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Crop Science 43:1380-1386 (2003)
© 2003 Crop Science Society of America

CROP ECOLOGY, MANAGEMENT & QUALITY

Root and Shoot Responses to Bird Cherry-Oat Aphids and Barley yellow dwarf virus in Spring Wheat

Walter E. Riedell*,a, Robert W. Kieckhefera, Marie A. C. Langhamb and Louis S. Heslera

a USDA-ARS, NPA, Northern Grain Insects Res. Lab., Brookings, SD 57006
b Plant Science Department, South Dakota State University, Brookings, SD 57007

* Corresponding author (wriedell{at}ngirl.ars.usda.gov)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
There is little information available that describes the effects of bird cherry-oat aphids (Rhopalosiphum padi L.) and Barley yellow dwarf virus (BYDV) on cereal plant root systems. This 2-yr field experiment was conducted to determine how spring wheat (Triticum aestivum L.) root characteristics, shoot characteristics, and grain yield respond to R. padi infestation, BYDV infection, or a combination of R. padi plus BYDV. Treatments were applied at the 2- to 3-leaf stage. When measured at anthesis, plants that received R. padi treatments (300 aphid days) had about a 30% greater total root length (as measured with a minirhizotron) than control plants. Plants that received BYDV as well as those that received R. padi plus BYDV had about a 40% decrease in total root length when compared with the control. Root system characteristics at different soil depths responded differently to BYDV infection across the 2 yr of the study. In the first year, BYDV-infected plants had less root length at all soil depths measured than plants that did not receive BYDV infection. In the second year, which was drier near anthesis, root lengths of both BYDV infected and uninfected plants were similar at the 0- to 18-cm soil depth, while at deeper soil depths, plants that had BYDV infection had less root length than plants that did not receive BYDV infection. Plants that received either R. padi or BYDV had fewer tillers, less tiller height, and less shoot dry weight than control plants. The lack of R. padi x BYDV interaction for above-ground variables suggests that shoots responded to BYDV in a similar manner regardless of whether or not they received R. padi treatment. Treatments consisting of R. padi infestation or BYDV infection reduced individual kernel weight by 8 or 22%, respectively. Infection with BYDV also caused a 53% reduction in the number of kernels per meter of row and a 62% reduction in total grain yield. We conclude that R. padi infestation caused acute injury to spring wheat plants while BYDV caused chronic injury. Most of the dependent variables measured in this experiment responded to BYDV in a similar manner regardless of whether or not the plants received R. padi treatment.

Abbreviations: BYDV, Barley yellow dwarf virus • DOY, day of year • PAR, photosynthetically active radiation


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BIRD CHERRY-OAT APHIDS, which are efficient vectors of BYDV (McPherson et al., 1986), commonly infest spring wheat in the northern Great Plains (Gill, 1980; Kieckhefer and Kantack, 1980). On finding a host plant, these insect pests probe leaf, leaf sheath, or stem tissues with their proboscises until they find a suitable tissue (usually phloem) from which to extract plant sap. Infestation by virus-free R. padi generates no dramatic damage symptoms on spring wheat plants (Kieckhefer et al., 1995).

The magnitude of yield loss caused by R. padi depends on the extent of the insect infestation (Kieckhefer et al., 1995), the timing of the infestation during the growing season (Kieckhefer and Gellner, 1988), the growth stage of the plant at infestation (Pike and Schaffner, 1985), and whether the R. padi transmit BYDV to the crop (McPherson et al., 1986). BYDV is a phloem-restricted Luteovirus obligately vectored by several species of aphids (Kolb et al., 1991). Symptoms of BYDV infection include leaf discoloration (yellow or red), leaf necrosis, stunting, and delay or lack of heading (Comeau, 1987; Hewings and D'Arcy, 1986; Riedell et al., 1999). BYDV is considered to be one of the most economically important diseases of cereal grain production in the world (Hoffman and Kolb, 1997).

