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Browsing by Author "Schwartz, Howard F., advisor"

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    Biology and over-winter survival of Iris yellow spot virus in Colorado
    (Colorado State University. Libraries, 2014) Szostek, Stephanie Aspen, author; Schwartz, Howard F., advisor; Bartolo, Michael, committee member; Cranshaw, Whitney, committee member; Tisserat, Ned, committee member
    Iris yellow spot virus (IYSV) (Family: Bunyaviridae, Genus: Tospovirus) and its insect vector, Thrips tabaci Lindeman, are of economic concern in onion (Allium cepa L.) growing regions worldwide. IYSV symptoms appear on onion foliage as tan or straw colored, elongate diamond shaped lesions. Accumulated lesions may coalesce on the foliage or girdle the scape, causing lodging and loss of seed. There is no evidence that Tospoviruses, including IYSV, are seed transmitted. Onion seed included in double antibody sandwich enzyme linked immunosorbent assays (DAS-ELISA) to detect IYSV occasionally yielded a positive result. IYSV was detected in the pedicels, petals, anthers, and fruits of onion flowers by reverse transcriptase polymerase chain reaction (RT-PCR). Onion seed collected from several cultivars of IYSV symptomatic plants was grown out under greenhouse and growth chamber conditions. IYSV was not detected in the six week old seedlings. Further investigation of onion seeds revealed IYSV could be detected in the seed coat, but not the emerging radicle. It is highly unlikely that IYSV can pass from the seed coat to the new plant during germination, and seeds remain an unlikely source of IYSV inoculum. Several weed species have been described as additional hosts and likely green bridges for IYSV survival, however, there is little work regarding the overwintering habits of T. tabaci and its potential to act as a source of inoculum during the following season. The results presented in this work close the loop, and show that both T. tabaci and IYSV are present near onion fields throughout the winter, T. tabaci will reproduce on several weed species, and larvae can acquire IYSV from non-allium sources. Thrips activity was monitored via sticky trap during the winter months from 2011 to 2013. Thrips activity appeared to cease once the average temperature fell below 0°C and resumed once the average temperature rose above 0°C. Onion cull piles were constructed, and while these piles provided an environment conducive to thrips survival, few live thrips were recovered from the piles after the onset of bulb decay. IYSV was detected by RT-PCR in live adult and larval thrips recovered from onion, Malva neglecta Wallr. (common mallow), Taraxacum officinale Weber in Wiggers (dandelion), Descurainia sophia (L.) Webb. Ex Prantl (flixweed), Lactuca serriola L. (prickly lettuce), and Tragopogon dubius Scop. (salsify) during the winters from 2010 to 2013. Of these plants, IYSV was detected in prickly lettuce and flixweed. These five weed species were grown from seed in the greenhouse and exposed to viruliferous thrips to further elucidate their potential role as green bridges. Of the five, IYSV was detected in salsify and the thrips larvae reared on this plant. Results indicate winter annuals play a role in onion thrips and IYSV over-winter survival, providing inoculum the next growing season, and that weed management during the winter may be warranted. IYSV distribution throughout onion leaves is uneven and patchy. A reverse transcription quantitative real time PCR (RT-qPCR) was developed to compare relative amounts of IYSV within leaves and between cultivars. The amount of IYSV was greatest at the lesion site itself and decreased as distance from the lesion increased. No statistically significant differences were found in the amount of IYSV between susceptible cultivar Granero and tolerant cultivar Advantage. This assay may be useful for additional comparative studies with other crops and viruses.
