Commercial Wheat Fields Research Articles

Monitoring wheat fields by RapidScan: accuracy and limitations

Simple active radiometer sensors, such as RapidScan, enable agronomic decision-making and phenotyping within commercial wheat fields and experiments. The objectives of this study were: 1 - to evaluate the accuracy of quantitative biomass and nitrogen uptake estimation by the RapidScan, and 2 - to evaluate yield loss estimation based on NDVI. The RapidScan sensor was used as an assessment tool in the following studies: (i) over 3 years, 518 wheat samples were monitored during the vegetative growth period for biomass and aboveground nitrogen uptake and (ii) wheat cultivars were tested in an additional 4 field experiments, which were scanned weekly and correlated with grain yield. Results showed that accurate biomass estimation is limited up to about 100 g DM m−2. Grain yield, actual and potential, estimation is highly affected by the emergence date. The results showed that the use of a proximal-sensing technique allows for rapid and accurate crop monitoring and yield estimation, but emphasizing limitations in future use as well.

Temporal Dynamics and Spatial Variation of Azoxystrobin and Propiconazole Resistance in Zymoseptoria tritici: A Hierarchical Survey of Commercial Winter Wheat Fields in the Willamette Valley, Oregon.

Fungicide resistance can cause disease control failure in agricultural systems, and is particularly concerning with Zymoseptoria tritici, the causal agent of Septoria tritici blotch of wheat. In North America, the first quinone outside inhibitor resistance in Z. tritici was discovered in the Willamette Valley of Oregon in 2012, which prompted this hierarchical survey of commercial winter wheat fields to monitor azoxystrobin- and propiconazole-resistant Z. tritici. Surveys were conducted in June 2014, January 2015, May 2015, and January 2016. The survey was organized in a hierarchical scheme: regions within the Willamette Valley, fields within the region, transects within the field, and samples within the transect. Overall, frequency of azoxystrobin-resistant isolates increased from 63 to 93% from June 2014 to January 2016. Resistance to azoxystrobin increased over time even within fields receiving no strobilurin applications. Propiconazole sensitivity varied over the course of the study but, overall, did not significantly change. Sensitivity to both fungicides showed no regional aggregation within the Willamette Valley. Greater than 80% of spatial variation in fungicide sensitivity was at the smallest hierarchical scale (within the transect) of the survey for both fungicides, and the resistance phenotypes were randomly distributed within sampled fields. Results suggest a need for a better understanding of the dynamics of fungicide resistance at the landscape level.

Variation in Puccinia graminis f. sp. tritici detected on wheat and triticale in South Africa from 2009 to 2013

To determine phenotypic diversity of Puccinia graminis f. sp. tritici (Pgt), the cause of stem rust of wheat, samples of infected stems were collected during 2009–2013 from commercial wheat fields, experimental plots, and rust trap nurseries across major wheat growing regions of South Africa (SA). Pgt races were identified based on their avirulence/virulence profiles on seedlings of 20 standard and five supplemental differential lines. Nine Pgt races were identified from 521 isolates pathotyped. Predominant races were TTKSF (2SA88, South African race notation) with 39%–85% frequency and BPGSC + Sr27,Kiewiet,Satu (2SA105) with 10%–20% frequency. Race TTKSF is virulent on major resistance genes such as Sr5, Sr6, Sr9e, and Sr38 and is one of the variants of the highly virulent Ug99 race group originally detected in East Africa. Race TTKSP (2SA106), also a member of the Ug99 lineage, was detected in 2009 and 2010. A new race virulent on Sr31, PTKST (2SA107), was detected in 2009. Two new races, TTKSF + Sr9h (2SA88 + Sr9h) and BFBSC (2SA108), were identified in 2010. Race TTKSF + Sr9h is similar to TTKSF except for its virulence on Sr9h. Race BFBSC appears related to Pgt races characterized by avirulence for Sr5 and often attacking triticale. Simple sequence repeat (SSR) analysis indicated that race BFBSC forms part of the non-Ug99 group of South African Pgt races. Despite some similarity in avirulence/virulence phenotype with the non-Ug99 races, BFBSC represents a third distinct genetic lineage within this group. Genes Sr13, 14, 22, 25, 26, 29, 32, 33, 35, 36, 37, 39, 42, and 43 that are effective against the new and other Pgt races can be used in resistance breeding in SA. Races like PTKST and TTKSF + Sr9h were also reported in other Southern African countries suggesting that they probably spread to SA from neighbouring regions. The new races are additions to nearly 30 Pgt races identified since the early 1980s, and suggest continued variability of the Pgt population in SA. Therefore, surveys should be conducted regularly to timely detect and manage new races, and utilize the latter in screening and identification of effective sources of resistance.