The effects of R. padi infestation or BYDV infection on aerial portions of small grains have been studied extensively. However, there is relatively little information available on how below-ground plant portions are affected by these two stress-causing organisms (Hoffman and Kolb, 1997; Riedell and Kieckhefer, 1995). Spring wheat infested with R. padi (300 aphid days) at the 2 leaf stage had 50% less root length than uninfested plants when measured at the 3- to 4-leaf stage (Riedell and Kieckhefer, 1995). These dramatic reductions in root length were not present when plants were again measured 30 d after the insects were removed. In oat (Avena sativa L.) and barley (Hordeum vulgare L.), the effects of BYDV infection on roots are more rapid and severe than on the shoots (Kainz and Hendrix, 1981; Kolb et al., 1991). BYDV-infected plants grown in aeroponic mist boxes had less root length and smaller root to shoot ratios than uninfected plants (Hoffman and Kolb, 1997). Haber and Comeau (1989) demonstrated that barley root growth was severely inhibited within 4 d after BYDV inoculation. There is no information in the literature that documents the interactions of R. padi infestation and BYDV infection on cereal plant root growth.

Because roots play a key role in plant growth and development by providing the shoot with water and nutrients from the soil, the reduction in water and nutrient uptake due to root stunting itself may be an important factor contributing to the reduction in grain yield suffered by plants infested with R. padi or infected with BYDV (Hoffman and Kolb, 1997). Reduced root length may also make damaged plants more susceptible to other physical stresses such as drought. Drought and BYDV both represent severe stresses when they occur separately; when they occur together, the combined effects on susceptible cultivars is devastating (Monneveux et al., 1989).

The use of the minirhizotron technique for evaluating root system growth and morphology (Box and Ramseur, 1993; Merrill et al., 1994) facilitates the study of R. padi and BYDV effects on cereal root growth under field conditions. This technique employs a miniature video camera to view and record root images in minirhizotrons (clear methyl methacrylate tubes placed in the ground) (Upchurch and Ritchie, 1983). Root length and other root characteristics present within the video images can be quantified by image analysis software (Cheng et al., 1990; Tremmel, 1995).

We felt that a field study of the influence of R. padi infestation and BYDV infection, alone and in combination, on root and shoot characteristics of spring wheat would be a step toward understanding the mechanism of yield loss in plants damaged by these two stress-causing organisms. Increased understanding of how plants are affected by these organisms might help breeders develop tolerant or resistant lines or may suggest new ways to manage the crop to reduce the yield loss to these organisms. Thus, the objective of this 2-yr field experiment was to study R. padi infestation and BYDV infection in spring wheat to determine how spring wheat root characteristics, shoot characteristics, and grain yield respond to these stress-causing organisms when they are applied alone and in combination.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Experiment Design, Crop Management, and Environmental Monitoring
The field study was conducted during the 1995 and 1996 growing seasons at the Eastern South Dakota Soil and Water Research Farm near Brookings, SD (44° 19' N lat., 96° 46' W long., and 500 m altitude). The Barnes (formerly Vienna) loam (fine-loamy, mixed Udic Haploboroll) soils at this location have an Ap1 horizon at about a 28-cm depth, an Ap2 horizon at a depth of about 54 cm, and an Bw1 horizon at a depth of about 78 cm (Maursetter et al., 1992). Experiments were conducted on spring wheat grown in plots that were managed under an annual soybean-spring wheat rotation. Plots following the wheat phase of the rotation received a fall chisel plow treatment (to a depth of about 25 cm) followed by a spring disk–harrow treatment (to a depth of about 10 cm) for seedbed preparation. Plots following the soybean phase received only a spring disk–harrow treatment. Weed control was accomplished by hand weeding of plots on 5 June 1995 (DOY 156) or chemical application [propanil plus MCPA ester; 3',4'-dichloropropionanilide plus (4-chloro-2-methylphenoxy)acetate] on 13 June 1996 (DOY 164). Crop production practices commonly used for soybean-spring wheat rotations in the northern Great Plains were used in this experiment.

Kernels of spring wheat (cv. Sharp) were planted (2 May 1995, DOY 122; 3 May 1996, DOY 133) at a rate of 90 kg ha-1 with a John Deere 750 drill with 19-cm-row spacing. Kernels were planted at a depth of 2.5 cm. Soil test-based (Gelderman et al., 1987) rates of N and P fertilizers (4000 kg ha-1 grain yield goal) were applied with the drill at planting time. Dry P fertilizer was placed directly in the seed furrow while liquid N fertilizer was placed in a subsurface inter-row band to a depth of about 15 cm below the soil surface (Gerwing and Gelderman, 1996). When the seedling coleoptiles began to emerge from the ground, areas of the field were selected for uniform germination and prepared for experimentation by placement of insect cages in a 4 x 4 arrangement with about 5 m between cages. Cages (1 m long x 1 m wide x 0.5-m-high enclosures of 14 x 18 wire mesh screen, the surface area of which contained about 20% wire and 80% open space) were used to exclude aphidophagous insects and, in the case of control plots, to compensate for possible variation in plant growth caused by screening (Kieckhefer and Kantack, 1988). Treatments are described below.

