{"id":1542,"date":"2018-12-20T19:42:39","date_gmt":"2018-12-20T19:42:39","guid":{"rendered":"https:\/\/agrilife.org\/texasrowcrops\/?p=1542"},"modified":"2018-12-20T19:42:39","modified_gmt":"2018-12-20T19:42:39","slug":"yellow-foliar-symptoms-in-fall-2018-wheat-west-texas","status":"publish","type":"post","link":"https:\/\/agrilife.org\/texasrowcrops\/2018\/12\/20\/yellow-foliar-symptoms-in-fall-2018-wheat-west-texas\/","title":{"rendered":"Yellow Foliar Symptoms in Fall 2018 Wheat (West Texas)"},"content":{"rendered":"

by Dr. Calvin Trostle, Extension Agronomy, TAMU Soil & Crop Sciences, Lubbock, (806) 723-8432, ctrostle@ag.tamu.edu\u00a0<\/a>; Dr. Reagan Noland, Extension Agronomy, TAMU Soil & Crop Sciences, San Angelo, (325) 657-7330, reagan.noland@ag.tamu.edu<\/a><\/p>\n

For wheat producers in the Texas High Plains and lower Rolling Plains\/Concho Valley Fall 2018 has brought field symptoms that have baffled some growers.\u00a0 Much of this is likely due to exceptionally large October rains.\u00a0 Essentially all fields south of Lubbock received at least 6\u201d of rain, a few fields received twice that.\u00a0 Rains north of Lubbock were somewhat less.\u00a0 Since a month ago the inquiries we are receiving are:<\/p>\n

    \n
  1. Yellow wheat south of Lubbock and in the lower Rolling Plains. This is the primary focus of this update\u2014remember that soil available nitrogen is in the nitrate form (NO3<\/sub>–<\/sup>) which is mobile in the soil and will move with soil water.<\/li>\n
  2. Leaf rust north of Lubbock. With colder temperatures this issue is greatly reduced.\u00a0 Though some fields indicated levels of leaf rust that were concerning, there were no fields that AgriLife Extension recommended fungicide treatment. \u00a0Perhaps most importantly for the Texas High Plains, leaf rust in the fall has no connection to leaf rust in the spring<\/u>.<\/li>\n<\/ol>\n

    Texas A&M AgriLife Extension plant pathologist Dr. Ken Obasa, Amarillo (806.677.5600, ken.obasa@ag.tamu.edu<\/a>) and others note that in order to have leaf rust in spring fields must be re-infected with spores blown in on winds.\u00a0 The annual cold of winter in the Texas High Plains is enough to kill the pathogen.\u00a0 For more about leaf rust review Dr. Obasa\u2019s comments at https:\/\/today.agrilife.org\/2018\/11\/08\/wheat-rust-showing-up-in-high-plains-but-treatment-not-recommended\/<\/a><\/p>\n

    We believe the primary culprit in field yellowing is the loss of plant available nitrate from the wheat root zone.\u00a0 As noted, nitrate is mobile in soil.\u00a0 When you factor in large rains, nitrate will move down with percolating water.\u00a0 For younger wheat, this will move the needed nitrate below the root zone.\u00a0 As conditions dry and the root volume expands downward wheat may catch up to this nitrate.\u00a0 The rate of mineralization or release of other forms of nitrogen in the soil\u2014either from organic forms or from ammonium (NH4<\/sub>+<\/sup>) from exchange sites on the negatively charged soil surface\u2014is insufficient, at least temporarily, to meet the needs of the growing wheat.\u00a0 This mineralization of soil N to nitrate is slower under cool conditions.<\/p>\n

    Producers can conduct soil and plant tissue tests, and some have, to assess N status of soil and plants.\u00a0 Analyses that we have either conducted ourselves or received from clientele, have pointed to low and very low soil N status.\u00a0 If a farmer does nothing wheat may grow out of the deficiency condition as the root volume expands downward.\u00a0 However, if no early season N (pre-plant, at-plant, etc.) was applied and you know your residual N fertility was low, then you have a situation that would likely benefit from added N (provided you can chemigate the N if irrigated, or get a rain or snow after making a broadcast granular N application).\u00a0 With all the rain received farmers are naturally reluctant to consider irrigating, but if you need to add up to 0.5\u201d of irrigation to chemigate N (or wash in broadcast N applications), do so.<\/p>\n