Seasonal population dynamics of wireworms in wheat crops in the Pacific Northwestern United States

Insect pests often exhibit predictable seasonal population dynamics in response to temperature and other environmental drivers. Understanding these dynamics is critical to developing effective integrated pest management strategies. Here we studied the seasonal phenology and feeding activity of two wireworm species that are major pests of wheat crops in the Pacific Northwestern United States, Limonius californicus and L. infuscatus. We conducted monthly sampling of the damaging larval stages of both species in commercial spring wheat fields in Washington and Idaho throughout 2013 and 2014. These data were used to model the seasonal phenology and feeding activity of each species in relation to soil temperature. We found larvae of both species were most abundant relatively early in the season, with total wireworms captures in soil cores declining as the season progressed. Larvae of both species were collected predominantly in the top 70 cm of the soil profile, suggesting that they primarily feed on plant roots and seeds up to this depth. While patterns of seasonal abundance of both species were similar, feeding activity varied significantly between the two species. Our results indicate that as spring moves into summer L. californicus feeds more aggressively, whereas the activity of L. infuscatus decreases as the crop season progresses. These differences might help explain why L. californicus is generally a more economically damaging pest that also threatens winter crops, while damage from L. infuscatus is generally limited to the spring. Accordingly, management strategies for each species should be tailored to their specific seasonal dynamics.

Occurrence and Survival of Apothecia of the Eyespot Pathogens Oculimacula acuformis and O. yallundae on Wheat Stubble in the U.S. Pacific Northwest

Eyespot is a chronic disease of wheat caused by Oculimacula yallundae and O. acuformis that results in premature ripening of grain, lodging, and reduced grain yield. Discovery of the sexual stage of these Oculimacula spp. in the Pacific Northwest (PNW) of the United States is relatively recent and the role of apothecia in the epidemiology of eyespot is unclear. Our goals were to determine whether and when apothecia of these Oculimacula spp. are found in the PNW, and monitor their ability to survive over summer and over winter. Seventy-three harvested commercial wheat fields in Idaho, Oregon, and Washington were surveyed for apothecia during spring and fall 2012 and spring 2013. Apothecia of both species were found in both spring and fall in 19% of fields. Apothecia survived on straw placed on the soil surface over the summer but not the winter. This is the first report of O. yallundae apothecia in commercial wheat fields in the PNW. Occurrence of apothecia in spring and fall demonstrates that sexual reproduction of both species occurs regularly in the PNW and at a time when ascospores could serve as primary inoculum for infection of winter wheat. Results of this study are consistent with previous population genetic studies that found high genotypic diversity of both eyespot pathogens in winter wheat fields and provides a baseline for understanding the influence of sexual reproduction on population dynamics and genetics of both pathogens.

Using Multispectral Imagery to Compare the Spatial Pattern of Injury to Wheat Caused by Russian Wheat Aphid1and Greenbug1

Abstract. The Russian wheat aphid, Diuraphis noxia (Kurdjumov), and greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), are important insect pests of wheat, Triticum aestivum L. Outbreaks of both pests in commercial wheat fields occur almost every year in the Great Plains of the United States. Infestations of both pests in wheat fields are clumped in distribution, and as a result so is the injury to plants in the field. The spatial pattern of patches of plants injured by the two aphid pests can be detected using airborne multispectral imaging. This study used multispectral imagery of wheat fields infested by the two species of aphids analyzed using spatial-pattern metrics with the purpose of determining differences between species in the spatial pattern of injury to wheat plants in fields. Analyses using Students t-tests revealed patches of wheat within fields injured by Russian wheat aphid were larger, more complex in shape, and more isolated than patches of wheat injured by greenbug. Our fin...

