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Agricultural News

Oklahoma Wheat Continues to Race Ahead of Most Disease Problems in 2012

Mon, 16 Apr 2012 07:53:43 CDT

Oklahoma Wheat Continues to Race Ahead of Most Disease Problems in 2012 The Oklahoma Wheat Crop continues to race toward maturity- and has been able to stay ahead of most disease problems all season long- the latest report from the weekend from Dr. Bob Hunger of Oklahoma State University seems to confirm that status. However, we are seeing some disease in many wheat fields across the state and invite you to read the specifics below in Hunger's report.

More disturbing are a couple of reports coming from Texas and Kansas. In Kansas, Plant Pathologist Dr. Erick De Wolf out of Kansas State says that "stripe rust is widely established in central Kansas this year. The weather has been conducive for disease development and the weather forecast appears to favor continued development. The disease is still at low levels in many fields, however, the severity of disease will likely increase dramatically in the next 10 days."

The news from Texas surrounds stripe rust and the possibility that we could see the pathogen changing and adapting and becoming more tolerant of warmer temperatures- which if that is the case- could spell more pressure from this newest variation of stripe rust in the next few years in Oklahoma.

Here's the latest report from Bob Hunger- followed by reports from his counterparts in Texas and Kansas:

Oklahoma: Friday (13-Apr) I visited the variety trial at Marshall (30 miles west of Stillwater) and at Lahoma (10 miles west of Enid). This was followed by visits to many fields north and northeast of Enid (Kremlin, Billings) and north of Ponca City (Tonkawa, Kildare and Newkirk). Wheat development varied across these locations. The earliest I saw had heads emerging and the latest was through flowering. On average, I would say wheat was in the middle of full bloom.

        Leaf rust (LR) pustules were found at nearly all stops, but were at low incidence along the entire route. In Stillwater, I have seen incidences in the 40-65% range, but not in all plots.

        Barley yellow dwarf (BYD) spots were present, but as in previous weeks, was not as severe as I have seen it in trials here at Stillwater. No or little dwarfing was associated with the spots, indicating spring infection.

        Powdery mildew (PM) was observed at many stops; mostly was on lower leaves. The exception to this was the variety trial at Marshall, where PM was severe (?65%) on lower leaves and could be found on F-1 leaves.

        Stripe rust (YR) at Marshall and Lahoma was severe on the varieties observed to be susceptible this year, including Armour, Garrison, Everest, and Pete. Most of the fields I stopped at along the route were Duster, where only an occasional YR stripe was observed. Be sure to read James Swarts and Erick De Wolf's commentaries below regarding stripe rust. I concur with their statements regarding race change, but have not observed stripe rust as severe on Jagger and its relatives as on the other varieties.

        Tan spot (TS), Septoria leaf blotch (SLB), physiological leaf spot (PLS): Leaf blight caused by these diseases (and the non-pathogen-caused PLS) were observed in many fields, especially in Duster. However, in some fields and trials minimal leaf blight due to these diseases and PLS was present. A good example of this was at Marshall and Lahoma, where SLB was readily found only on lower leaves of nearly all varieties. TS and PLS were mostly absent. In many fields visited north and northeast of Enid, the entire complex was common from lower to upper leaves at various levels of severity. On some of these, SLB and TS were highly suspected, but PLS also appeared to be involved. Leaf tip necrosis (LTN), which is associated with the leaf rust resistance gene Lr34 in Duster, exasperates the appearance. Correlation of specific environmental and/or fertility components with PLS remains elusive.

        Pyrenophora (the fungus that causes tan spot) was isolated from nearly all the samples collected last week from southwestern Oklahoma. Pseudothecia and pycnidia, which are the "resting bodies" of the fungi that cause TS and SLB, also were found on residue in these fields. However, I do not believe that all of the blight/spotting in these fields is due to TS and SLB because of the high incidence of blight on upper leaves and the low incidence of isolation.

Reports/excerpts of reports from other states:

Texas: Jim Swart (Entomologist (IPM), Texas AgriLife Extension 13-Apr-2012:

The regional wheat crop is maturing rapidly, with most varieties developing well ahead of normal. The latest maturing varieties are flowering, and the earliest varieties are well into the grain filling period. Russell Sutton, Research Associate with AgriLife Research, and I cannot remember a year where there were as many days difference in heading between the early and late maturing varieties.