    If you had good fertility status at planting and you have some yellow color you might be able to wait until typical late winter top-dressing (late February, early March), but we recommend that you ensure earlier topdress application before the burst of spring growth so sharp potential N deficiency conditions are avoided.<\/p>\n

    Though we are focusing on nitrogen deficiency as the likely factor in yellow fields and poor wheat growth there are other factors that may cause significant yellowing in wheat, including several plant diseases.\u00a0 We are still early enough in the cropping cycle that disease issues (other than leaf rust) appear to be minor.\u00a0 Virus testing of wheat tissue samples in Dr. Obasa\u2019s plant pathology lab in Amarillo to date have generally recorded at most low and undetectable levels of wheat streak mosaic virus, triticum mosaic virus, High Plains virus, and barley yellow dwarf mosaic virus.<\/p>\n

    AgriLife Extension Resource for Yellowing Wheat<\/em><\/p>\n

    For more scenarios that could lead to yellowing of wheat foliage, consult Texas A&M AgriLife\u2019s \u201cPotential Causes of Yellowing During the Tillering Stage of Wheat in Texas,\u201d ESC-040 (April 2016) at http:\/\/publications.tamu.edu\/WHEAT\/2016\/Yellow%20wheat%20publication.pdf<\/a><\/p>\n

    Field Descriptions of Famers\u2019 Yellow Wheat Fields<\/em><\/p>\n

    We will use five photographs from producers\u2019 fields to further outline the issues we have observed with yellowing in regional wheat fields.<\/p>\n

    Figure 1:\u00a0 This Castro Co. field ranges from spotty green vs. poor growth in the foreground, all poor and light color in the upper right, and the upper left is all green.\u00a0 Dr. Obasa commented that leaf rust would not create such a spotty appearance.\u00a0 No leaf rust pustules were observed.\u00a0 Some of the patchy green (at least for spots that are 1-2\u2019 in size) leads one to wonder if there is a general (temporary) deficiency of N, and the green spots are where grazing cattle in the past defecated or urinated.\u00a0 But the dark green area beyond the pickup to the left doesn\u2019t fit that scenario.\u00a0 We did not find larger field patterns that might indicate chemical drift or active residual herbicide.\u00a0 The October rainfall on this field was at least 5\u201d.<\/p>\n

    Roots in Fig. 1 plants look fine and no insects have been found. \u00a0White grubs are also not an issue.\u00a0 These cause expanding yellow spots, typically in fields that have been in continuous wheat for 7 years or more.\u00a0 Until it gets cold and grubs move deeper in the soil, grubs sever the root system just below the crown killing the plant.\u00a0 If they are present, you will easily find them in the transition between dead and normal plants.<\/p>\n

    \"pattern<\/a>

    Fig. 1. Spotty wheat growth, Castro Co., Texas. The pattern is puzzling as there is no apparent reason for poor, mixed, and good areas n the field. There is essentially no slope that would potentially pond water and lead to increase nitrate leaching potential (hence N deficiency). (Photo: Quillin consulting)<\/p><\/div>\n

    ** **\u00a0 **<\/strong><\/p>\n

    Figure 2.\u00a0 The field is characterized by sandy soil.\u00a0 So leaching potential for soil nitrate is high. The soil test reports comparing good and poor areas do not distinguish one area from the other (soil nitrate N was \u2018very low,\u2019 2 to 3 ppm).\u00a0 The farmer chemigated and put out dry nitrogen.\u00a0 Rainfall again was high at 5 to 8\u201d on different fields in the area.\u00a0 Typically, we expect more yellowing in low areas or small local depressions where rainwater may have ponded and so there was added leaching.\u00a0 The spottiness in Fig. 2 looks like the green areas might be where soils are slightly elevated thus run-off from a large rain moved to the side and percolated down elsewhere, leaving slightly more nitrate in the high areas.<\/p>\n

    \"Good<\/a>

    Fig. 2. Wheat on sandy soil, northern Lamb Co., TX. This and surrounding fields in the sandy area demonstrate spotty good growth. Soil and plant tissue tests of good and poor areas all report low N. (Photo: Marcus Sullivan)<\/p><\/div>\n