Resistance Allele Movement between Imazamox-Resistant Wheat and Jointed Goatgrass (Aegilops cylindrica) in Eastern Oregon Wheat Fields

Imazamox-resistant wheat varieties carry theImi1allele, which confers resistance to the imidazolinone (IMI) herbicide imazamox. This resistance trait allows the selective control of jointed goatgrass, a difficult-to-control winter annual grass weed. Allele movement between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. Hybrids (F1) of IMI-resistant wheat and jointed goatgrass were identified in 2008 in a commercial wheat field in Eastern Oregon. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of theImi1allele in wheat × jointed goatgrass hybrids. Using polymerase chain reaction assays we detected the presence of theImi1allele. A total of 128 sites were surveyed over the 2 yr. Of 1,548 plants sampled, 1,100 were positive for theImi1alelle and of those, 1,087 were heterozygous and 13 were homozygous for the allele. We assessed hybrid yield components and how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and the area or management practice in the commercial fields was determined. Nonagricultural sites or production of IMI-resistant wheat in consecutive years were two factors associated with a greater proportion of IMI-resistant hybrids. Our results demonstrate that theImi1allele is moving from IMI-resistant wheat to jointed goatgrass, producing resistant hybrids and backcross plants. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Therefore, it is important to implement field management practices that reduce IMI-resistant hybrid production and to effectively manage nonagricultural areas with jointed goatgrass infestations to prevent introgression of the IMI-resistance allele.

A Multiple Decrement Life Table Reveals That Host Plant Resistance and Parasitism Are Major Causes of Mortality for the Wheat Stem Sawfly.

This study investigated the dynamics of parasitism, host plant resistance, pathogens, and predation on the demography of wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), developing in susceptible (hollow stem) and resistant (solid stem) wheat hosts. This study is also the first to investigate the prevalence and impact of cannibalism on wheat stem sawfly mortality. Wheat stem sawflies were sampled in two commercial wheat fields over 4 yr from the egg stage through adult emergence, and multiple decrement life tables were constructed and analyzed. Cannibalism, host plant resistance, or unknown factors were the most prevalent factors causing egg mortality. Summer mortality of prediapause larvae ranged from 28 to 84%, mainly due to parasitism by Bracon cephi (Gahan) and Bracon lissogaster Muesebeck, cannibalism, and host plant resistance. Winter mortality ranged from 6 to 54% of the overwintering larvae, mainly due to unknown factors or pathogens. Cannibalism is a major cause of irreplaceable mortality because it is absolute, with only a single survivor in every multiple infested stem. Subsequent to obligate cannibalism, mortality of feeding larvae due to host plant resistance was lower in hollow stem wheat than in solid stem wheat. Mortality from host plant resistance was largely irreplaceable. Irreplaceable mortality due to parasitoids was greater in hollow stem wheat than in solid stem wheat. Host plant resistance due to stem solidness and parasitism in hollow stems cause substantial mortality in populations of actively feeding larvae responsible for all crop losses. Therefore, enhancing these mortality factors is vital to effective integrated pest management of wheat stem sawfly.

