Jacob A. Price

Effects of Wheat streak mosaic virus on Root Development and Water-Use Efficiency of Hard Red Winter Wheat

Greenhouse and field studies were conducted to determine the effects of Wheat streak mosaic virus (WSMV), a member of the family Potyviridae, on root development and water-use efficiency (WUE) of two hard red winter wheat (Triticum aestivum) cultivars, one susceptible and one resistant to WSMV. In the greenhouse studies, wheat cultivars were grown under three water regimes of 30, 60, and 80% soil saturation capacity. After inoculation with WSMV, plants were grown for approximately 4 weeks and then harvested. Root and shoot weights were measured to determine the effect of the disease on biomass. In all water treatments, root biomass and WUE of inoculated susceptible plants were significantly less (P < 0.05) than those of the noninoculated control plants. However, in the resistant cultivar, significance was only found in the 30 and 60% treatments for root weight and WUE, respectively. Field studies were also conducted under three water regimes based on reference evapotranspiration rates. Significant reductions in forage, grain yield, and crop WUE were observed in the inoculated susceptible plots compared with the noninoculated plots. Both studies demonstrated that wheat streak mosaic reduces WUE, which is a major concern in the Texas Panhandle because of limited availability of water.

Multiplex real-time RT-PCR for detection of Wheat streak mosaic virus and Tritcum mosaic virus

Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) are widespread throughout the southwestern Great Plains states. When using conventional diagnostics such as enzyme-linked immunosorbent assays (ELISA), these two viruses are commonly found together in infected wheat samples. Methods for molecular detection have been developed for wheat viral pathogens, but until recently no multiplex method for detection of both WSMV and TriMV within a single sample was available. Therefore, the objective of this study was to develop a multiplex real-time PCR technique for detection of both pathogens within a single plant sample. Specific primers and probe combinations were developed for detection of WSMV and TriMV, single and multiple reactions were run simultaneously to detect any loss in sensitivity during the multiplex reaction, as well as any cross-reaction with other common wheat viruses. The multiplex reaction was successful in detection of both pathogens, with little difference between single and multiplex reactions, and no cross-reaction was found with other common wheat viruses. This multiplex technique not only will be useful for diagnostic evaluations, but also as a valuable tool for ecological and epidemiology studies, and investigations of host/pathogen interactions, especially when the host is infected with both pathogens.

Wheat Streak Mosaic: A Classic Case of Plant Disease Impact on Soil Water Content and Crop Water-Use Efficiency

The role of soil water (or soil matric potential) in plant disease development is a well-documented phenomenon, especially in diseases caused by soilborne plant pathogens (7,11,14,19). For many plant pathogens, wet soil conditions or high soil matric potential promote hyphal growth, reproduction, spore dispersal, infection, and disease development (3,5–7,9–11). In sharp contrast to the abundance of information on the effect of soil water content on various plant diseases, the effect of plant diseases on soil water status and crop water-use efficiency is not well described. The significance of this effect may be less apparent in dryland agricultural production where rainfall is the only source of soil water; however, the effect has tremendous implications for irrigated agriculture in light of the dwindling water resources and the high cost of irrigation water (15–17). Plant diseases interfere with water intake (8,12), suggesting that there should be a greater amount of surplus water in diseased areas than in healthy areas of the field. If this is indeed the case, water-use efficiency would be low in diseased crops, and expenses associated with addition of more water (when in fact there is surplus water still in the soil) is a cost which could potentially be avoided. The implications of a plant disease effect on crop water-use efficiency is more significant for diseases which have characteristic patchy distribution, where large areas of the field are affected, than in sporadically infected plants which may not be amenable to sitespecific management. In this article, we describe the relationship between wheat streak mosaic (WSM) severity and soil water content as a prime example of the effect of a plant disease on soil water status and its implications for irrigated agriculture. Wheat streak mosaic virus (WSMV) is vectored by wheat curl mites (Aceria tosichella Keifer), which are typically blown into the field from nearby grasslands or fields with mite-infested volunteer wheat (4,20). WSM often exhibits severity gradients affecting over half of the field, with the heaviest infection toward the edges of the field (22). The disease is prevalent throughout the Texas Panhandle and much of the southwestern United States and is sometimes severe, drastically reducing both forage and grain yield (2,22). Infected plants exhibit symptoms ranging from chlorotic streaking and mosaic to complete chlorosis of leaves and stunting of the plants (20). A recent greenhouse study showed that plants infected with WSMV have poor root development and reduced water-use efficiency compared with uninfected plants (18). If diseased wheat cannot use water as efficiently as healthy wheat, blanket applications of irrigation water without recognizing specific requirements in different parts of the field is certainly an inefficient use of water. Diseased areas of the field are often infested with weeds because diseased plants cannot compete well with the weeds, and the unused soil moisture gets depleted by the weeds. If infected fields have greater soil water and lower wateruse efficiency than healthy fields, they don’t need as much water as healthy fields. This information can be vital to growers in making decisions whether to minimize or avoid water application depending on the disease severity. In light of rising energy costs, growers often question whether it is worthwhile to continue to irrigate fields affected with WSMV because information on the effect of the disease on crop water use is not available. In order to address this question, we conducted studies in growers’ fields to characterize the relation between wheat streak severity, soil water content, and water-use efficiency.

Triticum mosaic virus Isolates in the Southern Great Plains

In 2006, a previously unknown wheat (Triticum aestivum) virus was discovered in Western Kansas and given the name Triticum mosaic virus (TriMV). TriMV has since been found in wheat samples isolated all across the Great Plains. Even though it can infect singularly, TriMV is mostly found with Wheat streak mosaic virus (WSMV) as a co-infection. The potential for TriMV to cause economic loss is significant, but very little is known about the virus. The objective of this study was to survey the TriMV population for genetic variation by nucleotide sequencing of isolates across a geographical region. A secondary objective was to characterize the WSMV isolates that are being co-transmitted with TriMV. Fourteen different TriMV isolations were taken from locations in Texas, Oklahoma, and Kansas, and the coat protein cDNA was sequenced. Thirteen nucleotide differences were found in the TriMV isolates, of which three induce amino acid changes. WSMV isolates had 65 nucleotide changes when compared to WSMV Sydney81. Our results indicate the TriMV virus population has minimal amounts of sequence variation and no singular WSMV genotype is specifically associated with TriMV co-infection. Based on the isolates analyzed, it appears that the field population of TriMV is very homogeneous.

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.