    ** **\u00a0 **<\/strong><\/p>\n

    Figure 3.\u00a0 This Jones Co. field has noticeable green strips where tractor and drill equipment tires ran.\u00a0 This is one of two fields in Jones Co., and other fields with similar symptoms have been reported the week of Dec. 17 in Runnels Co.\u00a0 AgriLife Extension conducted soil and tissue sampling in this field for green strips vs. yellow areas.\u00a0 The wheat in the tracks had higher tissue N, but N was at least sufficient in yellow areas.\u00a0 (The average of three samples of track green vs. general yellow was 4.2% N vs. 2.8% N.\u00a0 Differences in soil N were not found (the opportunity to capture that difference may have already passed). \u00a0Local NRCS staff tested soil compaction with a penetrometer, but there was not enough resistance in any part of the field to show measurable compaction, as the field was deep-ripped prior to planting and soils were coarse-textured. \u00a0Our best guess is that shallow compaction from the wheels resulted in less N loss via leaching as some water may have moved to the side leaving a pocket of higher soil N under the wheel track.<\/p>\n

    \"\"<\/a>

    Fig. 3. Wheel tracks in two directions have greener wheat, Jones, Co. Texas. More firm soil in the tracks may have directed some leaching water to the side thus reducing potential downward movement of nitrate below the wheel track. (Photo: Reagan Noland)<\/p><\/div>\n

    **\u00a0 **\u00a0 **<\/strong><\/p>\n

    Figure 4.\u00a0 Seedling wheat from Dawson Co.\u00a0 Rainfall on this field was over 8\u201d in October and the field had a yellow appearance.\u00a0 Note the youngest leaf is darker green.\u00a0 This is a symptom of nitrogen behavior in wheat and other plants as N will be moved from older tissues to the growing point and newly emerging leaves.\u00a0 If the youngest leaf is also yellow then there is potentially acute N deficiency, or some other issue occurring.\u00a0 (Photo:\u00a0 Roy Paske)<\/p>\n

    \"\"<\/a><\/p>\n

     <\/p>\n

    Figure 5.\u00a0 Wheat sample from the field in Fig. 2 with reduced N status (soil N, \u2018very low\u2019; plant tissue N, \u2018low\u2019) on a sandy soil.\u00a0 Root mass looks good indicating that wheat has grown well, perhaps before heavy rains leached available nitrate.\u00a0 Also, the youngest leaf (orange arrow) is green indicating the plant response to N deficiency.\u00a0 (Photo:\u00a0 Marcus Sullivan)\"\"<\/a><\/p>\n

     <\/p>\n

    What do I do with current yellow wheat?<\/em><\/p>\n

    The guess work comes after you have documented low soil test nitrate and possibly low plant tissue N.\u00a0 As noted above, if you have not applied any N fertility to this season\u2019s wheat crop then you may consider if you should proceed with an N application or wait until you approach the typical late winter topdressing season in late February and early March.\u00a0 If your field has been yellow for some time and you don\u2019t notice much improvement, then nitrogen may be merited.<\/p>\n

    Management considerations also depend on your objectives for the wheat crop. The immediate risk with \u201cwaiting to see what happens\u201d is not as high with wheat being managed for grain, since most opportunity for fall tillering has passed, and the possibility exists that the wheat roots could still catch up to leached N. If the wheat is needed quickly for winter grazing, then immediate action may be justified (considering livestock production potential), especially if N has been leached beyond a recoverable range.<\/p>\n

    If you conclude that a light winter N application may be called for, review the principles of N top dressing in wheat at https:\/\/agrilifecdn.tamu.edu\/lubbock\/files\/2014\/02\/Wheat-N-Topdress-Trostle-Master-Feb2014-High-Plains.pdf<\/a> (PowerPoint) and https:\/\/lubbock.tamu.edu\/files\/2018\/02\/TopDress-N-Tips-for-Wheat-2018.pdf<\/a> (Word file).<\/p>\n

    Here is an additional tip that may give you confidence in how your wheat may respond to added nitrogen.\u00a0 You can do this two ways:<\/p>\n