ITEMS FROM THE UNITED STATES

OKLAHOMA

OKLAHOMA STATE UNIVERSITY
Department of Plant and Soil Sciences, 368 Ag Hall, Stillwater, OK 74078-6028, USA.

E.G. Krenzer, E.L. Smith, M.P. Anderson, B.F. Carver, T.F. Peeper, J.P. Kelley, K.T. Heap, A.E. Solie, J.B. Solie (Department of Biosystems and Agriculture Engineering), M.L. Franetovich, J. Roberts, S.B. Phillips, J. Chen, W.R. Raun, G.V. Johnson, D.A. Cossey, D.S. Murray, and R.B. Westerman.

1997 Oklahoma winter wheat crop.

E.G. Krenzer.

The power of wheat to recover from the extreme stresses of nature was proven again in 1997. This was the third consecutive year when a crisis for wheat production occurred over nearly the entire state. The severe freeze of 10-13 April was the culprit in 1997. Temperatures were low enough to predict catastrophic effects on the wheat.

Wheat yields statewide were better than expected and greatly appreciated by producers. Three years in a row with extremely different challenges to the Oklahoma wheat crop have left some asking "What next ?" One of the values of such extremes is data collection on cultivars and production practices across a broad range of conditions. Wheat cultivars that performed well on the average over the last 3 years probably can handle most circumstances that Oklahoma faces.

Wheat planting started well in 1996 until rains about 12 September initiated a wet period that lasted until 1 October. Producers who planted prior to 10 September had excellent wheat pasture. Fields not planted prior to the rains probably did not get planted until October. Early October plantings still produced adequate forage to graze, and cattle gains and profits were excellent. Because of cattle prices, some producers made more money on cattle grazing than on wheat than ever before.

Leaf rust was quite severe in November in many of the earlier-planted fields and had destroyed the lower leaves of all varieties at Marshall, OK, by mid-November. Much more discussion about leaf rust can be found in the HOW Newsletter Vol. 2(1), published in January 1997. The leaf rust overwintered and was a significant factor in limiting the recovery from freeze in many fields. Soilborne mosaic virus was more severe in early 1997 than we have observed in several years. Many producers discovered SBMV in fields where they have not noticed it previously.

Optimism of a great wheat crop continued until the freeze of 10-13 April. After the freeze, doom and gloom were prevalent. In the very southern part of the state, wheat was heading and temperatures were severe enough to sterilize flowers. Frequently, these fields looked very normal with green heads, but no seed developed. Some fields had so few seed set that they were not harvested. Other fields produced 5-15 bu/A primarily on very late tillers, which developed after the freeze. Test weight in these situations was much lower than normal.

In the southern two-thirds of the state, fields that were planted prior to 1 October and with early maturing varieties were in the boot stage of development when the freeze occurred. Complete heads were killed. When these heads emerged, they were white, making the damage obvious. Again, late tillers frequently developed. Fields with late tillers made harvesting decisions difficult, because the later tillers were green with grain moisture above 15 %.

The freeze hit the wheat in most other situations when it was jointing, with the head anywhere from 2-4 inches above the soil surface in the panhandle to just prior to boot stage in later-planted wheat in other areas. The closer the wheat was to the boot stage, the greater the damage and the more impact the freeze had on yield.

Wheat producers in Oklahoma and southern Kansas were fortunate that the weather was ideal after the freeze until maturity. Temperatures were seldom above the long-term average for the daily high or nightly low. Moisture distribution was excellent, and this combination allowed the wheat to maximize its ability to compensate for the freeze damage. This resulted in the current estimate of 178.2 million bushels of wheat on 5.4 million acres or 33 bu/A, the best yield per acre for Oklahoma since 1988.

Results from the free soil-sampling program offered by OSU in the summer of 1996 supported the theory that nitrogen had been accumulating in the subsoil as a result of several poor wheat crops. Many producers reduced the nitrogen applied to the 1996-97 crop and still recorded excellent yields. This reinforced the point that wheat can use the nitrogen in the zone from the base of the plow layer 24 inches deep. However, continued monitoring of the soil nitrate level is essential in planning for future wheat crops. Once the subsoil nitrogen reserve is depleted, higher nitrogen applications will be needed to maintain top yields.

Wheat breeding research

E.L. Smith.

Goals and objectives. Developing new cultivars and maximizing benefits from wheat production remain the goals of the OSU wheat breeding research program. Wheat leaf rust, SBMV, and greenbugs are three of the most important biotic constraints to wheat production in the state. Recently, the wheat industry has emphasized grain quality (test weight and 1,000-kernel weight) as an important characteristic in the market place. More attention will be given to this characteristic in the breeding program. Other concerns in the breeding program include end-use quality (e.g., H2O absorption, protein, mixing tolerance, and flour yield); acid soil tolerance; and grazing potential. Promising genetic resources from eastern Europe are being used extensively in the OSU wheat breeding program. We recently have exchanged wheat germplasm lines with Romania and southern Ukraine.

New cultivar release. In the spring of 1997, the Oklahoma Agricultural Experiment Station released 2174, a HRWW, which traces to Pioneer Seeds, Inc. germplasm. 2174 is a medium-maturity, awned, tall semidwarf cultivar that flowers a couple of days later than Jagger, similar to 2163. Grain yields are similar to or better than those of 2163, Karl 2137, Custer, and Jagger. In 1995-96, 2174 yielded 1.6 bu/A more than 2137 in 15 Oklahoma tests. Test weight is better than those of all the above except Tonkawa, which has outstanding test weight. Plant height is slightly taller than those of Tonkawa and Jagger. Coleoptile length is expected to be longer than those of most semidwarf cultivars.

Disease resistance is a strength of 2174, because it is resistant to SBMV, leaf rust, powdery mildew, and tan spot. Tolerance to low pH appears to be better than average, but not as good as that of 2163 or 2137. 2174 is expected to be best adapted to central and north central Oklahoma.

Leaf rust resistance at the molecular level.

M.P. Anderson and R.M. Hunger.

Breeders and pathologists worldwide have borne the brunt of the work required for rust control. They have done this without knowing a great deal about the infection process at the molecular level. In this project, we seek to isolate and identify genes that are turned on during infection using a relatively new technique known as the differential display. The differential display is highly sensitive in that it is capable of isolating fragments of differentially expressed single-copy genes. These fragments can be sequenced and compared with genes of known function. Significant matches can provide information leading to clues about subtle aspects of the infection mechanism. To date, we have sequenced over 20-gene fragments, many showing little homology to genes of known function or character. However, three of the gene fragments were strongly homologous to known genes, including: 1) a fungal growth factor, 2) a calcium-binding protein, and 3) a carbohydrate-responsive gene. A cDNA library was constructed recently to isolate and sequence the complete genes. More sequence information will lead to further insight into the probable functions of each gene. We hope that mechanistic knowledge behind the infection process, gained through this approach, eventually will enable us to design novel methods for rust control.

Wheat genetics research.

B.F. Carver and E.G. Krenzer.