Disease Risk, Spatial Patterns, and Incidence-Severity Relationships of Fusarium Head Blight in No-till Spring Wheat Following Maize or Soybean

The first large-scale survey of Fusarium head blight (FHB) in commercial wheat fields in southern Brazil was conducted over three years (2009 to 2011). The objectives were to: (i) evaluate whether increased FHB risk is associated with within-field maize residue; (ii) determine the spatial pattern of FHB incidence; and (iii) quantify the relationship between FHB incidence and severity. FHB was assessed in a total of 160 fields between early milk and dough. Incidence ranged from 1.0 to 89.9% (median = 25%) and severity from 0.02 to 18.6% (median = 1.3%). FHB risk was neither lower nor higher in wheat following maize than in wheat following soybean. Only 18% of fields were classified as having aggregated patterns of FHB-symptomatic spikes. A binary power law description of the variances was consistent with an overall random pattern of the disease. These results conform with the hypothesis that FHB epidemics in southern Brazil are driven by sufficient atmospherically-transported inoculum from regional sources. The incidence-severity relationship was coherent across growing season, growth stage, and previous crop; one common fitted curve described the relationship across all observations. Estimating severity from incidence may be useful in reducing the workload in epidemiological surveys.

An approach to precise nitrogen management using hand-held crop sensor measurements and winter wheat yield mapping in a Mediterranean environment.

Regardless of the crop production system, nutrients inputs must be controlled at or below a certain economic threshold to achieve an acceptable level of profitability. The use of management zones and variable-rate fertilizer applications is gaining popularity in precision agriculture. Many researchers have evaluated the application of final yield maps and geo-referenced geophysical measurements (e.g., apparent soil electrical conductivity-ECa) as a method of establishing relatively homogeneous management zones within the same plot. Yield estimation models based on crop conditions at certain growth stages, soil nutrient statuses, agronomic factors, moisture statuses, and weed/pest pressures are a primary goal in precision agriculture. This study attempted to achieve the following objectives: (1) to investigate the potential for predicting winter wheat yields using vegetation measurements (the Normalized Difference Vegetation Index—NDVI) at the beginning of the season, thereby allowing for a yield response to nitrogen (N) fertilizer; and (2) evaluate the feasibility of using inexpensive optical sensor measurements in a Mediterranean environment. A field experiment was conducted in two commercial wheat fields near Seville, in southwestern Spain. Yield data were collected at harvest using a yield monitoring system (RDS Ceres II-volumetric meter) installed on a combine. Wheat yield and NDVI values of 3498 ± 481 kg ha−1 and 0.67 ± 0.04 nm nm−1 (field 1) and 3221 ± 531 kg ha−1 and 0.68 ± 0.05 nm nm−1 (field 2) were obtained. In both fields, the yield and NDVI exhibited a strong Pearson correlation, with rxy = 0.64 and p < 10−4 in field 1 and rxy = 0.78 and p < 10−4 in field 2. The preliminary results indicate that hand-held crop sensor-based N management can be applied to wheat production in Spain and has the potential to increase agronomic N-use efficiency on a long-term basis.

High spectral and spatial resolution hyperspectral imagery for quantifying Russian wheat aphid infestation in wheat using the constrained energy minimization classifier

The effects of insect infestation in agricultural crops are of major ecological and eco- nomic interest because of reduced yield, increased cost of pest control and increased risk of environmental contamination from insecticide application. The Russian wheat aphid (RWA, Diuraphis noxia) is an insect pest that causes damage to wheat (Triticum aestivum L.). We pro- posed that concentrated RWA feeding areas, referred to as hot spots, could be identified and isolated from uninfested areas within a field for site specific aphid management using remotely sensed data. Our objectives were to (1) investigate the reflectance characteristics of infested and uninfested wheat by RWA and (2) evaluate utility of airborne hyperspectral imagery with 1-m spatial resolution for detecting, quantifying, and mapping RWA infested areas in commercial winter wheat fields using the constrained energy minimization classifier. Percent surface reflec- tance from uninfested wheat was lower in the visible and higher in the near infrared portions of the spectrum when compared with RWA-infested wheat. The overall classification accuracies of >89% for damage detection were achieved. These results indicate that hyperspectral imagery can be effectively used for accurate detection and quantification of RWA infestation in wheat for site-specific aphid management. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JRS.8.083661)

Quantifying potential yield and lodging-related yield gaps for irrigated spring wheat in sub-tropical Australia