Soil temperature and matric potential were measured in the plots at 15-, 30-, and 60-cm depths over the course of the experiment. Soil temperatures (measured between 800 and 1000 h) were monitored with temperature probes attached to biophenometers (OmniData, Inc., Logan, UT) while tensiometers (Irrometer, Inc., Riverside, CA) were used to record soil matric potential.

Rhopalosiphum padi and BYDV Treatment Protocols
Four experimental treatments (control, bird cherry-oat aphid infestation, BYDV infection, and a combination of aphids and BYDV) were evaluated in a randomized complete block design with four replications. Nonviruliferous colonies of aphids used in these experiments originated from nymphs deposited on Parafilm (American National Can Co., Greenwich, CT) membranes by adults collected from natural populations in the field. Nymphs were collected and transferred to caged colony plants (‘Hazen’ barley). Colonies were maintained in growth chambers at 20°C, 225 µmol s-1 m-2 PAR, and 16 h light/8 h dark in the insect rearing facility at the Northern Grain Insects Research Laboratory at Brookings, SD (Kieckhefer and Gellner, 1992). To produce viruliferous aphids, nonviruliferous aphids were transferred from colony plants to BYDV-infected (PAV strain) ‘Coast Black Oat’ plants and allowed to feed and reproduce several weeks in the greenhouse.

All treatments were established when the wheat crop was in the 2- to 3-leaf stage (15 May 1995, DOY 135; 30 May 1996, DOY 150). Nonviruliferous aphids for the bird cherry-oat aphid infestation treatment were introduced into the appropriate field cages by randomly distributing aphid-infested leaves from two laboratory colonies within each cage. The mean number of aphids per stem, estimated by counting the number of aphids on 10 stems per cage, was monitored three times per week. Plants were sprayed with acephate (O,S-dimethyl acetylphos-phoramidothioate) systemic insecticide to the point of runoff when the total aphid day accumulation reached 300 (e.g., 30 aphids per stem for 10 d; Kieckhefer et al., 1995). The combined aphid and BYDV treatment (R. padi + BYDV) was accomplished in the same manner with viruliferous aphids. For both R. padi and R. padi + BYDV treatments, aphids were present in cages an average of 13 d in 1995 and for an average of 14 d in 1996. Control plants received an acephate insecticide treatment at the same time aphids were removed from the 300 aphid day treatments. The BYDV treatment was accomplished by randomly distributing aphid-infested leaves from two viruliferous aphid colonies into each BYDV field cage. The plants were treated with acephate insecticide after a 48-h period.

Cages, which were present on all plants (including controls) for the same length of time, were removed about 20 d after the aphid treatments were started. There were no natural aphid infestations in the plots during the experiments.

Plant Measurements and Statistical Analyses
A Bartz Technology (Santa Barbara, CA) minirhizotron system was used to characterize root distribution in the soil profile. Cellulose acetate butyrate minirhizotron tubes (1.85 m long; 5.7-cm o.d.; 5.1-cm i.d.) were installed 22 d after planting in 1995 (24 May 1995, DOY 144) and 16 d after planting in 1996 (29 May 1996, DOY 149). In both years, tubes were installed perpendicular to the crop row from the south side of each cage. A soil probe equipped with a reverse taper bit (5.7-cm diam) was used to bore minirhizotron tube observation holes. A metal bit guide was used to assure that tube holes were at an angle of 45° from vertical. Holes were reamed with a soil probe equipped with a straight taper bit (5.7-cm diam). Minirhizotron tubes were inserted by hand. Approximately 30 cm of the top portion of each tube was left protruding from the soil surface to accommodate the minirhizotron camera. Care was taken to minimize light penetration down the side of the tubes. Soil was removed to a 5-cm depth around the top of the soil surface–tube interface, and the protruding portion of each tube was covered with opaque tape. Soil was then replaced. The tube tops were capped with aluminum cans whose cut ends were taped to the tube outer wall.

