Monitoring the Incidence, Distribution, and Transmission of the Iris Yellow Spot and Tomato Spotted Wild Viruses in Georgia Vidalia Onions

NON-TECHNICAL SUMMARY: IYSV is a major disease of onion worldwide and has recently been detected in Georgia. TSWV has also been detected and is the first report of this virus in onion. Symptomology, virus detection, and thrips acquisition/transmission are unknown in Georgia. This project will help elucidate the nature of IYSV and TSWV in Georgia in regard to distribution, thrips vectors, and symptomology.

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APPROACH: Obj. 1: Plant samples will be tested virus by DAS-ELISA. DAS-ELISA will be conducted according to manufacturer's protocol. Results will be read with an automated microplate reader. RT-PCR will be conducted on virus samples as well using gel electrophoresis. Obj. 2: ACP-indirect ELISA will be used to detect the virus in thrips. Again, ELISA procedures will be conducted according to the manufacturer's protocol. Thrips will be ground in PBS buffer and the suspension will be added to a test well of an ELISA plate. Plates will be incubated in a moist chamber overnight. Plates will be rinsed PBS-tween buffer, then dried. A blocking solution will be added to each well and incubated at room temperature and dried again. A monoclonal antibody suspension will be added as above and incubated. After washing plates, the conjugate will be added and plates will be incubated. After washing plates, substrate will be added and incubated. Obj. 3: RT-PCR will be conducted with extracted RNA from samples transported to the laboratory as above and compared with RT-PCR from plant tissue homogenates processed immediately in the field and spotted on FTA cards. For the latter, the virus suspension will be applied to the FTA paper in the field. FTA samples will be stored at room temperature in the dark until needed. RT-PCR will be conducted on the RNA captured on the FTA paper by following the company protocol. Obj. 4: Digital images will be made of plants with various symptoms and stored according to sample ID. ELISA and PCR will be conducted to verify virus type. Results from viral detection and identification will be compared with digital images and types of symptoms will be noted for each virus and for mixed infections. Obj. 5: Colonies of T. tabaci, F. occidentalis and F. fusca will be established and maintained on virus-free hosts in rearing chambers. Once IYSV infected plants are identified, test colonies will be reared on virus-infected and virus-free hosts. Adults will be introduced on these hosts and allowed to feed and oviposit. Adults will be removed from these plants after a week and placed on virus-free plants and allowed to feed for at least a week. The plants on which the adults feed will be held and tested for virus infection to verify acquisition and transmission of IYSV. The initial virus-infected and virus-free plants will be held to allow for thrips to develop to adults. These adults will be transferred to virus-free plants and allowed to feed for one week. These plants will be held and tested for development of IYSV. Infection will be evaluated by visual assessment and confirmed with ELISA and PCR. Obj. 6: Regional sampling for onion tospoviruses and thrips species will be conducted by monitoring eight commercial fields in Toombs and Tattnall Counties. Fields will be sampled every two weeks from February through harvest. Fields will be searched for thirty minutes, adults will be collected, and vials containing thrips will be labeled and taken to the laboratory for identification. Thrips will be examined under a stereo-microscope for identification. Sub-samples will be mounted on slides and examined under a compound microscope to verify species.
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PROGRESS: 2005/09 TO 2007/09 <BR>
In 2007, we tested 970 onion leaf samples for IYSV. 900 of the samples were tested in the hail simulation study. In addition, we tested sprouts of 24 cull onions that had been discarded in a field. 934 onion samples were tested for TSWV, including the 900 samples from the hail simulation study. Of these samples, 18 samples including one cull onion tested positive for IYSV, and 11 samples tested positive for TSWV. The 900 samples were the total for the hail simulation study collected over a 4-week period. Spiny sowthistle (Sonchus asper) from Tattnall Co. was confirmed with ELISA, RT-PCR and sequencing as a host of IYSV. In a survey across Georgia we collected 184 spiny sowthistles of which 21% tested positive with ELISA for IYSV. We confirmed three of these sowthistles with RT-PCR. ELISA positive sowthistles were found to the north, northwest and southwest of the Vidalia onion region which was considered the source of IYSV in Georgia. No sowthistles tested positive to the south and east of the Vidalia onion region. Overall outputs indicated variability in the amount of IYSV observed from year to year, the weeds hosts it infects, and how it may spread.
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IMPACT: 2005/09 TO 2007/09 <BR>
Overall the impact of this work has determined the source of IYSV and has given us insight as to how it spreads.