Winter Wheat Cultivars with Temperature-Sensitive Resistance to Wheat streak mosaic virus Do Not Recover from Early-Season Infections

Molecular Cytogenetic Identification of a Novel Wheat-Thinopyrum ponticum 1JS (1B) Substitution Line Resistant to Powdery Mildew and Leaf Rust

Thinopyrum ponticum (2n = 10x = 70) is a wild relative of wheat with high tolerance to both biotic and abiotic stresses; it has been wildly used in wheat genetic improvement. A disomic substitution line named SN19647 was derived from a cross between Triticum aestivum and the wheat-Th. ponticum partial amphiploid SNTE20 (2n = 8x = 56). It was evaluated for disease resistance and characterized via sequential fluorescence in situ hybridization (FISH)-genomic in situ hybridization (GISH) and molecular markers. The results showed that SN19647 carried resistance to both powdery mildew and leaf rust. It contained 42 chromosomes with a pair of wheat chromosome 1B replaced by a pair of J^S chromosomes from Th. ponticum. In addition to chromosomal substitution events, structural variation also occurred on wheat chromosomes 2A, 5A, 6B, and 7B. Based on marker analysis, 19 markers specific to the J^S chromosome were obtained, of which seventeen markers belonged to homoeologous group one. These results indicated that SN19647 was a 1J^S (1B) substitution line. Compared with the known 1J^S (1D) substitution line CH10A5, it was found that 17 markers generated different specific bands to Th. ponticum, confirming the novelty of the 1J^S chromosome in SN19647. Therefore, SN19647, resistant to powdery mildew and leaf rust, was a novel 1J^S (1B) substitution line that can be used in wheat genetic improvement.

Management of the Wheat Curl Mite and Wheat Streak Mosaic Virus With Insecticides on Spring and Winter Wheat

The wheat curl mite (WCM, Aceria tosichella, Keifer) is an eriophyid mite species complex that causes damage to cereal crops in the Northern Great Plains by feeding damage and through the transmission of plant viruses, such as wheat streak mosaic virus. Insecticide treatments were evaluated in the greenhouse and field for efficacy at managing the WCM complex on wheat. Treatments tested were carbamates, organophosphates, pyrethroids, a neonicotinoid seed treatment, mite growth inhibitors, and Organic Materials Review Institute-approved biocontrols, soaps, and oils. Treatment with carbamates, organophosphates, and pyrethroids decreased WCM in greenhouse trials compared with untreated controls 14 days after infestation. The seed treatment, mite growth inhibitors, and organic pesticides did not reduce WCM populations effectively and consistently. The timing of application was tested using a sulfur solution as the experimental treatment. Treating plants with sulfur seven days after mite infestation reduced mites compared with the untreated control. In contrast, prophylactically applied sulfur and sulfur applied 14 days after mite infestation were not effective. When tested under field conditions with plots infested with viruliferous mites, there was no yield difference detected between untreated control plots and plots sprayed with insecticides. Select carbamates, organophosphates, and pyrethroids have a potential for use in greenhouse mite management when appropriate.

Exploiting the genome of Thinopyrum elongatum to expand the gene pool of hexaploid wheat

One hundred and thirty four introgressions from Thinopyrum elongatum have been transferred into a wheat background and were characterised using 263 SNP markers. Species within the genus Thinopyrum have been shown to carry genetic variation for a very wide range of traits including biotic and abiotic stresses and quality. Research has shown that one of the species within this genus, Th. elongatum, has a close relationship with the genomes of wheat making it a highly suitable candidate to expand the gene pool of wheat. Homoeologous recombination, in the absence of the Ph1 gene, has been exploited to transfer an estimated 134 introgressions from Th. elongatum into a hexaploid wheat background. The introgressions were detected and characterised using 263 single nucleotide polymorphism markers from a 35 K Axiom^® Wheat-Relative Genotyping Array, spread across seven linkage groups and validated using genomic in situ hybridisation. The genetic map had a total length of 187.8 cM and the average chromosome length was 26.8 cM. Comparative analyses of the genetic map of Th. elongatum and the physical map of hexaploid wheat confirmed previous work that indicated good synteny at the macro-level, although Th. elongatum does not contain the 4A/5A/7B translocation found in wheat.