Wheat plants were at the anthesis (Tottman et al., 1979) development stage (DOY 189 in 1995, DOY 187 in 1996) when a portable color video camera attached to an index handle system was used to view roots at specific depths in the minirhizotron tubes. Images of roots at the soil interface of the minirhizotron tube were viewed with a high resolution monitor and recorded with a high resolution 8-mm video cassette recorder. Videotaped root images from each index frame were captured in digital form with the AV digitizer built into the Macintosh computer. Digitized root images were analyzed by RooTracker software (Tremmel, 1995). Digital root images for each index frame were loaded into the computer and the mouse was used to trace root images. Analysis of tracings by computer software provided total root length and average root diameter for each index frame. Actual soil depth in the vertical plane was calculated from index frames recorded at 45° from vertical by means of the Pythagorean Theorem. Root data were summed (length) or averaged (width) over 9-cm actual depth increments before statistical analysis (Merrill et al., 1994; Nickel et al., 1995).

The number of tillers per meter of row was extrapolated from tiller counts made in the four quadrants in each cage (e.g., four measurements per cage) at the anthesis development stage. Four plants were then removed (one from each cage quadrant) and the height of the primary tiller was measured. Stems and leaves of these plants were separated, pooled, placed into a forced-air oven at 60°C for 72 h, and weighed. The number of kernels per meter of row, individual kernel weight, and grain yield were measured at plant maturity. Care was taken to avoid those regions of the cage that were sampled at anthesis.

Field data were analyzed with PROC MIXED in SAS ({alpha} = 0.1; SAS Institute, 1999) appropriate for a factorial experiment conducted in a randomized block over two growing seasons. We considered years to be fixed and treatment and replication to be random. Data obtained from the minirhizotron were further analyzed with PROC MIXED over depths and years with depth as a factor. Given the high spatial variability found in field-gathered root data (Box, 1996), we decided that a statistical probability of 10% or less ({alpha} = 0.1) would allow rejection of the null hypothesis in this study (Little and Hills, 1978).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Growing Season
Statistical analyses revealed that all dependent variables examined were significantly affected by year. Differences in growing season climate must have played an important role in causing these year effects. Air temperatures in July and August were above normal in 1995 and below normal in 1996. Above normal precipitation accompanied warmer air temperatures during July and August in 1995, while precipitation levels were considerably lower than normal in July and near normal in August in 1996 (National Climatic Data Center 1995, 1996).

Soil matric potentials from the beginning of the measurement period to about 10 d before anthesis were very similar in 1995 and 1996, especially at the 30- and 60-cm depth (Fig. 1). During this same time period, soil temperatures at all depths measured were also very similar in 1995 and 1996. Soil matric potential, as recorded from about 10 d before anthesis until the end of the experiment, stayed relatively constant in 1995 and gradually became more negative in 1996. Soil temperatures were cooler in 1995 than in 1996 during the 10 d that preceded anthesis. After anthesis, soil temperatures were warmer in 1995 than in 1996. These temperature trends were especially evident at the 15-cm depth (Fig. 1).



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  		Fig. 1. Soil matric potential and soil temperature at three depths (15, 30, and 60 cm below soil surface) during the growing season in each of the 2 yr of the experiment. The average date where the spring wheat crop reached anthesis is marked.

 
Even with these yearly differences in growing season weather and soil characteristics, there were no significant three-way interactions (R. padi x BYDV x year) among dependent variables. This observation suggests that dependent variables responded to main effect treatments in similar ways across years. Thus, results of statistical analysis across years will be presented and discussed.

Treatment Effects on Wheat Root Characteristics at Anthesis
Root width, averaged over all soil depths as observed with the minirhizotron tube, was not affected by R. padi infestation but was reduced (P = 0.01) by BYDV infection (Table 1). The lack of a significant interaction (P = 0.56) between R padi and BYDV for root width suggests that plants responded to BYDV in the same manner regardless of whether or not they received R. padi treatments.


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  		Table 1. Single and combined influences of bird cherry oat aphid infestation and BYDV infection on field grown spring wheat root system characteristics over 2 yr. Root characteristics were measured with a minirhizotron at the anthesis development stage.