Approximately 15% of global wheat production (40% of the wheat produced by developing countries) occurs in irrigated, low-rainfall regions of the subtropics. Irrigated wheat is uncommon in sub-tropical Australia because cotton is more profitable, but high grain prices in 2008 triggered a record area of irrigated wheat production. Unfortunately, widespread lodging occurred and yields were lower than expected, although the losses directly attributable to lodging were unknown.Crop models have not been previously validated for calculating potential yield and yield gaps of mechanised, lodging-susceptible production systems. This study aimed to determine potential yield, farmer-realised yield, and water use requirements of lodging-prone irrigated wheat in sub-tropical Australia, using field monitoring and the APSIM simulation model. Yield data were obtained in 2008 and 2009 from 16 commercial irrigated wheat fields and multiple treatments from four irrigated experiments, and used to assess APSIM's ability to predict grain yield of irrigated wheat.APSIM predicted biomass production satisfactorily in 2008 but substantially over-predicted grain yield of lodged fields. The mean difference (yield gap) between APSIM-estimated potential yield and farmer-realised yield was 1.0tha−1 in non-lodged fields, and 2.7tha−1 in lodged fields. The average effect of lodging was therefore a decrease of 1.7tha−1; the difference between the yield gap calculated for lodged and non-lodged fields. Yield gaps in lodged fields were highly correlated with soil N content at sowing, with most of the lodged fields having more than 300kgha−1 of soil N at sowing to a depth of 90cm.In 2009, management of commercial fields aimed to avoid lodging by using the canopy management technique of in-season N application. APSIM generally under-predicted biomass production and yield in these fields, suggesting that the N uptake parameters in APSIM may require adjustment. Observed yields from fields of a quick-maturing cultivar that experienced little lodging were simulated accurately when N was assumed to be non-limiting.The simulation study found that the potential yield of irrigated spring wheat in sub-tropical Australia is currently between 8 and 9tha−1, and average growing season evapotranspiration of such crops was approximately 500–550mm. Potential yield of furrow-irrigated fields with one or two-metre wide beds were 16% and 8% lower, due to the area occupied by the 60cm wide unsown furrows between irrigation beds.This study demonstrated the value of using validated simulation models for estimating potential yield and yield gap analysis. Yield gap analysis also enabled the identification of agronomic characteristics (high levels of soil residual N) which probably contributed to lodging. Further investigation is required to determine whether alternative varieties or the canopy management technique of in-season N application can increase the potential yield of irrigated spring wheat above the levels reported here.

First Report of Resistance to QoI Fungicides in North American Populations of Zymoseptoria tritici, Causal Agent of Septoria Tritici Blotch of Wheat.

