Quantification of Yield Loss Caused by Triticum mosaic virusand Wheat streak mosaic virus in Winter Wheat Under Field Conditions

Byamukama, E., Wegulo, S. N., Tatineni, S., Hein, G. L., Graybosch, R. A., Baenziger, P. S., and French, R. 2014. Quantification of yield loss caused by Triticum mosaic virus and Wheat streak mosaic virus in winter wheat under field conditions. Plant Dis. 98:127-133. Triticum mosaic virus (TriMV) and Wheat streak mosaic virus (WSMV) infect winter wheat (Triticum aestivum) in the Great Plains region of the United States. The two viruses are transmitted by wheat curl mites (Aceria tosichella), which also transmit High Plains virus. In a field study conducted in 2011 and 2012, winter wheat cultivars Millennium (WSMV-susceptible) and Mace (WSMV-resistant) were mechanically inoculated with TriMV, WSMV, TriMV+WSMV, or sterile water at the two-leaf growth stage. Chlorophyll meter (soil plant analysis development [SPAD]) readings, area under the SPAD progress curve (AUSPC), grain yield (=yield), yield components (spikes/m2, kernels/spike, 1,000-kernel weight), and aerial dry matter were determined. In Millennium, all measured variables were significantly reduced by single or double virus inoculation, with the greatest reductions occurring in the double-inoculated treatment. Among the yield components, the greatest reductions occurred in spikes/m2. In Mace, only AUSPC was significantly reduced by the TriMV+WSMV treatment in 2012. There was a significant (P ≤ 0.05), negative linear relationship between SPAD readings and day of year in all inoculation treatments in Millennium and in the TriMV+WSMV treatment in Mace. There were significant (P ≤ 0.05), positive linear relationships between yield and SPAD readings and between yield and aerial dry matter in Millennium but not in Mace. The results from this study indicate that under field conditions, (i) Mace, a WSMV-resistant cultivar, is also resistant to TriMV, and (ii) double inoculation of winter wheat by TriMV and WSMV exacerbates symptom expression and yield loss in a susceptible cultivar. Triticum mosaic virus (TriMV), the type member of the genus Poacevirus in the family Potyviridae (22), is a newly discovered virus that infects wheat (Triticum aestivum L.) (16). It was first isolated in Kansas in 2006 from wheat plants with mosaic symptoms (16) and subsequently shown to be transmitted by the wheat curl mite (WCM, Aceria tosichella Keifer) (15). Recent field surveys indicated that TriMV is widely distributed in the Great Plains region of the United States (2,3). These surveys also showed that Wheat streak mosaic virus (WSMV), the type member of the genus Tritimovirus in the family Potyviridae and which also is transmitted by the WCM (18), is the predominant virus infecting wheat in the Great Plains region. A survey of the central Great Plains states of Colorado, Kansas, Nebraska, and South Dakota revealed a high frequency (91%) of co-infection of wheat by TriMV and WSMV (3). The highly frequent occurrence of double infection of wheat by TriMV and WSMV implies that there is potential for exacerbated yield loss in wheat under natural field conditions. In greenhouse experiments, it was demonstrated that double infections of wheat with TriMV and WSMV induced disease synergism with severe leaf deformation, bleaching, and stunting (21). Previously, we showed that both single and double infections of WSMV-susceptible wheat cv. Millennium with TriMV and WSMV under greenhouse conditions significantly reduced chlorophyll content, number of tillers per plant, shoot and dry weight, and total nitrogen and carbon content relative to noninfected plants (4). However, the greatest reductions occurred in the double infection treatment. In contrast, single and double infections of WSMV-resistant cv. Mace with TriMV and WSMV had no significant effect on all of these yield determinants except nitrogen content (4). In the same study, TriMV caused more severe symptoms than WSMV in both cultivars, suggesting that TriMV has the potential to cause greater yield loss than WSMV. These studies show the potential negative effects on yield determinants and yield when a susceptible wheat cultivar is co-infected with TriMV and WSMV. Previous research has shown that WSMV by itself can cause significant yield losses in wheat (6,17,19). The economic impact of these losses, some of which can approach 100% (6), can be devastating to farmers and the wheat industry. An economic analysis conducted by Velandia et al. (23) showed a marginal loss (or marginal profit reduction) due to WSMV of up to $464.5/ha. This loss was higher in irrigated wheat than in dryland wheat due to the reduced water use efficiency that results when wheat is infected by WSMV (12,23). Recently, Seifers et al. (14) reported that TriMV caused significant yield and volume weight reductions in the wheat cultivars Danby, RonL, and Jagalene, but not in the wheat breeding line KS96HW10-3. Because TriMV is a newly discovered virus, research is needed, especially under field conditions, to elucidate the effects of its synergistic interactions with WSMV on yield reduction in wheat. We report here the effects of single (TriMV or WSMV) and double (TriMV+WSMV) inoculation on yield and yield components of WSMV-susceptible Millennium and WSMVresistant Mace under field conditions in two growing seasons. Materials and Methods Virus inoculum source, maintenance, and inoculum preparation. WSMV Sidney 81 isolate and a TriMV isolate were maintained on winter wheat cv. Millennium (PI 613099) in the greenhouse. The WSMV Sidney 81 isolate was obtained from an infectious cDNA clone whose in vitro–generated RNA transcripts were inoculated to wheat seedlings at the single-leaf stage (5). The TriMV isolate was originally obtained from wheat plants collected from Red Willow County, NE and confirmed to be TriMV by imCorresponding author: S. N. Wegulo, E-mail: [email protected] Accepted for publication 20 August 2013 http://dx.doi.org/10.1094 / PDIS-04-13-0419-RE © 2014 The American Phytopathological Society