disease and Insect Diagnostic Lab – photos below taken by Rick).
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.