The G143A mutation in cytb (cytochrome b gene) is associated with high levels of resistance to quinone outside inhibitor (QoI or strobilurin) fungicides that disrupt electron transport during cellular respiration (1). The G143A mutation in Zymoseptoria tritici (synonyms: Mycosphaerella graminicola and Septoria tritici), the causal agent of septoria tritici blotch of wheat (Triticum aestivum), was first reported in Europe in 2001 (1). Although Z. tritici has a global distribution (3), G143A mutants of Z. tritici have not been reported outside of Europe. We used PCR-RFLP (4) to estimate the frequencies of G143A mutants in Z. tritici populations at two locations in the Willamette Valley of western Oregon: the Hyslop Crop Science Field Research Laboratory (Hyslop Farm, HF), Benton County (44°37'52.85″ N, 123°11'55.19″ W) and research plots planted in a commercial wheat field in Washington County (45°33'58.53″ N, 123°00'11.78″ W) (North Valley Farm, NVF). Isolates originated from flag leaf collections from two cultivars ('Bobtail' and 'Tubbs 06') made in April and June of 2012 from plants in a replicated fungicide-treatment experiment, with isolates collected from both sprayed and unsprayed plots. Sixteen of the 169 isolates (9.5%) from HF possessed the G143A mutation (7 of 132 isolates from plots not receiving a QoI fungicide and 9 of 37 isolates collected from plots receiving two applications of the QoI azoxystrobin). One hundred forty six of the 175 isolates (83.4%) from NVF were G143A mutants (101 of 129 isolates from plots receiving no QoI fungicide and 45 of 46 isolates from plots receiving two applications of azoxystrobin). Results of phenotypic assays of a subset of 10 isolates from each location (5 mutants, 5 wild types from each location; 20 isolates altogether) supported a high level of resistance to azoxystrobin only in the G143A mutants. All 10 G143A mutants developed colonies after 8 days of growth on YMA plates amended with SHAM (2) and 1 ppm or 10 ppm azoxystrobin, with nine and eight G143A mutant isolates developing colonies on plates amended with 1 ppm and 10 ppm azoxystrobin, respectively. None of the wild-type isolates developed colonies on plates amended with SHAM and 1 ppm azoxystrobin, nor on plates amended with SHAM and 10 ppm azoxystrobin. All 20 isolates developed colonies on YMA plates lacking azoxystrobin, and treatments produced identical results across three replicates. These results are consistent with findings of higher levels of azoxystrobin resistance in G143A mutants compared to wild types in European populations (1). Isolates from HF and NVF differ in their previous exposure to QoI fungicides. The majority of the wheat area at HF is planted to breeding plots that are not sprayed with fungicide. Plots at NVF were planted in a commercial wheat field in a county where most wheat fields were treated with two to three applications of strobilurins each year over the past 4 years. Future monitoring for G143A mutants of Z. tritici throughout its range in North America will be necessary to assess whether strobilurin resistance will spread via wind-dispersal of ascospores or emerge de novo in treated fields. In Europe, stobilurins were first applied to wheat in 1996. G143A mutants of Z. tritici emerged de novo several times (4) and were widespread by 2007.

Remote Monitoring of Wheat Streak Mosaic Progression Using Sub-Pixel Classification of Landsat 5 TM Imagery for Site Specific Disease Management in Winter Wheat

Wheat streak mosaic (WSM), caused by Wheat streak mosaic virus is a viral disease that affects wheat (Triticum aestivum L.), other grains, and numerous grasses over large geographical areas around the world. To improve disease management and crop production, it is essential to have adequate methods for monitoring disease epidemics at various scales and multiple times. Remote sensing has become an essential tool for monitoring and quantifying crop stress due to biotic and abiotic factors. The objective of our study was to explore the utility of Landsat 5 TM imagery for detecting, quantifying, and mapping the occurrence of WSM in irrigated commercial wheat fields. The infection and progression of WSM was biweekly assessed in the Texas Panhandle during the 2007-2008 crop years. Diseased-wheat was separated from uninfected wheat on the images using a sub-pixel classifier. The overall classification accuracies were >91% with kappa coefficient between 0.80 and 0.94 for disease detection were achieved. Omission errors varied between 2% and 14%, while commission errors ranged from 1% to 21%. These results indicate that the TM image can be used to accurately detect and quantify disease for site-specific WSM management. Remote detection of WSM using geospatial imagery may substantially improve monitoring, planning, and management practices by overcoming some of the shortcomings of the ground-based surveys such as observer bias and inaccessibility. Remote sensing techniques for accurate disease mapping offer a unique set of advantages including repeatability, large area coverage, and cost-effectiveness over the ground-based methods. Hence, remote detection is particularly and practically critical for repeated disease mo- nitoring and mapping over time and space during the course of a growing season.

Influences of crop sequence, rainfall and irrigation, on relationships between Gaeumannomyces graminis var. tritici and take-all in New Zealand wheat fields

