Geographical Distribution and Survival of Iris yellow spot virus in Spiny Sowthistle , Sonchus asper , in

Nischwitz, C. , Srinivasan, R. , Sundaraj, S. , Mullis, S. W. , McInnes, B. , and Gitaitis, R. D. 2012. Geographical distribution and survival of Iris yellow spot virus in spiny sowthistle, Sonchus asper, in Georgia. Plant Dis. 96:1165-1171. Iris yellow spot virus (IYSV) has occurred in Georgia since 2003. IYSV is transmitted by onion thrips, Thrips tabaci. During a weed survey in the Vidalia onion-growing zone (VOZ), spiny sowthistle (Sonchus asper) was identified as a host for IYSV. Spiny sowthistle is widespread in Georgia, and this presented an opportunity to study the natural spread of IYSV and assess its potential role in IYSV epidemiology. From 2007 to 2009, during the spring season, 2,011 sowthistle samples were collected from various counties within and outside the VOZ. The samples were tested for IYSV infection by enzyme-linked immunosorbent assay and confirmed by reverse-transcription polymerase chain reaction and sequencing. IYSV sequences from sowthistle were 98 to 99% identical to onion IYSV sequences from onion originated from Georgia. By the third year, IYSV-infected sowthistle plants were found in 79% of the counties in the VOZ and in 61% of the sampled counties in all directions, except to the east of the VOZ. Furthermore, thrips-mediated transmission assays confirmed that T. tabaci can efficiently transmit IYSV from onion to sowthistle. Sowthistle also supported T. tabaci survival and reproduction. These findings demonstrate that sowthistle plants can serve as an IYSV inoculum source and as a thrips reservoir. Iris yellow spot virus (IYSV) is a member of the Tospovirus genus in the family Bunyaviridae (37). IYSV is known to infect plants belonging to at least sixteen families (2,12,15– 17,21,26,28,38,39,44). Under favorable conditions, IYSV can severely impact onion production and yield (17,18). IYSV has been reported in onion in the United States (9,10,12,14,22– 24,32,34,40,46) and worldwide (3–8,13,19,21,25,27,29,31,33, 35,41–44,49). In Georgia, IYSV was first found in onion in the Vidalia region in 2003 (34). Phylogenetic analysis of the IYSV isolates from Georgia and elsewhere indicated a high N-gene (capsid protein gene) sequence homology between IYSV isolates from Georgia and Peru (38). This revealed that the virus was most likely introduced into the Vidalia region through imported Peruvian onion. Onion growers in the Vidalia onion zone (VOZ) often resort to repacking imported Peruvian onion during the off-season to minimize downtime for their equipment and may have inadvertently introduced IYSV into Georgia by discarding imported onion bulbs in cull piles (38). IYSV is transmitted by onion thrips, Thrips tabaci (Lindeman) (Thysanoptera: Thripidae), in a persistent, circulative, and propagative manner (35). Onion thrips and tobacco thrips, Frankliniella fusca (Lindeman), are two common thrips species found on onion in Georgia (48). The vector status of F. fusca is currently being investigated but preliminary findings indicate that F. fusca can also transmit IYSV (R. Srinivasan, unpublished data). Since its first detection in Georgia in 2003, IYSV has been found in low levels every year in onion. Onion plants are grown as a winter crop in Georgia. The seedbeds are planted in September to October; transplants are set in November to January and harvested in April to May. This indicates that the virus has to survive on alternate hosts during summer. Typically, numerous other crops that are hosts of another Tospovirus sp. (Tomato spotted wilt virus) are cultivated in summer. To date, none of the commonly grown crops in and around the VOZ, such as soybean, cotton, peanut, tomato, and pepper, have tested positive for IYSV. Across the United States, several weeds have tested positive for IYSV (15,16,45). Susceptibility of other hosts has been reported by mechanical inoculation (1,17). In 2007, numerous weed species were collected from areas adjacent to an onion cull pile and from other areas in the VOZ, and were tested for IYSV. Only one plant species, spiny sowthistle, Sonchus asper (L.) Hill, tested positive by enzyme-linked immunosorbent assay (ELISA). The presence of the virus was confirmed with reverse-transcription polymerase chain reaction (RT-PCR) followed by partially sequencing the coat protein gene (36). Other weed species tested included Oenothera laciniata Hill, Verbena rigida Spreng. , Solidago canadensis L. , Amaranthus retroflexus L. , Wahlenbergia marginata (Thunb.) A. DC. , Lonicera flava Sims, Passiflora incarnata L. , Richardia scabra L. , and Portulaca pilosa L. Sowthistle plants are native to Europe and were introduced into North America. Two species are found in Georgia: spiny sowthistle (Sonchus asper) and common sowthistle (S. oleraceus (L.)). They are both considered annuals. Spiny sowthistle is more common along roadsides, fences, and railroads and in waste areas and was the only sowthistle species tested. The objectives of this study were to (i) conduct a multiyear survey across Georgia to determine the extent of the spread of IYSV in sowthistle within and outside of the VOZ and (ii) evaluate the ability of sowthistle to serve as a suitable host for T. tabaci and as an inoculum source for IYSV. Materials and Methods Sample collection. In 2007, the survey of spiny sowthistle plants included sites in 42 counties both within and outside the VOZ (Fig. 1). The sample size was a minimum of three samples per site. This was increased to a minimum of five samples per site in 2008 and 25 samples per site in 2009; however, in 2008 and 2009, sampling was reduced to one site per county. Total samples per year were 184, 308, and 1,519 for 2007, 2008, and 2009, Corresponding author: C. Nischwitz, E-mail: claudia.nischwitz@usu.edu Accepted for publication 28 February 2012. http://dx.doi. org/10.1094 / PDIS-09-11-0747-RE © 2012 The American Phytopathological Society