More than 50 % of the wheat acreage in Oklahoma may be used for the dual purpose of forage and grain production in the same crop season, yet no cultivars to date were bred specifically under a dual-purpose management system. This discrepancy between selection environment and target environment prompted an investigation, beginning with the 1996-97 crop season, to determine the impact that breeding in a grain-only management system (selection environment) has on yield response in a dual-purpose management system (target environment). Although the results obtained from experiments at the Wheat Pasture Center at Marshall, OK, are only preliminary, they do suggest that future selection pressure must be applied in a dual-purpose environment. At Marshall, a historical set of cultivars widely grown in Oklahoma since the introduction of Turkey was evaluated for genetic gain in grain yield under grain-only and graze-plus-grain conditions. Rates of genetic gain in the two environments were noticeably different. Under grain-only conditions, yield increased at a per-annum rate of 13 kg/ha (1919 to 1997), equivalent to about 1 % per year. More surprising was the lack of a significant increase in yield over time when genetic gain was measured in the dual-purpose environment (3 kg/ha/year). Further, average yield decreased by almost one-half in the dual-purpose environment, even though grazing was terminated in early February before first-hollow-stem stage.

Our hypothesis is that grazing, in combination with an earlier planting date produces a series of stresses on the wheat plant that go unchecked in a selection program dedicated to improving yield under grain-only conditions. This research will continue to further test that hypothesis and identify the stress factors that apparently suppress the increased yield potential of contemporary cultivars.

Wheat cultivar testing.

E.G. Krenzer.

The freeze of 10-13 April, 1997, reduced yields the most in fields where the wheat was headed or nearly headed. Early maturing cultivars suffered greater yield loss from the freeze than late maturing cultivars. Tonkawa and AgriPro Tomahawk showed more leaf rust resistance than other commercially available cultivars evaluated. Jagger, Custer, Agseco 7853, AgriPro Tomahawk, and 2163 showed the highest grain yields across the range of stresses received in the wheat variety trials in the last 3 years. Among the high-yielding varieties, test weight averaged across the last 3 years ranged from 54.9 lb/bu for 2163 to 57.6 lb/bu for Agseco 7853.

Wheat forage data were collected by clipping with a sickle-bar mower. Forage yields often are doubled when comparing the lowest yielding variety (1,100 lb/A) with the highest (2,000 lb/A) for autumn forage. We have been using a 10:1 ratio for forage to beef conversions. Thus, a 90 lb/A difference in beef production should be expected. For 5 years we have compared Agseco 7853 and 2180 in grazing trials, and the average difference in beef production was 35 lb/A. We conclude that the clipping data are not estimating beef differences well enough. Beginning in the autumn of 1997, forage estimates will be based on hand clippings to the soil surface. Trials at two locations will be conducted to compare the hand clippings and the sickle bar mower method. Data also will be collected at the Wheat Pasture Research Unit to evaluate the conversion ratio for forage to beef when clipping to the soil surface.

Weed control research.

A.E. Solie, T.F. Peeper, M.L. Franetovich, J. Roberts, J.P. Kelley, and J.B. Solie (Department of Biosystems and Agricultural Engineering.

Seed treatments applied during wheat harvest for cheat [Bromus secalinus (L.)] control. During wheat harvest, the majority of cheat seed passes through grain combines, providing the opportunity to catch and spray the seed with herbicides before returning it to the field. Preliminary research in 1995 demonstrated that seed-applied trifluralin reduced cheat emergence up to 95 %.

In 1996, an experiment assessed three rates (0.5 X, 1 X, and 1.5 X) of five herbicides. Cheat seed was treated in a small rotating drum mixer and planted in replicated field plots. Trifluralin and pendimethalin at 1X rates reduced cheat emergence 58-84 % at one location, but results were variable. Herbicide-seed mixing times in the rotating drum mixer (15, 30, and 60 seconds) did not affect control. However, increasing spray volume from 1.5 to 5.6 GPA, based on the harvesting collection area, improved control. To confine spray and provide continuous material flow, a grain auger was fitted with five 8001 flat-fan nozzle tips. When the auger was compared with the rotating drum mixer, all 1 X and 1.5 X treatments applied in 10 GPA of water carrier reduced cheat emergence 97 %.

In 1997, experiments compared three carrier volumes used with the nozzle-equipped auger (2.5, 5, and 10 GPA). Control increased with carrier volume. However, increasing volume above 10 GPA (680 ml/min) caused occasional auger plugging, thus 10 GPA was considered the maximum practical carrier volume. Neither nonionic surfactant nor a crop oil improved control with auger-applied trifluralin.

Effect of tillage system and seeding practice on jointed goatgrass infestations in wheat. Three field experiments were established in 1996 and continued in 1997, and three additional experiments were established in 1997, to evaluate multiple cultural options for suppression of jointed goatgrass (Ae. cylindrica) in winter wheat. Treatments for all experiments included three primary tillage systems (moldboard plow, offset disk, and sweep plow with a 5-foot V-blade; two wheat row spacings (4 and 8 inch); and four wheat seeding rates (37, 64, 86, and 135 lbs/A). Wheat stand counts were recorded following planting for all experiments to establish true seeding rates. Jointed goatgrass density was determined by plant counts following planting in 1996 and prior to planting in 1997.

In three of the six experiments, moldboard plow tillage significantly reduced jointed goatgrass emergence prior to seeding compared to the disk and sweep plow in 1997. In two of the six locations, the moldboard plow resulted in significantly less jointed goatgrass emergence than the disk or the sweep plow following seeding in 1997. At one of three locations, wheat seeded with 8-inch row spacing had a significantly lower yield than wheat seeded with 4-inch row spacing in moldboard-plow and disk-tillage treatments.

Evaluation of herbicides for field bindweed control. Field research is being conducted at two locations in north central Oklahoma to evaluate herbicide programs for field bindweed control on hard red winter wheat fields. Quinclorac + 2,4-D (Paramount) was applied sequentially in 1995, 1996, and 1997. Glyphosate + 2,4-D (Landmaster BW) and dicamba + 2,4-DLV also were applied sequentially each autumn. Glyphosate was applied in the autumn of 1995 as a standard. Postharvest (June-July) control programs included picloram + 2,4-DLV, and quinclorac + 2,4-D. One program included dicamba applied in June 1996, followed by a sequential application of dicamba in July 1997. Control varied with location and was less than desired (Table 1). Postharvest applied Paramount was ineffective, while fall applications look promising.

Bromus control in winter wheat with MON 37500. Cheat is a serious winter annual grass weed in Oklahoma wheat. Current herbicides for cheat do not give consistent control and can cause wheat injury. MON 37500 is an experimental sulfonylurea herbicide that is being evaluated for Bromus species control in wheat.

In the autumn of 1996, three field experiments were established in Oklahoma to evaluate MON 37500 for selective cheat control. The experimental design for each site was a randomized complete block with four replicates. Treatments included preemergence, 2­3 leaf wheat, tillered wheat, January, and February applications. MON 37500 rates were 0.15, 0.37, 0.44, and 0.50 oz ai/A. A standard treatment was included at each application timing, and an untreated control was included at all sites. Standard treatments were 0.57 oz ai/A of chlorsulfuron + metsulfuron applied preemergence, 0.3 oz ai/A of chlorsulfuron + metsulfuron + 3 oz ai/A metribuzin applied to 2-3 leaf wheat, and 0.38 lb ai/A of metribuzin at the tillered, January, and February timings. All herbicide treatments were applied with a C02 backpack sprayer in 20 gallons/A total volume. Postemergence MON 37500 treatments were applied with 50 % v/v NIS. Wheat injury in the cheat experiments, pooled over three locations, was less with MON 37500 than the standard treatment. Cheat control pooled over three locations was generally excellent and was greater with MON 37500 than the standard at three of five timings.