In a study, over three growing seasons, of Gaeumannomyces graminis var. tritici (Ggt) and take-all disease in commercial wheat fields (that were in their first, second, third or fourth year of consecutive wheat) in New Zealand, Ggt concentrations in soil (the amount of Ggt DNA measured using quantitative PCR) and take-all incidence and take-all index (TAI) were least at the start of a wheat sequence and stabilized in third and fourth consecutive wheat crops. Median Ggt concentrations in soil increased 10-fold during a first wheat crop, reaching 78 pg Ggt DNA/g soil after harvest. Growing season rainfall and position in a continuous wheat crop sequence were most closely associated with TAI and post-harvest soil Ggt concentrations, but other factors were also associated with inoculum and disease. Inoculum concentrations, disease incidence and TAI were greater where the frequency of crops susceptible to Ggt (wheat, barley or triticale) was greatest in the crop rotations and where crops were irrigated. Irrigation in particular was associated with high post-harvest soil Ggt concentrations in the driest of the three growing seasons assessed, when environmental conditions were least favourable for disease development. There was evidence of take-all decline following two wheat crops where environmental conditions had enabled severe epidemics to occur. Our study showed that frequency of cereal crops in the rotation, length of breaks between host crops, irrigation and growing season rainfall interact over the course of a cropping sequence to influence TAI when wheat is sown.

Virulence of Pyrenophora tritici-repentis in the Southern Cone Region of South America

Pyrenophora tritici-repentis induces tan spot, one of the most important fungal diseases of wheat. At least eight races of the pathogen are known to occur based on their virulence on a wheat differential set. In 2009 and 2010, 87 commercial wheat fields were sampled for tan spot in the Southern Cone region of South America, namely Argentina, Brazil and Uruguay. An average of three isolates from each field were tested for their virulence on the differential wheat hosts ‘Glenlea’, 6B365, 6B662, ‘Salamouni’, ‘Coulter’ and 4B1149. Of 273 isolates tested, 52.4% were classified as race 1 and 47.6% were classified as race 2, with no other races identified. This low level of pathogenic diversity was somewhat expected, since the bread wheat grown in this region has a narrow genetic diversity with respect to tan spot resistance. To fully assess virulence patterns in the Southern Cone, use of this differential set is recommended. To our knowledge, this is the first report of the race structure of P. tritici-repentis in South America.

Predicting Take-All Severity in Second-Year Wheat Using Soil DNA Concentrations of Gaeumannomyces graminis var. tritici Determined with qPCR.

The lack of accurate detection of Gaeumannomyces graminis var. tritici inoculum in soil has hampered efforts to predict the risk of severe take-all for wheat growers. The current study used a molecular method to quantify soil G. graminis var. tritici concentrations in commercial wheat fields in New Zealand and to compare them with the proportion of crops surpassing the thresholds for visible and moderate to severe take-all over three growing seasons. The study evaluated a soil G. graminis var. tritici DNA-based take-all prediction system developed in Australia, with four take-all risk categories. These categories were found to be useful for predicting disease severity in second wheat but did not clearly separate risk between fields in medium- and high-risk categories. A sigmoidal relationship was identified between inoculum concentration and the proportion of fields exceeding the two disease thresholds. A logistic response curve was used to further examine this relationship and evaluate the boundaries between take-all risk categories. G. graminis var. tritici boundaries between medium- and high-risk categories were clustered near or within the upper plateau of the relationship. Alternative G. graminis var. tritici boundaries for a three-category system were identified that provided better separation of take-all risk between categories. This information could improve prediction of the risk of severe take-all.

Monitoring regional wheat yield in Southern Spain using the GRAMI model and satellite imagery

The worldwide increase in the demand for food and the limited available resources to produce it make it necessary to develop tools which allow estimation of crop production, thereby helping to manage the way food is produced, stored and distributed. GRAMI, a model developed to simulate the growth and yield of grain crops and capable of using remotely sensed information, was applied to the semiarid region of Southern Spain. The aim of this study was to demonstrate a methodology for using the GRAMI model, along with satellite remote sensing data, to estimate regional crop yields, and to assess the accuracy of the resulting yield estimates. Crop-specific model parameters (light-use efficiency, crop phenological stage and yield partitioning factor) were evaluated using information collected from 29 durum and bread wheat experimental plots in order to verify the performance of the model in this region spectral radiometry measurements were taken for each plot throughout the growing season to obtain experimental relationships between the normalized difference vegetation index (NDVI) and leaf area index (LAI). This relationship was used to estimate crop LAI for the within-season calibration of GRAMI from satellite remote sensing data. Forty-nine commercial wheat fields were chosen in 2008 and 2009 to validate the model. Information from meteorological stations was used in running the model, and satellite image data were used along with the LAI–NDVI relationship to provide estimates of crop LAI for within-season calibration of the model. Yield data for comparison with model estimates were obtained for each field from the farmers. For the validation study, the average amount that the yield of an individual field was over- or under-estimated was 884 and 852kgha−1 for the 2008 and 2009 seasons, respectively. The absolute errors between the average estimated and average observed yield values were 5.44% and 6.86% for the 2008 and 2009 seasons, respectively. For each of the 2 years, the average estimated yield was not significantly different from its corresponding average observed yield. Based primarily on its ability to accurately estimate the average yield for a set of fields and its reliance on readily available weather and remote sensing data, the GRAMI model, verified for a region and calibrated using satellite image data, appears to be a practical and appropriate option for operationally monitoring regional crop yields with a reasonable degree of accuracy.

