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Biology and over-winter survival of Iris yellow spot virus in Colorado




Szostek, Stephanie Aspen, author
Schwartz, Howard F., advisor
Bartolo, Michael, committee member
Cranshaw, Whitney, committee member
Tisserat, Ned, committee member

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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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onion thrips
Thrips tabaci
Allium cepa
Iris yellow spot virus


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