Incidence of Mite-Vectored Viruses of Wheat in the Texas High Plains and Interactions With Their Host and Vector

Mite-vectored virus diseases of wheat are common throughout the Great Plains and cause significant economic losses to growers each year. These diseases are caused by Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and Wheat mosaic virus (WMoV), all of which are transmitted by the wheat curl mite (WCM), Aceria tosichella Keifer. New wheat cultivars with tolerance or resistance to WSMV have been released recently, but their widespread cultivation and potential impact on mite-transmitted virus incidence in the Texas Panhandle was unknown. A total of 648 symptomatic wheat samples were collected from 26 counties, predominately in the Texas Panhandle, and tested by enzyme-linked immunosorbent assay (ELISA) for WSMV, TriMV, and WMoV. Samples that tested negative by ELISA were subsequently tested by real-time quantitative PCR (qPCR) for each virus. Approximately 93% of the samples tested by ELISA were positive for WSMV, 43% were positive for TriMV, and 7% were positive for WMoV. Eleven samples tested positive only for TriMV, but none were positive only for WMoV. When samples that tested negative for the different viruses by ELISA were retested by real-time qPCR, detection of each virus was significantly increased. When results of the ELISA test and qPCR were combined, 100% of the 648 samples tested positive for WSMV, approximately 94% were positive for TriMV, and 23% were positive for WMoV. This demonstrated that the incidence of TriMV in the Texas High Plains is much greater than previously reported. The fact that real-time qPCR revealed over a 2-fold increase in the incidence of TriMV and a 3-fold increase in WMoV demonstrated that the ELISA test, which is commonly used by diagnostic laboratories in the Great Plains, should not be used for studies requiring a high degree of sensitivity and accuracy in virus detection. After initial virus infection status was determined, samples that tested positive for WSMV and TriMV were further observed for WCM infestation. A total of 292 samples were inspected and a total of 101 mites were collected from 40 tillers. Individual mites and the tillers from which they were recovered were tested by real-time qPCR to determine how copy numbers of WSMV and TriMV in mites and host tissue compared, and whether the WSMV/TriMV copy number ratio in individual mites was similar to that of the host tissue from which they were collected. In all mites and tillers tested, the WSMV copy number was always higher than that of TriMV and copy numbers of both viruses were always higher in plant tissue than in mites. Although there was a significant correlation between the WSMV/TriMV copy number ratio in plant tissue and in associated mites, the correlation coefficient was very low (r = 0.31, P = 0.0248). In the majority of comparisons, the WSMV/TriMV ratio was higher in individual mites than in the tiller from which they were recovered. The reason for this increase is unknown but indicates that mites may preferentially acquire WSMV from tillers coinfected with WSMV and TriMV, a finding that could have significant implications for virus transmission and disease epidemiology.

Development and characterization of wheat-sea wheatgrass (Thinopyrum junceiforme) amphiploids for biotic stress resistance and abiotic stress tolerance

Development of a complete wheat-Thinopyrum junceiforme amphiploid facilitated identification of resistance to multiple pests and abiotic stress derived from the wild species and shed new light on its genome composition. Wheat production is facing numerous challenges from biotic and abiotic stresses. Alien gene transfer has been an effective approach for wheat germplasm enhancement. Thinopyrum junceiforme, also known as sea wheatgrass (SWG), is a distant relative of wheat and a relatively untapped source for wheat improvement. In the present study, we developed a complete amphiploid, 13G819, between emmer wheat and SWG for the first time. Analysis of the chromosome constitution of the wheat-SWG amphiploid by multiple-color genomic in situ hybridization indicated that SWG is an allotetraploid with its J₁ genome closely related to Th. bessarabicum and Th. elongatum, and its J₂ genome was derived from an unknown source. Two SWG-derived perennial wheat lines, 14F3516 and 14F3536, are partial amphiploids and carry 13 SWG chromosomes of mixed J₁ and J₂ genome composition, suggesting cytological instability. We challenged the amphiploid 13G819 with various abiotic and biotic stress treatments together with its emmer wheat parent. Compared to its emmer wheat parent, the amphiploid showed high tolerance to waterlogging, manganese toxicity and salinity, low nitrogen and possibly to heat as well. The amphiploid 13G819 is also highly resistant to the wheat streak mosaic virus (temperature insensitive) and Fusarium head blight. All three amphiploids had solid stems, which confer resistance to wheat stem sawflies. All these traits make SWG an excellent source for improving wheat resistance to diseases and insects and tolerance to abiotic stress.