        USG 3295 SRWW, a variety that had previously shown good resistance to both leaf and stripe rust, is showing heavy stripe rust infections in some fields. Based on reports across the southern wheat belt, this is likely a reflection of a race change in the stripe rust pathogen. At the first sign of this infection, we suggested that growers consider a fungicide application on this variety, and many fields were sprayed last week.

Infection from the stripe rust pathogen (Puccinia striiformis) has slowed in the past few days, but infection could recur if weather conditions cool down a bit. Stripe rust is typically a cool weather pest, but some reports suggest this new race may be tolerant to warmer conditions than the rust race we have seen in past years.

        Leaf rust (Puccinia recondita) infection levels remain low, even in susceptible varieties. Jackpot HRWW, a variety that has been highly susceptible to leaf rust, still shows low infection levels in the mid to upper leaf canopy. I expect infection levels to increase as the season progresses.

        Glume blotch (Stagonospora nodorum) is more prevalent this year than usual, likely triggered by the warm, wet, humid conditions of the past month. The tebuconazole that was sprayed earlier for stripe and leaf rust is still providing flag leaf and head protection, but unsprayed fields are at greater risk. Unfortunately, there are no fungicides that are labeled to be applied after flowering, so this option is unavailable except in the very latest maturing varieties. This is a weather driven pathogen that will subside if we experience dry weather (it is spread by splashing rain drops).

      Some local suppliers have been disseminating misinformation on foliar fungicides. Claims are being made that Alto lasts "twice as long as tebuconazole", and produces "5 more bushels per acre than tebuconazole". This is simply not true. Since the research trial that is being used to misinform growers is based on our work from last year, I will clarify the results of this experiment. There were no differences in leaf rust control between any of the fungicides in this study. There were also no differences in yield, bushel weight, and thousand kernel weight between the fungicide treatments. TebuStar® 3.6 L, Alto® 100 SL, Quilt®, Quilt Xcel®, Prosaro® 421 SC, TwinLine®, and Tilt® all provided better leaf protection than the untreated plots but none were different from one another. I am including the leaf rating table from this study for your information. This rating was made at physiological maturity, 43 days after the single applications. The variety used was highly susceptible to rust, and would not be recommended to plant commercially.

Kansas: Dr. Erick De Wolf (Wheat Extension Pathologist, Kansas State University): 13-Apr:

The wheat in Kansas is now heading and beginning to flower in Southeast and South Central Kansas.   Wheat in central KS is now at the boot stage and will likely begin to head soon. In fact, it is likely that some fields are already beginning to head out in central region of the state. As we move north and west in the state, the wheat is moving toward flag leaf emergence to boot stages of development.

My own scouting and reports from other KSU agronomists and agents indicates that stripe is generally at low levels in many fields throughout central Kansas. The stripe rust was generally limited to the F-2 and F-1 leaves and less than 1% incidence. This afternoon; however, I have received reports that stripe rust has now moved to the flag leaf and the severity of disease has increased dramatically in some fields. Here are some specific reports: Stripe rust was reported on the flag leaf in Montgomery, Labette, Crawford and Wilson counties (Southeast, KS). Stripe rust was reported on the upper canopy (F-1 and Flag) with a noted increase in the incidence in many fields this past week in Saline, McPherson, Harvey, Reno and Ellsworth county (central KS). Stripe rust was observed on the flag leaf in Sedgwick county (South central, KS) with severe stripe rust developing in a field of Armour wheat in the southeast portion of this county. I have also seen stripe rust in north central KS including Cloud, Mitchell counties. The disease appears to be limited to the F-2 and F-1 leaves currently in north central KS.

Varieties with the Jagger based pedigree that have Yr17 are being affected by stripe rust. Varieties such as Everest, Armour, and TAM111 are also being affected by stripe rust this year. This strongly suggests that the stripe rust population has changed to overcome these sources of genetic resistance.

My assessment of the situation is that stripe rust is widely established in central Kansas this year. The weather has been conducive for disease development and the weather forecast appears to favor continued development. The disease is still at low levels in many fields, however, the severity of disease will likely increase dramatically in the next 10 days.   I think there is high risk of severe yield loss to stripe rust for wheat in at least the eastern 2/3 of Kansas. Based on my current information I believe there is at least a moderate risk of severe disease in western KS. I will attempt to get more information about western Kansas next week.



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