 
Total root length (calculated as the sum of root lengths over all minirhizotron tube depths) was significantly greater (P = 0.06) in plants that received R. padi treatments when compared with plants that did not receive R. padi treatments (Table 1). Total root length was significantly less in plants that received BYDV treatments when compared with plants that did not receive BYDV treatments (Table 1). A significant R. padi x BYDV interaction (P = 0.09) suggests the effects of R.. padi treatments were different in plants that received BYDV treatments than in those that did not. Treatment averages were plotted to determine the source of this interaction (Fig. 2). Plants that received R. padi treatment had about a 30% greater total root length than control plants while plants that received BYDV treatment had about a 40% decrease in total root length when compared with control (Fig. 2). Plants that received both R. padi and BYDV treatments also had about a 40% reduction in total root length. Thus, the significant R. padi x BYDV interaction (P = 0.09) occurs because the increase in root length as caused by R. padi treatment was not present in plants that received R. padi + BYDV treatment. Injury to plants is usually thought to result in stress and damage that causes reductions in growth and yield. Under certain permissive conditions, injury to plants results in overcompensation that results in increased growth or yield (Pedigo et al., 1986). Thus, the proliferation of roots in spring wheat injured by R. padi in the absence of BYDV infection is likely a form of overcompensation.



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  		Fig. 2. A plot of the R. padi x BYDV interaction for total root length as measured with a minirhizotron. The dependent variable represents the total length of root that intersected with the minirhizotron tube in the top 55 cm of the soil profile. Root measurements were made at the anthesis crop development stage.

 
These results extend the findings of previous experiments where root system characteristics from spring wheat plants damaged by R. padi feeding were measured. Under field conditions, spring wheat plants infested with R. padi (300 aphid days) at the 2-leaf stage had a significant reduction in root length density in the upper 30 cm of the soil profile compared with uninfested plants when measured between 4 and 20 d after the insects were removed (Riedell and Kieckhefer, 1995). In this same study, there were no significant differences in root length density between R. padi infested or uninfested plants when measured 30 d after the insects were removed. Taken together, these observations suggest that the initial dramatic reductions in root growth caused by R. padi infestation are temporary and that spring wheat root systems can recover once the insects are removed.

Further analysis of minirhizotron root length data with soil depth as a factor revealed a significant BYDV x soil depth x year interaction (P = 0.09). This interactions suggests that spring wheat root system characteristics at different soil depths responded differently to BYDV infection across the 2 yr of the study. In 1995, which had greater soil moisture starting at about 10 d before anthesis at all depths than 1996 (Fig. 1), plants that had BYDV infection had less root length at all soil depths measured than plants that did not receive BYDV infection (Fig. 3). In 1996, root lengths of both BYDV infected and uninfected plants in the upper portions of the soil profile (0- to 9- and 9- to 18-cm depths) were similar, while at deeper soil depths, plants that had BYDV infection had less root length than plants that did not receive BYDV infection (Fig. 3). Thus, it appears that plants that did not receive BYDV infection were able to respond to the drying soil conditions present in 1996 by increasing root growth at deeper depths in the soil profile. Infection by BYDV seemed to prevent this compensatory response to this abiotic stress.



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  		Fig. 3. A plot of the BYDV x soil depth x year interaction for root length as measured with a minirhizotron. This plot compares the root length of plants that were infected with BYDV (BYDV and R. padi + BYDV treatments) with plants that were not (control and R. padi treatments). Root length represents the sum of the total length of root that intersected with the minirhizotron tube over 9-cm depth increments. Root measurements were made at the anthesis crop development stage.

 
Treatment Effects on Wheat Shoot Characteristics at Anthesis
On average, shoots of experimental plants reached the anthesis development stage on 8 July 1995 (DOY 189) and 6 July 1996 (DOY 187). Plants receiving the BYDV and the R. padi + BYDV treatments tended to reach anthesis at a later time than the other treatments (data not shown). This delay in anthesis caused by BYDV and R. padi + BYDV treatments is consistent with observations seen under greenhouse conditions (Riedell et al., 1999).