Impact of Timing and Method of Virus Inoculation on the Severity of Wheat Streak Mosaic Disease

Wheat streak mosaic virus (WSMV), transmitted by the wheat curl mite Aceria tosichella, frequently causes significant yield loss in winter wheat throughout the Great Plains of the United States. A field study was conducted in the 2013-14 and 2014-15 growing seasons to compare the impact of timing of WSMV inoculation (early fall, late fall, or early spring) and method of inoculation (mite or mechanical) on susceptibility of winter wheat cultivars Mace (resistant) and Overland (susceptible). Relative chlorophyll content, WSMV incidence, and yield components were determined. The greatest WSMV infection occurred for Overland, with the early fall inoculations resulting in the highest WSMV infection rate (up to 97%) and the greatest yield reductions relative to the control (up to 94%). In contrast, inoculation of Mace resulted in low WSMV incidence (1 to 28.3%). The findings from this study indicate that both method of inoculation and wheat cultivar influenced severity of wheat streak mosaic; however, timing of inoculation also had a dramatic influence on disease. In addition, mite inoculation provided much more consistent infection rates and is considered a more realistic method of inoculation to measure disease impact on wheat cultivars.

Relationships Between Early Wheat Streak Mosaic Severity Levels and Grain Yield: Implications for Management Decisions

Wheat streak mosaic (WSM) caused by Wheat streak mosaic virus, which is transmitted by the wheat curl mite (Aceria tosichella), is a major yield-limiting disease in the Texas High Plains. In addition to its impact on grain production, the disease reduces water-use efficiency by affecting root development. Because of the declining Ogallala Aquifer water level, water conservation has become one of the major pressing issues in the region. Thus, questions are often raised as to whether it is worthwhile to irrigate infected fields in light of the water conservation issues, associated energy costs, and current wheat prices. To address some of these questions, field experiments were conducted in 2013 and 2016 at two separate locations to determine whether grain yield could be predicted from disease severity levels, assessed early in the spring, for potential use as a decision tool for crop management, including irrigation. In both fields, disease severity assessments started in April, using a handheld hyperspectral radiometer with which reflectance measurements were taken weekly in multiple plots in arbitrarily selected locations across the fields. The relationship between WSM severity levels and grain yield for the different assessment dates were determined by fitting reflectance and yield values into the logistic regression function. The model predicted yield levels with r² values ranging from 0.67 to 0.85 (P < 0.0001), indicating that the impact of WSM on grain yield could be fairly well predicted from early assessments of WSM severity levels. As the disease is normally progressive over time, this type of information will be useful for making management decisions of whether to continue irrigating infected fields, especially if combined with an economic threshold for WSM severity levels.

Wheat Genotypes With Combined Resistance to Wheat Curl Mite, Wheat Streak Mosaic Virus, Wheat Mosaic Virus, and Triticum Mosaic Virus