In the autumn of 1996, eight field experiments were conducted to evaluate the effects of MON 37500 application timing on wheat injury, cheat control, and wheat yield. Treatments were MON 37500 at 0.5 oz ai/A applied at 3-week intervals from preemergence to the second week in March plus an untreated control. Plot size was '8 x 25 ft' with 4 or 6 replications. All treatments were applied with a C02 backpack sprayer in 20 gallons/A total volume with 50 % NIS. Minor wheat injury occurred at some sites in the fall when treatments were applied prior to substantial tillering. Cheat control was slightly higher with the autumn and March treatments than the January and February treatments, when the cheat was not as actively growing. The preemergence treatment controlled less cheat than postemergence treatments. All MON 37500 treatments increased wheat yield, but the preemergence treatment increased yield less than the postemergence treatments. Timing of postemergence treatments did not affect wheat yield.

Winter wheat and cheat response to foliar nitrogen applications.

S.B. Phillips, J. Chen, W.R. Raun, G.V. Johnson, D.A. Cossey, D.S. Murray, and R.B. Westerman.

Growing winter wheat cultivars in a weed-free environment is necessary for optimum grain yield. Cheat (Bromus secalinus L.) is an important grass weed in winter wheat in Oklahoma. Wheat grain yield losses can exceed 40 % in fields heavily infested with cheat. A 2-yr field experiment was initiated in the autumn of 1994 at Stillwater, OK, to evaluate the influence of N rate and source of foliar fertilizer on the growth of winter wheat and cheat. Foliar fertilizer solutions evaluated included urea-ammonium nitrate (UAN), ammonium hydroxide, and ammonium sulfate. Three wheat cultivars (Tonkawa, Longhorn, and Jagger) also were evaluated. Foliar N was applied after winter wheat had completed flowering but 1-2 wk prior to cheat flowering. Yield of wheat, grain protein, and yield of cheat were determined after harvest. Cheat seeds also were collected for germination tests. Wheat grain yields were not reduced by foliar-applied N following wheat flowering, whereas wheat grain protein increased significantly (1-3 % protein). Both UAN and ammonium hydroxide solutions significantly desiccated immature cheat heads and reduced cheat seed production. This response resulted in cheat yield being significantly reduced by UAN and ammonium hydroxide applications. Linear-plateau models indicated that foliar-applied UAN and ammonium hydroxide at a rate of 12 lb/A can result in cheat reduction (percent germination * cheat yield versus check) of 55 %. This reduction in the cheat population could prove to be beneficial to subsequent winter wheat crops. Similar differences in flowering between weed and crop in other production systems may reveal additional windows of opportunity for applying foliar fertilizers aimed specifically at weed control.

One field experiment was established in the autumn of 1994 at Stillwater, OK, on a Norge loam (fine mixed, thermic Udertic Paleustoll). The experimental design employed was a randomized complete block with two replications. Main plot size was '8.5 by 100 ft', and subplots were '8.5 by 10 ft' in both years. In the 1994-95 crop season, two winter wheat varieties (Tonkawa and Longhorn) and one foliar application (urea ammonium nitrate (UAN) were used in a complete factorial arrangement of treatments. In the 1995-96 crop season, winter wheat cultivars (Tonkawa and Jagger) and three foliar applications (UAN, ammonium hydroxide (NH4OH), and ammonium sulfate ((NH4)2SO4) were evaluated. In the autumn of 1994, the entire experimental area was fertilized with 90 lb/A of diammonium phosphate (18-46-0) broadcast and incorporated preplant. Cheat was dribble applied to the entire area at a seeding rate of 45 lb/A. The wheat was sown using a conventional grain drill at a seeding rate of 80 lb/A. Foliar N was applied to the Tonkawa plots on 11 May and to the Longhorn plots on 16 May, after flowering had taken place in each wheat cultivar but prior to cheat flowering. Foliar applications were made using a logarithmic sprayer that was calibrated to deliver 18.93 gal/A. By constantly diluting the liquid fertilizer in a fixed volume canister while traveling at a speed of 3.1 mi/hr, rates were reduced by half every 10 ft. The sprayer was equipped with 6-11002 degree tip nozzles on 1.7-ft centers. In the 1994-95 crop year, three passes over each plot were made, thus delivering a total volume of 56.79 gal/A. In 1995-96, two passes were made (37.86 gal/A). For all foliar applications, the spreader X-77 (manufactured by ORTHO, St. Paul, MN) was applied at a rate of 0.13 oz/gal of solution. Using the sprayer discussed, N rates ranged from 0.2 to 146 lb/A for foliar N-fertilizer solutions evaluated from 1994 to 1996. In the 1995-96 growing season, the seeding rate for winter wheat was 70 lb/A, whereas the seeding rate for cheat remained at 45 lb/A. No preplant fertilizer treatments were applied in the autumn of 1995. Foliar N fertilizer was applied on 9 May, 1996, to both the Tonkawa and Jagger plots. Foliar N application dates were determined for all cultivars in both years after 20 random wheat heads from each cultivar and 20 random cheat heads were selected and examined under a microscope to assess complete wheat flowering, but incomplete cheat flowering. During the 1994-95 crop year, cheat and wheat were harvested every 10 ft using a self-propelled combine, whereby the blower and sieves were set to collect the cheat seed and all other fine materials in the bin. In the 1995-96 crop year, cheat and wheat were harvested every 5 ft. The harvested samples were cleaned with a small seed cleaner to separate cheat seed, wheat seed, and other material. Yields of wheat and cheat were determined after cleaning. Total N analyses of wheat grain samples were accomplished using dry combustion. Grain protein content was calculated by multiplying percent N in the grain by 5.7. Cheat reduction was calculated as the following:

Cheat reduction (%) = 1 - CG (%) * CY / B

where CG is the percentage of cheat germination, CY is the yield of cheat, B is the product of the highest percentage cheat germination and the yield of cheat where no foliar N was applied. One hundred cheat seeds from each treatment were placed on wet paper and refrigerated at 39°F for 5 days, then placed in a germination chamber (77°F). A germination count was performed after 7 days. Wheat and cheat yields, cheat reduction, and wheat grain protein were evaluated using two-segment linear-plateau models. Linear-plateau programs were adapted using the NLIN procedure outlined in SAS. Equations for the linear-plateau models were y = b0 + b1 [min (X, A)], such that b0 is the Y-intercept, and b1 is the slope of the line up to where X (N rate) = A (point where the combined residuals were at a minimum). Best estimates for b0, b1, and the point of intersection (joint for linear and plateau portions, defined here as the critical N rate) were obtained from the model, which minimized combined residuals. Combinations of possible values of b0, b1, and the point of intersection were evaluated (holding the other two constant) and ultimately resulted in the highest coefficient of determination. Results from regression are reported on the means over replications.