Accumulation of the antibiotic phenazine-1-carboxylic acid in the rhizosphere of dryland cereals.

Natural antibiotics are thought to function in the defense, fitness, competitiveness, biocontrol activity, communication, and gene regulation of microorganisms. However, the scale and quantitative aspects of antibiotic production in natural settings are poorly understood. We addressed these fundamental questions by assessing the geographic distribution of indigenous phenazine-producing (Phz(+)) Pseudomonas spp. and the accumulation of the broad-spectrum antibiotic phenazine-1-carboxylic acid (PCA) in the rhizosphere of wheat grown in the low-precipitation zone (<350 mm) of the Columbia Plateau and in adjacent, higher-precipitation areas. Plants were collected from 61 commercial wheat fields located within an area of about 22,000 km(2). Phz(+) Pseudomonas spp. were detected in all sampled fields, with mean population sizes ranging from log 3.2 to log 7.1 g(-1) (fresh weight) of roots. Linear regression analysis demonstrated a significant inverse relationship between annual precipitation and the proportion of plants colonized by Phz(+) Pseudomonas spp. (r(2) = 0.36, P = 0.0001). PCA was detected at up to nanomolar concentrations in the rhizosphere of plants from 26 of 29 fields that were selected for antibiotic quantitation. There was a direct relationship between the amount of PCA extracted from the rhizosphere and the population density of Phz(+) pseudomonads (r(2) = 0.46, P = 0.0006). This is the first demonstration of accumulation of significant quantities of a natural antibiotic across a terrestrial ecosystem. Our results strongly suggest that natural antibiotics can transiently accumulate in the plant rhizosphere in amounts sufficient not only for inter- and intraspecies signaling but also for the direct inhibition of sensitive organisms.

Diversity in genetic structure and chemotype composition of Fusarium graminearum sensu stricto populations causing wheat head blight in individual fields in Germany

Fusarium head blight (FHB) is one of the most destructive diseases of wheat. Twelve small commercial wheat fields (size 1–3 hectares) were sampled in Germany for Fusarium populations at three spots per field with 10 heads each. PCR assays using generic primers confirmed 338 isolates as F.graminearum sensu stricto (s.s.) (64.9%) out of 521 Fusarium spp. that were further analyzed. Populations of F. graminearum s.s. in Germany contain three types of trichothecenes with a dominancy of 15-acetyldeoxynivalenol chemotype (92%) followed by 3-acetyldeoxynivalenol chemotype (6.8%) and a few isolates of nivalenol chemotype (1.2%). All these isolates were genotyped using 19 microsatellite loci. The 12 populations showed a high genetic diversity within the small scale sampling areas resulting in 300 different haplotypes. Genetic diversity within populations (71.2%) was considerably higher than among populations (28.8%) as shown by analysis of molecular variance. Gene flow (Nm) between populations ranged from 0.76–3.16. Composition of haplotypes of one population followed over 2 years changed considerably. No correlation between genetic and geographical distance was found. In conclusion, populations of F. graminearum s.s. in Germany display a tremendous genetic variation on a local scale with a restricted diversity among populations.