The wheat curl mite, Aceria tosichella Keifer, (WCM) is a global pest of bread wheat that reduces yields significantly. In addition, WCM carries Wheat streak mosaic virus (WSMV, family Potyviridae, genus Tritimovirus), the most significant wheat virus in North America; High Plains wheat mosaic virus (HPWMoV, genus Emaravirus, formerly High plains virus); and Triticum mosaic virus (TriMV, family Potyviridae, genus Poacevirus). Viruses carried by WCM have reduced wheat yields throughout the U.S. Great Plains for >50 yr, with average yield losses of 2-3% and occasional yield losses of 7-10%. Acaricides are ineffective against WCM, and delayed planting of winter wheat is not feasible. Five wheat breeding lines containing Cmc4, a WCM resistance gene from Aegilops tauschii, and Wsm2, a WSMV resistance gene from wheat germplasm CO960293-2 were selected from the breeding process and assessed for phenotypic reaction to WCM feeding, population increase, and the degree of WSMV, HPWMoV, and TriMV infection. Experiments determined that all five lines are resistant to WCM biotype 1 feeding and population increase, and that two breeding lines contain resistance to WSMV, HPWMoV, and TriMV infection as well. These WCM-, WSMV-, HPWMoV-, and TriMV-resistant genotypes can be used improve management of wheat yield losses from WCM-virus complexes.

Effect of Temperature on Wheat Streak Mosaic Disease Development in Winter Wheat

Temperature is one of the key factors that influence viral disease development in plants. In this study, temperature effect on Wheat streak mosaic virus (WSMV) replication and in planta movement was determined using a green fluorescent protein (GFP)-tagged virus in two winter wheat cultivars. Virus-inoculated plants were first incubated at 10, 15, 20, and 25°C for 21 days, followed by 27°C for 14 days; and, in a second experiment, virus-inoculated plants were initially incubated at 27°C for 3 days, followed by 10, 15, 20, and 25°C for 21 days. In the first experiment, WSMV-GFP in susceptible 'Tomahawk' wheat at 10°C was restricted at the point of inoculation whereas, at 15°C, the virus moved systemically, accompanied with mild symptoms, and, at 20 and 25°C, WSMV elicited severe WSMV symptoms. In resistant 'Mace' wheat (PI 651043), WSMV-GFP was restricted at the point of inoculation at 10 and 15°C but, at 20 and 25°C, the virus infected systemically with no visual symptoms. Some plants that were not systemically infected at low temperatures expressed WSMV-GFP in regrowth shoots when later held at 27°C. In the second experiment, Tomahawk plants (100%) expressed systemic WSMV-GFP after 21 days at all four temperature levels; however, systemic WSMV expression in Mace was delayed at the lower temperatures. These results indicate that temperature played an important role in WSMV replication, movement, and symptom development in resistant and susceptible wheat cultivars. This study also demonstrates that suboptimal temperatures impair WSMV movement but the virus rapidly begins to replicate and spread in planta under optimal temperatures.

Physiological Responses of Hard Red Winter Wheat to Infection by Wheat streak mosaic virus

Wheat streak mosaic virus (WSMV) causes significant yield loss in hard red winter wheat in the U.S. Southern High Plains. Despite the prevalence of this pathogen, little is known about the physiological response of wheat to WSMV infection. A 2-year study was initiated to (i) investigate the effect of WSMV, inoculated at different development stages, on shoot and root growth, water use, water use efficiency (WUE), and photosynthesis and (ii) understand the relationships between yield and photosynthetic parameters during WSMV infection. Two greenhouse experiments were conducted with two wheat cultivars mechanically inoculated with WSMV at different developmental stages, from three-leaf to booting. WSMV inoculated early, at three- to five-leaf stage, resulted in a significant reduction in shoot biomass, root dry weight, and yield compared with wheat infected at the jointing and booting stages. However, even when inoculated as late as jointing, WSMV still reduced grain yield by at least 53%. Reduced tillers, shoot biomass, root dry weight, water use, and WUE contributed to yield loss under WSMV infection. However, infection by WSMV did not affect rooting depth and the number of seminal roots but reduced the number of nodal roots. Leaf photosynthetic parameters (chlorophyll [SPAD], net photosynthetic rate [Pn], stomatal conductance [Gs], intercellular CO2 concentration [Ci], and transpiration rate [Tr]) were reduced when infected by WSMV, and early infection reduced parameters more than late infection. Photosynthetic parameters had a linear relationship with grain yield and shoot biomass. The reduced Pn under WSMV infection was mainly in response to decreased Gs, Ci, and SPAD. The results of this study indicated that leaf chlorophyll and gas exchange parameters can be used to quantify WSMV effects on biomass and grain yield in wheat.