Wheat and cheat responses to foliar N fertilizers, 1994-95. Foliar-applied UAN had no effect on wheat grain yields. This finding agreed with results of previous studies that showed no grain yield response to foliar applied N at or near anthesis. However, linear-plateau models for foliar N rate versus wheat grain protein were significant. Significant protein increases were observed at N rates up to 15 lb N/A for the Longhorn plots, as shown by the joint value from the linear-plateau model. Increases in grain protein ranged from 1 to 3 % as a result of applying foliar N when compared to plots that did not receive foliar N applications. Linear-plateau models for foliar N rate versus cheat yield and cheat reduction were all significant. Three days after foliar N solutions were applied, dissected cheat heads revealed substantial desiccation of stamens and stigma branches. In addition, severe burn on the leaves of wheat and cheat could be observed in the field at the high N rates, when compared to plots that did not receive foliar N. Desiccation caused leaves to drop and hastened cheat physiological maturity. This in turn reduced harvestable cheat seed, which confirmed our hypotheses that foliar application of N fertilizer 1 to 2 wk before cheat flowering could desiccate immature cheat heads and reduce seed set. Cheat yields decreased significantly at low N rates, but this was variable over variety. Cheat reduction ranged from 47 to 55 % when foliar UAN was applied at rates between 8.2 and 12.6 lb N/A prior to cheat flowering for both cultivars.

Wheat and cheat responses to foliar N fertilizers, 1995-96. In the 1995-96 crop year, results similar to 1994­95 were found, whereby wheat yields showed no response to applied foliar N and did not differ over N source. Linear-plateau models for foliar N rates versus wheat grain protein content were significant for all sources, excluding the (NH4)2SO4 application. Similar to results in 1994-95, critical N rates ranged from 13 to 23 lb N/A. Wheat grain protein increased as much as 4 % as a result of applying foliar N. Linear-plateau models for foliar N rates versus cheat yield were all significant, excluding the UAN foliar treatment. Cheat yields were decreased from 357 to 116 lb/A with foliar N rates of 4.4 lb/A when compared with plots that did not receive foliar N applications. Cheat reduction was variable in 1995-96 depending on N source. Foliar UAN applied at a rate of 12 lb N/A resulted in a 55 % cheat reduction in 1995, although a similar 19 lb N/A rate was required for a 52 % cheat reduction in 1996. A 70 % cheat reduction was achieved when NH4OH was applied at 0.7 lb N/acre. Critical N rates from linear-plateau models were not entirely consistent for the two cultivars. However, excellent cheat reduction was achieved at low N rates in both cultivars. Rates of 0.7 lb N/acre, using NH4OH, provided 70-80 % cheat reduction. Cheat reduction ranged from 47-67 % when foliar (NH4)2SO4 solution was applied at rates between 0.4 and 5.1 lb N/A. Increased foliar N fertilizer, prior to cheat flowering, generally decreased cheat yield and increased cheat reduction.

Winter wheat grain protein increased 1-4 % when foliar N fertilizer was applied after wheat flowering. Grain protein was maximized in the 1994-95 crop year at a foliar N rate of 15 lb/A. In the 1995-96 growing season, linear-plateau models also indicated that wheat grain protein increased 4 % with foliar N rates between 13 and 23 lb/A when compared to plots not receiving foliar N following wheat flowering. Foliar N applied after wheat flowering did not affect wheat yields in either year. Cheat yield and cheat reduction showed significant responses to foliar N applications. Cheat yield significantly decreased with increased foliar applied N fertilizer prior to cheat flowering. Foliar UAN applied at rates of 12 and 19 lb N/A achieved over a 50 % reduction in cheat. Ammonium hydroxide applied at a rate of 0.7 lb N/A resulted in a 70 % cheat reduction. Linear-plateau models suggest that 5.1 lb/A was the critical N rate necessary for a 67 % cheat reduction using (NH4)2SO4 foliar solution. The responses of wheat and cheat to foliar N application in this study indicate that foliar application of N fertilizer can be used to effectively increase winter wheat protein and to decrease cheat yield. This decrease in cheat yield should be beneficial to subsequent winter wheat crops. Similar differences in flowering between weed and crop in other production systems may reveal additional windows of opportunity for applying foliar fertilizers aimed specifically at weed control.

Departmental changes.

The Department of Agronomy was renamed the Department of Plant and Soil Sciences in 1997. No personnel changes have occurred.

Publications.

Gavi F, Basta NT, and Raun WR. 1997. Wheat grain cadmium as affected by long-term fertilization and soil acidity. J Environ Qual 26:265-271.

Gavi F, Raun WR, Basta NT, and Johnson GV. 1997. Effect of sewage sludge and ammonium nitrate on wheat yield and soil profile inorganic nitrogen accumulation. J Plant Nutr 20(2&3):203-218.
Johnson JP, Carver BF, and Baligar VC. 1997. Expression of aluminum tolerance transferred from Atlas 66 to hard winter wheat. Crop Sci 37:103-108.

Johnson JP Jr, Carver BF, and Baligar VC. 1997. Productivity in Great Plains acid soils of wheat genotypes selected for aluminum tolerance. Plant and Soil 188:101-106.

Kanampiu FK, Raun WR, and Johnson GV. 1997. Effect of nitrogen rate on plant nitrogen loss in winter wheat varieties. J Plant Nutr 20(2&3):389-404.

Krenzer G. 1997. Economic impact of grazing termination in a wheat-grain stocker cattle enterprise. OSU Cooperative Ext. Service, PT 97-5, Vol. 9, No. 5.

Krenzer G, Austin R, and Luper C. 1997. Wheat forage variety trials. 1996-97. OSU Cooperative Ext. Service, PT 97-32, Vol. 9, No. 32.

Krenzer G, Strickland G, Hunger B, Austin R, and Luper C. 1997. Grain yield wheat variety trails 1996-97. OSU Cooperative Ext. Service, PT 97-33, Vol. 9, No. 33.

Raun WR, Johnson GV, Hattey JA, Taylor SL, and Lees HL. 1997. Nitrogen cycle ninja, a teaching exercise. J Nat Resour Life Sci Educ 26:39-42.

Smith EL, Carver BF, Hunger RM, Sherwood JL, Ward RG, and Jordan BG. 1997. Registration of OK91P648, an acid-soil tolerant wheat germplasm. Crop Sci 37:296-297.

Taylor SL, Johnson GV, and Raun WR. 1997. A field exercise to acquaint students with soil testing as a measure of soil fertility status and field variability. J Nat Resour Life Sci Educ 26:132-135.

DEPARTMENT OF ENTOMOLOGY AND PLANT PATHOLOGY

127 Noble Research Center, Stillwater, OK 74078, USA.

R.M. Hunger, L.J. Littlefield, L.L. Singleton, J.L. Sherwood, and M.E. Payton (Department of Statistics).

Barley yellow dwarf virus.