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    Iris yellow spot virus in Colorado onions: a survey of its spatial distribution and techniques to manage the pest
    (Colorado State University. Libraries, 2007) Fichtner, Scott M., author; Schwartz, Howard F., advisor; Hill, Joe, advisor
    The Iris yellow spot virus (IYSV) is a new, and at times devastating pathogen of alliums throughout the U.S. as well as many other countries. Since its discovery in Colorado in 2001, IYSV has been identified in nearly all of the major onion producing regions within the state. The severity of this virus disease appears to fluctuate from one year to the next but incidence continues to increase with newly infested fields identified each year. With the number of outbreaks on the rise and the inadequacy of current control strategies, new management techniques as well as novel chemistry pesticides have become a major focus for the management of this virus disease and its thrips vector. Our trials demonstrated the use of reflective materials such as straw or silver reflective mulch can result in a reduction in thrips populations by as much as 69% on onions and a reduction of nearly 9% in IYSV disease incidence. Additionally, Entrust (Spinosad) and Aza-Direct (Neem extract) were found to work as well as or better than conventional materials such as Warrior (Pyrethroid) and Lannate (Carbamate). To better understand the epidemiology of the IYSV pathogen, we also conducted an extensive survey at several locations along the Colorado Front Range and Western Slope. In our surveys, we collected information including thrips populations and incidence of IYSV using 0.2 ha grids developed using mapping software (MapInfo) creating several randomly chosen plots in each field. With data collected on several sampling dates, we attempted to identify a spatial correlation of within field spread of the virus during the growing season. The levels of positive spatial autocorrelation from our survey locations were minimal, this leads us to believe that secondary outbreaks of the disease are occurring in a random fashion across the field.
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    Physiological responses of onion germplasms to Iris yellow spot virus and onion thrips (Thrips tabaci)
    (Colorado State University. Libraries, 2012) Boateng, Charles Osei, author; Schwartz, Howard F., advisor; Cranshaw, Whitney, committee member; Tisserat, Ned, committee member; Bartolo, Michael, committee member
    Onion (Allium cepa L.) is the most economically important monocot outside of the grasses. Onion thrips (Thrips tabaci), a cosmopolitan and polyphagous insect, infests and damages onion crops grown between sea level and 2,000 m. In recent years, onion thrips has emerged as the principal vector of the economically important Iris yellow spot virus (IYSV, family Bunyaviridae). Any attempt to significantly reduce the economic impact of this pest/pathogen will require a multifaceted approach designed in an IPM framework. Host plant resistance is an important foundation to the success of such approaches, and in an effort to find sources of resistance to IYSV and onion thrips, 137, 104 and 84 onion germplasms, respectively, were evaluated in 2009, 2010 and 2011 in northern Colorado, near Fort Collins. Sixteen, 18 and 11 germplasms, respectively, were selected in 2009, 2010 and 2012 for exhibiting acceptable level of tolerance/resistance to the two pests. Of these, PIs 264320, 546140, and 546192 were selected in both 2009 and 2010, and PIs 258956 and 546188 were selected in all three years of the evaluation. Selected germplasms were included as candidates for the onion translational genomics component of this national endeavor. Greenhouse experiments were carried out to study the effects of five treatments; Healthy Control (HC), Spray (S), Thrips Only (T), IYSV Only (V) and Thrips + IYSV (TV), on the growth, physiology and productivity of Colorado 6, Talon and Salsa Red onion cultivars. Seasonal mean net photosynthesis (A) and late season light response curves were higher in Colorado 6 than in Talon. Late season curves were significantly lower in TV, V and T than in HC and S in all cultivars. Seasonal mean growth rate was significantly higher in Colorado 6 than in Talon. Among the treatments, seasonal mean growth rate was in the decreasing order of HC, V, TV and T in 2009; and S, HC, V, T and TV in 2011. In one of three years, yield was significantly higher in Colorado 6 than in Salsa Red and Talon. Biomass partitioning pattern revealed that Talon had the highest harvest index among the cultivars. TV had the lowest bulb yield among the treatments in all three years of the study, and caused 14-60% yield loss in Colorado 6, 14-46% loss in Talon, and 17-48% loss in Salsa Red. Seasonal dynamics of IYSV titer in leaves of Colorado 6 and Talon indicated that virus titer for TV and V were consistently higher in Talon than in Colorado 6. Titer was in the increasing order of middle, top and base leaf sections. In naturally infected pre-bulb plants, virus titer increased from Leaf 1 (outer leaf) to Leaf 4 (inner leaf) after which it declined in subsequent younger leaves. Virus was not detected in dead leaves, bulb scales, basal plates or roots. In post-bulb plants, virus titer distribution was considerably non-uniform, with no apparent trend within or between leaves. Sources of resistance to IYSV and onion thrips exist in the onion gene pool. The search for these vital sources should continue so that the sustainability of the U.S. onion industry may be achieved through the use of efficient, reliable and environmentally safe integrated disease management strategies.