Plants that received the R. padi treatment had fewer tillers, less tiller height, and less shoot dry weight at anthesis than plants that did not receive R. padi treatment (Table 2). Plants that received BYDV treatment also had significantly fewer tillers, less tiller height, and less shoot dry weight at anthesis than plants that did not receive the BYDV treatment (Table 2). The lack of statistically significant interactions between R. padi and BYDV treatments for these dependent variables suggests that plants responded to BYDV in the same manner regardless of whether or not they received R. padi treatments.


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  		Table 2. Single and combined influences of bird cherry oat aphid (R. padi) infestation and BYDV infection on shoot characteristics of field grown spring wheat plants harvested at the anthesis development stage over 2 yr.

 
Treatment Effects on Yield and Yield Components
Grain harvested from plants that received the R. padi treatment had significantly less individual kernel weight than grain from plants that did not receive R. padi treatment (Table 3). Treatment by R. padi did not significantly affect the number of kernels per meter of row or the total grain yield. The effects of R. padi on spring wheat yield and yield components as recorded in our experiment were much less than those reported by others. Kieckhefer and Kantack (1980) found that R. padi treatment (25 aphids per stem for 10 d) applied at the 2-leaf stage resulted in a 70% reduction in the number of kernels per foot of row, a 35% reduction in the individual kernel weight, and an 80% reduction in total grain yield. Papp and Mesterházy (1996) determined that yield loss in plants damaged by R. padi was strongly related to reductions in seed number whereas reductions in individual kernel weight were much less dramatic.


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  		Table 3. Single and combined influences of bird cherry oat aphid (R. padi) infestation and BYDV infection on yield components and grain yield of field grown spring wheat plants over 2 yr.

 
In the present experiment, plants that received BYDV treatment had significantly fewer kernels per meter of row, less individual kernel weight, and dramatically less total grain yield when compared with plants that did not receive BYDV treatment. These results are in agreement with Gill (1980) who documented a 59% reduction in seed number, a 36% reduction in individual kernel weight, and a 69% loss in total grain yield in spring wheat plants infected with BYDV under field conditions. The lack of statistically significant interactions between R. padi and BYDV treatments for yield and yield components suggests that plants responded to BYDV in the same manner regardless of whether or not they received R. padi treatments (Table 3).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Insects and disease, which routinely reduce crop quality and yields, cause decreased agricultural profitability. Thus, the elucidation of how these biotic stresses interact is of much theoretical and practical importance. Theoretically, biotic stress interactions are of interest because they provide insight into interrelationships between physiological processes. Practically, biotic stress interactions are important because they indicate best management practices for multiple stresses (Higley et al., 1993).

In the present study, R. padi infestation damage (likely caused by removal of assimilates from the phloem tissues) resulted in plant stress at the 2- to 3-leaf stage. Plant damage caused by R. padi resulted in enhanced root growth and reduced shoot growth at the anthesis development stage, as well as decreased individual kernel weights. The R. padi stress and damage responses, in terms of total yield, were acute and reversible in that plants recovered over time after the insects were removed with insecticide. BYDV damage (likely caused by disruption of phloem assimilate translocation) resulted in plant stress from the time of infection and beyond. Plant damage caused by BYDV was characterized as reduced root growth and reduced shoot growth at the anthesis development stage, as well as decreased yield and yield components. The BYDV stress and damage responses, in terms of total yield, were chronic and irreversible in that plants did not recover over time. The BYDV stress and damage responses overshadowed the lesser effects of R. padi on most of the dependent variables measured.


   ACKNOWLEDGMENTS
 
The authors thank Dave Schneider, Erika Zink, Eric Beckendorf, and Kurt Dagel for able technical assistance and Paul Evenson for statistical analysis assistance. Mention of commercial or proprietary products does not constitute endorsement by the USDA. USDA offers its program to all eligible persons regardless of race, color, age, sex, or national origin.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cooperative investigations of the USDA-ARS and South Dakota Agric. Exp. Stn., Brookings SD. Journal Series no 3323. Research supported in part by a grant from the South Dakota Wheat Commission.

Received for publication July 24, 2002.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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W. E. Riedell, S. L. Osborne, and A. A. Jaradat
Crop Mineral Nutrient and Yield Responses to Aphids or Barley Yellow Dwarf Virus in Spring Wheat and Oat
Crop Sci., July 30, 2007; 47(4): 1553 - 1560.
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