BYDV was not as severe in Oklahoma during 1996-97 as during 1995-96. In plots located near Stillwater for studying the efficacy of Gaucho (imidacloprid, Gustafson, Inc.) seed treatment to reduce aphids and BYDV, the incidence of greenbugs was much less than during 1996-97, but the incidence of bird cherry-oat aphids was much higher. Gaucho significantly reduced the numbers of aphids in these plots at all treatments tested (1, 2, and 3 fl oz/cwt), although the number of aphids in plots treated with 1 fl oz nearly equaled the number of aphids in the control plots by March, 1997. BYDV was present in the plots, but a late, killing freeze and severe WSBMV greatly confounded rating of BYDV symptoms and the measurement of yield parameters as affected by aphids and BYDV.

Wheat leaf rust.

Wheat leaf rust was extremely severe and widespread in Oklahoma during 1996-97. Autumn 1996 was characterized by abundant rainfall and mild temperatures, which resulted in a severe epidemic of leaf rust. The first killing frost for much of the state did not occur until mid to late November, 1997, and was followed by a relatively mild and wet winter, which facilitated the overwintering of leaf rust in Oklahoma. Favorable climatic conditions continued during the spring, and this, combined with the large number of acres of wheat planted to wheat cultivars either susceptible or moderately susceptible to leaf rust (Karl, Karl 92, and 2163) resulted in a severe epidemic of leaf rust in Oklahoma. A freeze in mid-April appeared to decimate the Oklahoma wheat crop and decreased interest in applying fungicides to control leaf rust. However, nearly ideal climatic conditions after the freeze resulted in an outstanding wheat crop, and trials showed yield increases of 10-15 bu/A on susceptible cultivars resulting from a single application of Tilt at growth stage 8 of Feeke's scale (E. Krenzer, Plant & Soil Sciences, O.S.U., personal communication).

Seedling (greenhouse) and adult plant (field) reactions to wheat leaf rust of entries in the 1997 Southern Regional Performance Nursery (SRPN) were evaluated and are presented (Table 1). Seedling reaction was determined using a mixture of P. recondita f. sp. tritici urediospores collected in spring, 1995, from field plots of the HRWW cultivars Danne, Chisholm, and Karl. The avirulence/virulence formula of this urediospore mixture was determined by three replicate inoculations of a set of single-gene differentials. First leaves of 10-15 seedlings of each SRPN entry were inoculated by brushing with infected Danne seedlings. Following inoculation, SRPN entries were kept in a mist chamber at 68-72°F for 24 hr and then moved to greenhouse benches. Leaf rust reaction was determined 10-12 days later (Stakeman. USDA Bull. #E617, 1962, 53 pp). Adult plant reaction to leaf rust was determined in field plots planted on the 11 October, 1996, near Stillwater (STW) and the 25 October, 1996, near Altus (ALT), OK. The experimental design was a randomized complete block with three replications per entry. Plots consisted of six, 10-ft rows. Seed was planted 1-inch deep at 1 bu/A, and recommended management practices were followed for each location. Leaf rust severity was assessed on the 5 May, 1997 (STW), and the 29 April, 1997 (ALT), using a disease index of 1-9, where 1-3, 4-6, and 7-9 indicate increasing leaf rust severity within resistant, intermediate, and susceptible categories, respectively. Nurseries were harvested on the 19 June, 1997 (STW), and the 25 June, 1997 (ALT), and yield (bu/A) was calculated.

Based on a comparison of seedling and field ratings presented in Table 1, 17 entries (05-09, 19-24, 29, 31, 34, 35, 37, and 38) appear to have only adult plant resistance. Only one entry (10) appears to have seedling and adult plant resistances to leaf rust. Five entries (03, 15, 16, 17, and 18) exhibited severe leaf rust in the field (average severity rating at Stillwater and Altus between 6.6-9.0) and yielded an average of 35.4 bu/A. Twenty-one entries (01, 02, 04, 11-14, 19, 25-28, 30, 32, 33, 36, 39-42, and 45) exhibited a moderate leaf rust severity (average rating of the two locations between 3.6-6.5) and yielded an average of 47.2 bu/A. Nineteen entries (05-10, 20-24, 29, 31, 34, 35, 37, 38, 43, and 44) had a high level of leaf rust resistance (average rating of the two locations between 1.0-3.5) and yielded an average of 53.9 bu/A.

Table 1. Seedling (greenhouse) and adult plant (field) reactions to wheat leaf rust of entries in the 1997 Southern Regional Performance Nursery. Avirulence/virulence formula of urediospore mixture used in inoculations: 17 19 26 / 1 2a 2c 3 3ka 9 11 16 24 30. Seedlings rated according to Stakeman's system for rating rust reaction (USDA Bull. #E617, 1962, 53 pp.). Severity values are the average of three replications rated on a scale of 1-9, with 1-3 = resistant, 4-6 = intermediate, and 7-9 = susceptible. Yield values are the averages of three replications in bu/A. Values followed by an asterisk (*) are significant at P = 0.05).

Entry	Selection number	Seedling reaction			Adult plant leaf rust reaction
					Stillwater		Altus
		rep 1	rep 2	rep 3	severity	yield	severity	yield
01	CI1442	3+	3+	3+	5.3	25.0	4.0	16.6
02	CI13996	3	3+	3+	6.0	32.3	5.0	32.1
03	PI49559 4	3+c	X;3-	3+	8.7	30.8	9.0	33.7
04	OK9361 7	3	3+c	3+	5.7	49.3	4.7	46.2
05	OK94P5 49	3c	3+	X;3-	2.0	50.0	2.0	49.3
06	OK94P4 61	3+	3+	3+	1.7	41.9	1.7	32.3
07	TX91D6 825	3+	3+	3+	1.0	64.7*	2.0	57.6
08	TX91D6 856	X;3-	3+	3+	1.3	56.9	1.3	72.2*
09	HBG03 58	3c	3c	3+	1.3	55.0	2.0	45.2
10	TX94V2 327	;	X;3-	3c	2.0	56.8	1.0	66.7*
11	TX94V3 329	3+	3+	3-c	6.3	38.6	5.7	59.1
12	TX95V4 926	3+	3+	3+	5.3	47.0	5.7	48.9
13	TX95V4 933	3c	3+	3+	6.0	35.5	4.7	58.7
14	TX95V5 332	3c	3+	3c	7.3	34.7	4.3	48.3
15	TX94V2 130	3+	3+	3+	8.0	36.2	9.0	29.0
16	CO9104 24	3+	X;3-c	X;3	6.0	53.8	8.0	43.3
17	CO9206 96	3c	3-c	3+	7.3	33.9	8.0	35.1
18	CO9407 00	3+	X;3-	3+c	7.3	27.6	6.7	30.7
19	KS94H 147	X;3-	3+	3-c	5.7	57.6	5.7	56.4
20	KS9410 64-6	3+	4	X;3-	2.3	43.7	2.0	39.0
21	KS9409 35-125- 5+	3c	3+	3+c	1.7	59.3*	2.0	61.9
22	KS85W 663-11- 6+	3+c	3+	4	2.0	55.2	1.1	51.6
23	KS84W 063-9-3 9+	X;3-	3+c	4	1.0	68.1*	1.3	66.5*
24	N95L15 8	3+	3+	3+c	2.7	57.8*	1.7	60.2
25	NE9340 5	X;3-	3+	X;3-c	4.0	46.5	4.0	43.2
26	NE9342 7	3c	3+	X;3-c	4.3	51.7	3.0	56.8
27	NE9349 6	3	X;3-	X;3-	4.3	44.2	4.0	41.9
28	NE9463 2	3+	3+	3+	4.7	53.2	2.7	63.5*
29	W94-04 2	3+	3+	X;3c	2.3	56.0	2.0	62.5*
30	W94-13 7	3+	X;3-	3+	4.0	54.8	6.0	46.3
31	W94-32 0	4	3+	3+	3.3	54.6	2.7	61.1
32	W94-24 5	3+	3+	3+	7.0	42.6	6.3	43.3
33	W94-43 5	3+c	3+c	3+c	4.3	48.7	4.7	52.5
34	WX94-3 504	3+	3+	3+	3.0	38.2	2.7	51.8
35	WX94-1 604	3+	3+	3+	3.0	50.4	1.0	36.9
36	XH1877	3+	3c	3+	4.3	58.7*	3.3	53.0
37	XH1881	X3+c;	3+	3+	2.0	63.5*	1.3	65.9*
38	WX95-2 401	3+	3+c	3+	3.3	46.6	2.7	44.9
39	T89	3+	3+	3+	6.7	36.4	6.3	42.3
40	T86	3+	3+c	X;3-	4.3	55.5	5.3	63.2*
41	T93	3+	4	3+	6.0	42.8	5.7	42.5
42	T94	3+	3+	3+	6.0	49.2	4.0	43.3
43	G1594	4	X;3-c	3+	4.3	50.0	2.0	50.0
44	G1720	3+	3+	3c	4.3	49.7	2.7	52.9
45	G12017	3+	3+	3+	5.0	55.8	3.3	61.5
Average					---	48.3	---	49.3
LSD (P = 0.05)					1.5	10.3	1.6	9.7
CV (%)					21.0	13.2	24.8	12.1

Wheat soilborne mosaic virus.

Reaction to WSBMV was determined for wheat entries in the 1997 Southern Regional Performance Nursery (SRPN) and the 1997 Northern Regional Performance Nursery (NRPN) (Table 2). The trial was conducted near Stillwater, OK, in a Norge loam soil. Soil tests were used to ensure adequate fertilization (N-P-K) and pH for a production goal of 40 bu wheat/A. The experimental design was a randomized complete block with three replications (three, 2-ft rows per entry). Seeds were planted about 1.0 in. deep at a rate of 24 seeds/2-ft row on 09 October, 1996. Rows of Vona (WSBMV-susceptible, WSSM-susceptible), Sierra (WSBM-resistant, WSSM-susceptible) and Hawk (WSBM-resistant, WSSM-resistant) were planted between reps to monitor the presence and distribution of WSBM and WSSM. Glean (chlorsulfuron, 0.33 oz/A in 24 gal) was applied on 14 November, 1996, to control weeds, and Tilt (propiconazole, 4 oz/A in 22 gal) was applied on 14 April, and 10 May, 1997, to control foliar diseases. Entries were assessed for symptoms on 3 March, 1997, using a visual assessment (VA) index of 1-4, where 1 = no stunting, no mosaic, 2 = slight stunting and/or slight mosaic, 3 = moderate stunting and/or slight mosaic, and 4 = severe stunting and/or severe mosaic. Young foliage was collected from each row of each entry on 5 March, 1997, for evaluation by ELISA (Hunger et al. 1991. Crop Sci 31:900-905). The trial was harvested on 7 June (SRPN) and 11 June (NRPN).

WSBM was distributed uniformly as indicated by the susceptible checks. WSSMV was detected by ELISA (absorbance values > 0.30) in 11 of 39 Vona, Hawk, and Sierra samples. Vona samples (n = 32) had VA indices of 4 and values from ELISA for WSBMV from 1.12 to 1.63 (mean = 1.34). Sierra and Hawk samples (n = 18) had VA indices of 1 or 2 and values from ELISA for WSBMV from 0.01 to 0.09 (mean = 0.04) with four exceptions (0.13. 0.24, 0.80, and 1.17). Twenty-five SRPN entries and seven NRPN entries were resistant to WSBM (VA & 2.0 and values from ELISA consistent with no detectable virus or virus concentrations less than those of the susceptible check cultivar Vona). The 25 SRPN entries identified as WSBMV-resistant had an average yield of 95.5 gm (sd = 30.0) and an average 1,000-kernel weight of 30.1 gm (sd = 3.1) compared to an average yield of 33.1 gm (sd = 21.7) and a 1,000-kernel weight of 23.1 gm (sd = 3.1) for the 20 WSBMV-susceptible entries. The seven NRPN entries identified as WSBMV-resistant had an average yield of 92.5 gm (sd = 20.5) and an average 1,000-kernel weight of 26.8 gm (sd = 2.8) compared to an average yield of 26.1 gm (sd = 11.5) and an average 1,000-kernel weight of 19.8 gm (sd = 2.4) from the 28 WSBMV-susceptible entries. These results indicate significant effects of WSBMV on yield and seed quality and also demonstrate the effectiveness of this nursery to screen for reaction to WSBMV.

Ultrastructure of Polymyxa graminis.

Studies to elucidate the ultrastructure of the vector of WSBMV, P. graminis, throughout its life cycle in wheat roots are in their fourth year. Following is a brief summary of the studies in wheat roots, based on interference contrast, laser confocal, scanning electron, and transmission electron microscopy of plants grown in P. graminis-infested field soil.

The first stage to appear following infection is wall-less plasmodia, 10-12 days after planting (DAP). Development beyond that can follow two divergent paths, the zoosporogenic path or the resting spore path. In some instances the zoosporogenic path is followed initially, producing motile zoospores that reinfect the root cells, subsequently leading to the formation of resting spores. In other instances, development proceeds directly along the resting spore path, leading to the formation of resting spores, thus bypassing the zoospore stage. What controls these different paths is not known.

In the case of zoosporogenic development, thin-walled, elongated zoosporangia that contain numerous exit tubes are common by 18-30 DAP. Cytoplasm within zoosporangia cleaves to form up to several hundred biflagellate zoospores. Those spores swim from zoosporangia through exit tubes into adjacent cortical cells, into intercellular space, or to the outside of the root. They then reinfect to initiate secondary infections.

Depending on whether or not the zoosporogenic stage has been bypassed, the resting spore stage becomes evident anytime between 18 and 40 DAP. Sporogenic plasmodia, i.e., naked plasmodia that develop into resting spores, never develop an enclosing wall, as do the zoosporogenic plasmodia that become zoosporangia. Sporogenic plasmodia cleave synchronously and completely to form up to 200-300 primordial resting spores, depending on the size of the plasmodium. The entire mass of the wall-less sporogenic plasmodium is converted into a mass of resting spores. Initially resting spores are tightly packed, thin-walled, and angular in shape. As resting spores mature, they increase somewhat in size, their wall thickness increases, and interstitial spaces form between the expanding resting spores in the sporosorus. Sporosori remain held together by the incomplete separation of adjacent resting spore walls. Resting spores contain four wall layers over ~ 75 % of their surface plus a fifth, innermost layer restricted to ca. 25 % of the spore surface that faces the outside of the sporosorus. That outer-facing region of the resting spore that contains the fifth wall layer is also the area through with germination occurs. Studies are planned to examine the ultrastructure of resting spore germination.

Breeding for disease resistance.

Incorporation of genetic resistance from various sources into hard red winter wheat genotypes continues to be pursued. Sources of the disease resistance include wheat lines from Eastern Europe (leaf rust), lines from South Africa (leaf rust), emmer (leaf rust and WSBMV), lines from Oklahoma (leaf rust), lines from Kansas (WSMV and leaf rust], lines from Indiana (leaf rust), and a line from the USDA-ARS (WSMV). Backcrossing is being used to transfer the resistance into a HRWW background, with crosses made during the winter of 1997-98 representing the third backcross. Hence, many lines will move into the field in the 1998-99 season for testing.

Wheat root rots.

1996-97 crop-season tests. Seed treatments were evaluated for control of sharp eyespot and Fusarium root rot in wheat planted early (mid-September) and late (mid-October) at Perkins and Lahoma, OK. Six seed treatments of three fungicides (LSO68, Baytan-Captan, and RTU Vitavax-Thiram) each alone and each in combination with Kodiak (Bacillus subtilus) were compared to an untreated control. Disease incidence and severity data were taken along with determinations of test weight (#/bu) and yield (bu/A). Similarly at Stillwater, 14 seed treatments (Agro-T, Agro-T + We119A, Agro-T + We082, Agro-T + We120C (High and Low), We120C (High and Low), Agro-T + Raze + O.C., Granox-plus, Granol-nm, Vitavax 200, Raxil/Thiram, Dividend + Apron XL, and Agro-T + Apron XL) were evaluated and compared to an untreated control with a mid-October planting date.

1996-97 crop-season results. All experiments were subjected to a late-season freeze after heading, and the severity of damage as estimated at the time was much less than anticipated. Overall recovery of the crop was enhanced greatly by cool, wet, and favorable weather conditions that supported the development and maturation of new tillers after the freeze. Similarly, these conditions also suppressed the damage from root rot pathogens. At Lahoma and Perkins, severity and disease incidence were usually higher for the seed treatments as compared to the untreated checks. Sharp eyespot was the major pathogen at these locations. At Stillwater, wheat treated with most of the compounds and combinations performed better than the untreated control, which suggests that these compounds are worth further testing in our environment. Overall, grain yields were much higher at these locations as compared to the previous year. Sharp eyespot was more prominent than Fusarium spp. at all locations, which may be explained by the cool and wet conditions after the freeze. In areas of chronic root rot disease pressure, cultural control by delaying planting until after 15 October is an effective alternative to early planting, especially in fields known to be prone to root rot. This type of decision is cost effective, because it does not require major dollar investments by the producer. Planting in mid-October when soil temperatures at planting depth are lower than 77°F offers a mechanism for escaping seedling infection by this group of root rot pathogens. By contrast with early planting (1 September), we know that soil temperatures can be 87°F or greater and can result in greater potential for root rot infection and damage. For a more detailed discussion of control measures see OSU Extension Fact Sheet. No. 7622.

Departmental/personnel changes.

Dr. John Sherwood became the head of the Plant Pathology Department at the University of Georgia in June, 1997. The position he vacated as plant virologist was filled with the hiring of Dr. Jeanmarie Verchot. Dr. Verchot received her Ph.D. from Texas A&M and has been employed as a postdoctoral researcher in England until hired at Oklahoma State. Dr. Verchot should be on campus about 1 March, 1998.

The Department of Plant Pathology merged with the Department of Entomology in September, 1997. Dr. Russ Wright, Head of Entomology, became the Head of the combined departments, which was named the Department of Entomology and Plant Pathology.

Publications.

Hunger RM, Sherwood JL, Siegerist WC, Myers L, and Payton ME. 1997. Reaction of wheat genotypes to wheat soilborne mosaic (WSBM), 1996. Biol & Cul Test for Control of Plant Disease 12:123.

Hunger RM, Sherwood JL, Krenzer G, Mulder P, and Payton M. 1997. Evaluation of Gaucho 480F seed treatment to control aphids and barley yellow dwarf (BYD) in hard red winter wheat, 1996. Fungicide & Nematicide Tests 52:326.

Littlefield LJ, Delfosse P, Whallon JH, Hassan ZM, Sherwood JL, and Reddy DVR. 1997. Anatomy of sporosori of Polymyxa graminis, the vector of Indian peanut clump virus, in roots of Sorghum bicolor. Can J Plant Path 19:281-288.

 

PLANT SCIENCE RESEARCH LABORATORY, USDA-ARS
1301 N. Western St., Stillwater, OK 74075, USA.
http://www.csrl.ars.usda.gov/pswcrl/pswcrl.htm

C.A. Baker, J.D. Burd, N.C. Elliott, M.H. Greenstone, D.B. Hays, S.D. Kindler, D.W. Mornhinweg, D.R. Porter, K.A. Shufran, J.A. Webster, and Y.C. Zhu.

Host plant resistance/germplasm enhancement.

Efforts are ongoing to develop wheat germplasm resistant to RWA, greenbug, and bird cherry-oat aphid (BCOA). Thirty-nine advanced wheat lines derived from 12 different RWA-resistance sources were planted for seed increase and evaluation in preparation for germplasm release. Field evaluations of 423 winter lines were made in Stillwater during 1997. Approximately 30 % of these were selected for further evaluation. Over 400 lines were planted for 1997-98 field evaluation in Stillwater. Over 50 lines also will be planted in irrigated and dryland field tests in Goodwell, OK. Additional topcrossing of 71 RWA-resistant wheat lines to high performance recurrent parents continued. This material includes resistance genes derived from at least 24 different resistance sources. Topcrossed seed was screened for resistance to RWA, and resistant plants were transplanted to the greenhouse for increase and progeny testing. Homozygous RWA-resistant lines will be selected for germplasm release. Development of 'resistant x resistant' populations continued in order to study genetic diversity of RWA resistance.

Eleven RWA clones collected from five states were evaluated for biotypic variation on four wheat and two barley germplasm entries. Ten years after its detection in the U.S. and before the release of resistant cultivars, the RWA has not exhibited any biotypic variation in the U.S.

Screening for resistance to BCOA utilizing a newly developed root-based bioassay continued. Over 2,000 wheat accessions from the ARS National Small Grain Collection were tested for resistance. Tolerance was detected in 126 accessions. Seed of selected accessions is being increased for additional testing. Research is ongoing to confirm the validity of the seedling root-based BCOA resistance assay.

Over 4,550 wheat accessions were tested for resistance to biotype E greenbug. Five possible sources of resistance were identified and will undergo additional testing.

Biological control.

Modeling. Progress was made toward the development of an outbreak-risk prediction system for the RWA. A novel method, based on the use of multi-temporal MSS imagery and a supervised classification algorithm, was used in conducting land cover classification. Use of the method increased classification accuracy over that of traditional classification methods.

Studies of three coccinellid species suggested that differences in dispersal rates and the degree of plasticity in sex ratio and size among generations are life history traits that vary among species and provide a means for species to exploit agricultural landscapes that vary spatially and temporally.

Evaluation of aphid natural enemy effectiveness. The first phase of a project to distinguish three parasitoid species that were released against the RWA has been completed successfully, through the development of polymerase chain reaction assays that can distinguish Aphelinus varipes, A. albipodus, and two geographic strains of A. asychis. The method also is able to distinguish parasitized and nonparasitized RWAs.

Preliminary taxonomic analysis of spiders that were collected quantitatively from Halt and Tam 107 wheat has been completed. Densities did not differ between Halt and Tam 107. Taxonomic diversity was quite high. The relatively even representation across spider families is in sharp contrast to the situation in Great Britain, where one family can constitute 71-97 % of all individuals.

Integrated pest management.

Cereal insect genetic resource library (CIGRL). A Cereal Insect Genetic Resource Library (CIGRL) has been established to facilitate proactive research on the genetics of cereal pests and their natural enemies on a global level. The CIGRL will arrange for the collection, preservation, and dissemination of insect genetic material so as to promote and coordinate interactive research with scientists at other locations. A primary mission of the CIGRL will be to advance the implementation of economically and environmentally sound pest management strategies through the genetic integration of host plant resistance and biological control. Currently, the CIGRL contains over 5,000 entries from 12 different countries.

Aphid genetics. To assess the mechanism by which greenbugs generate and maintain genetic diversity, we induced the greenbug holocycle and conducted both intra- and inter-clone matings to study the inheritance of the intergenic spacer (IGS) in the offspring. Results suggest that periodic sexual reproduction is a primary mechanism for the generation and maintenance of genetic variability in greenbug populations.

Aphid ecology. Field surveys were conducted in Oklahoma, Kansas, Colorado, Nebraska, Wyoming, and Montana, from mid-June through mid-October to assess the oversummering ecology of RWA populations during the intervening period between cereal crops. Three distinct regional populations of RWAs were identified based upon their independent responses to endemic host phenological conditions and local weather. Successful dispersal from maturing cereal hosts to non-cultivated oversummering hosts appeared to be primarily of short distances. The highest densities of oversummering aphids were within 25 m of previously infested fields. RWA oversummered successfully in all three regions; however, aphids in the southern region did not oversummer if volunteer wheat was not present during August. Many of the grass species that have been deemed as suitable RWA hosts in greenhouse screening trials were not colonized by RWA in the field. The most important oversummering hosts were volunteer wheat and barley, followed by Canada wildrye (E. canadensis), crested wheatgrass (Ag. cristatum), and squirreltail (Sitanion hystrix).

Aphid-plant interactions. Collaborative research with Oklahoma State University was initiated that will focus on physiological, biochemical, and genetic aspects of aphid-host interactions. Research will employ a molecular approach to describe the biological aspects of host suitability. Aphid stylectomy procedures will be used to collect phloem exudates for analysis to determine potential constituents that confer antibiotic resistance.

Evaluation of alternate hosts and determination of economic injury levels. A second season test on the economic threshold of greenbugs on winter wheat was completed. Current thresholds are outdated since none relate aphid numbers/tiller to yield loss, control costs, and monetary value of the wheat. Our studies utilized control cost, monetary value of the crop, and number of aphids/tiller. We will continue to evaluate the economic injury level of greenbugs and other cereal aphids.

Knowledge specifically defining host relations between grasses and greenbug biotypes occurring in North America is surprisingly limited in view of the potential implications of grasses as alternate hosts for these biotypes, and as possible reservoirs for important aphid-vectored plant viruses. Susceptibility of nine cool-season grasses to eight of the nine identified greenbug biotypes were determined. Overall, all biotypes are capable of sustaining themselves on all grasses tested, eventually resulting in death to the plant, with the exception of Canada bluegrass and smooth bromegrass.

Yellow sugarcane aphid (YSCA) has been an increasingly important pest of cereal grains. As with most of the important cereal aphids that are pests of both cereal grains and native grasses, this species is probably native to warm-season grasses and has moved to relatives of these grasses. The increased importance of YSCA in the past decade on cereal grains coincides with the establishment of the Conservation Reserve Program (CRP). The widespread use of native and Old World bluestems in CRP planting may be contributing to the increased damage to cereal grains by YSCA.

Greenbugs collected from 19 counties in Oklahoma from 1996-97 wheat crops and from seven counties from the 1997 sorghum crops were identified to biotype and also to susceptibility to phosphate insecticides. Biotype K, a recently identified greenbug biotype that will kill biotype I-resistant grain sorghum was not identified in any of 26 collections from wheat but did occur in two of the 14 collections from sorghum. This is important information because commercial grain sorghum companies currently are developing biotype I-resistant grain sorghum varieties. Six collections made from wheat had resistance to phosphate insecticides. None of the collections from sorghum had phosphate insecticide resistance.

Personnel.

Dr.Yu Cheng Zhu has been a postdoctoral research associate in our laboratory from August, 1997, through March, 1998. He has been working on the development of molecular methods for distinguishing closely related species of parasitic wasps attacking RWA and other cereal aphids. He recently has accepted a postdoctoral research associate position at the USDA-ARS Grain Marketing and Production Research Center in Manhattan, KS.

Publications.

Baker CA, Mirkes KA, Webster JA, and Porter DR. 1997. A new technique for screening for bird cherry-oat aphid resistance in wheat and barley. Agron Abstr p. 87.

Elliott NC. 1997. Ecology of the Russian wheat aphid in landscape and regional scales. Proc N Cent Br Entomol Soc Am. Pp. 23-24.

Elliot NC, Michels GJ, Jr., Kieckhefer RW, and French BW. 1997. Sequential sampling for adult coccinellids in wheat. Entomol Exp Appl 84:267-273.

Hays DB and Porter DR. 1997. The role of defense gene expression in resistant and susceptible isolines of barley in response to virulent and avirulent biotypes of greenbug. Plant Physiol 114:226.

Michels GJ, Jr., Elliot NC, Romero RL, and French BW. 1997. Estimating populations of aphidophagous coccinellidae (Coleoptera) in winter wheat. Environ Entomol 26:4-11.

Miller HL and Porter DR. 1997. A technique to quantitatively measure the leaf streaking symptom of Russian wheat aphid infestation. Crop Sci 37:278-280.

Mornhinweg DW, Porter DR, Jones BL, and Webster JA. 1997. Field resistance of barley to Russian wheat aphid. Agron Abstr p. 87.

Porter DR, Burd JD, Shufran KA, Webster JA, and Teetes GL. 1997. Greenbug biotypes: They just keep coming! Proc 1997 Sorghum Conf and 20th Bien Grain Sorghum Res Util Conf. Pp. 66-70.

Porter DR, Burd JD, Shufran KA, Webster JA, and Teetes GL. 1997. Greenbug (Homoptera: aphididae) biotypes: selected by resistant cultivars or preadapted opportunists? J Econ Entomol 90:1055-1065.

Porter DR and Webster JA. 1997. Greenbug resistance profiles in wheat. Agron Abstr p. 73.

Shufran KA, Peters DC, and Webster JA. 1997. Generation of clonal diversity by sexual reproduction in the greenbug, Schizaphis graminum. Insect Molec Biol 6(3):203-209.

Shufran KA, Burd JD, and Webster JA. 1997. Biotypic status of Russian wheat aphid (Homoptera: Aphididae) populations in the United States. J Econ Entomol 90:1684-1689.