Green Foxtail (Setaria viridis), A Naturally Infected Grass Host of Iris yellow spot virus in Utah

Importance of Transplanted Onions Contributing to Late-Season Iris yellow spot virus Epidemics in New York

Iris yellow spot virus (IYSV) is an economically significant tospovirus of onion transmitted by onion thrips (Thrips tabaci Lindeman). IYSV epidemics in onion fields are common in New York; however, the role of various habitats contributing to viruliferous onion thrips populations and IYSV epidemics is not known. In a 2-year field study in New York, the abundance of dispersing onion thrips, including those determined to be viruliferous via reverse-transcriptase polymerase chain reaction, was recorded in habitats known to harbor both IYSV and its vector. Results showed that viruliferous thrips were encountered in all habitats; however, transplanted onion sites accounted for 49 to 51% of the total estimated numbers of viruliferous thrips. During early to midseason, transplanted onion sites had 9 to 11 times more viruliferous thrips than the other habitats. These results indicate that transplanted onion fields are the most important habitat for generating IYSV epidemics in all onion fields (transplanted and direct-seeded) in New York. Our findings suggest that onion growers should control onion thrips in transplanted fields early in the season to minimize risk of IYSV epidemics later in the season.

Iris yellow spot virus (Tospovirus: Bunyaviridae): from obscurity to research priority

TAXONOMY

Iris yellow spot virus (IYSV) is in the genus Tospovirus, family Bunyaviridae, with a single-stranded, tri-segmented RNA genome with an ambisense genome organization. Members of the other genera in the family infect predominantly vertebrates and insects.

GEOGRAPHICAL DISTRIBUTION

IYSV is present in most Allium-growing regions of the world.

PHYSICAL PROPERTIES

Virions are pleomorphic particles of 80-120 nm in size. The particle consists of RNA, protein, glycoprotein and lipids.

GENOME

IYSV shares the genomic features of other tospoviruses: a segmented RNA genome of three RNAs, referred to as large (L), medium (M) and small (S). The L RNA codes for the RNA-dependent RNA polymerase (RdRp) in negative sense. The M RNA uses an ambisense coding strategy and codes for the precursor for the GN /GC glycoprotein in the viral complementary (vc) sense and a non-structural protein (NSm) in the viral (v) sense. The S RNA also uses an ambisense coding strategy with the coat protein (N) in vc sense and a non-structural protein (NSs) in the v sense.

TRANSMISSION

The virus is transmitted by Thrips tabaci Lindeman (Order: Thysanoptera; Family: Thripidae; onion thrips) and with less efficiency by Frankliniella fusca Hinds (tobacco thrips). HOST: IYSV has a relatively broad host range, including cultivated and wild onions, garlic, chives, leeks and several ornamentals. Some weeds are naturally infected by IYSV and may serve as alternative hosts for the virus.

SYMPTOMS

IYSV symptoms in Allium spp. are yellow- to straw-coloured, diamond-shaped lesions on leaves and flowering scapes. Diamond-shaped lesions are particularly pronounced on scapes. As the disease progresses, the lesions coalesce, leading to lodging of the scapes. In seed crops, this could lead to a reduction in yield and quality. Early to mid-season infection in bulb crops results in reduced vigour and bulb size.

CONTROL

Resistant varieties are not available, but a limited number of accessions with field tolerance have been identified. Integrated disease management tactics, including sanitation, crop rotation, thrips management, maintenance of optimal plant vigour, soil fertility, irrigation and physical separation of bulb and seed crops, can mitigate the effect of the disease. Virus code: 00.011.0.85.009 Useful link: http://www.alliumnet.com/.

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

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.

Tospoviruses in the Mediterranean area

Tospoviruses are among the most serious threats to vegetable crops in the Mediterranean basin. Tospovirus introduction, spread, and the diseases these viruses cause have been traced by epidemiological case studies. Recent research has centered on the close relationship between tospoviruses and their arthropod vectors (species of the Thripidae family). Here, we review several specific features of tospovirus-thrips associations in the Mediterranean. Since the introduction of Frankliniella occidentalis in Europe, Tomato spotted wilt virus (TSWV) has become one of the limiting factors for vegetable crops such as tomato, pepper, and lettuce. An increasing problem is the emergence of TSWV resistance-breaking strains that overcome the resistance genes in pepper and tomato. F. occidentalis is also a vector of Impatiens necrotic spot virus, which was first observed in the Mediterranean basin in the 1980s. Its importance as a cause of vegetable crop diseases is limited to occasional incidence in pepper and tomato fields. A recent introduction is Iris yellow spot virus, transmitted by the onion thrips Thrips tabaci, in onion and leek crops. Control measures in vegetable crops specific to Mediterranean conditions were examined in the context of their epidemiological features and tospovirus species which could pose a future potential risk for vegetable crops in the Mediterranean were discussed.

Sources of Iris yellow spot virus in New York

Iris yellow spot virus (IYSV) has been found consistently in commercial dry bulb onion fields throughout New York State since 2006. Yearly recurrence of IYSV may result from annual reintroductions of the virus or persistence of the virus in overwintering host plants. To identify potential sources of IYSV, we surveyed onion transplants imported into New York as well as volunteer onion plants and weeds using a double-antibody sandwich enzyme-linked immunosorbent assay. IYSV was not found in any of 1,097 transplant samples tested in 2007 but 4 of 760 (0.53%) transplant samples tested positive in 2008. IYSV was found in volunteer onion plants in 3 of 10 (30%) onion fields sampled in 2007, in 4 of 27 (15%) onion fields sampled in 2008, and in 6 of 12 (50%) onion cull piles sampled in 2008. In all, 4 of 17 weed species (i.e., chicory [Cichorium intybus], common burdock [Arctium minus], curly dock [Rumex crispus], and dandelion [Taraxacum officinale]), were confirmed to be infected with IYSV using serological and molecular testing methods. IYSV may be reintroduced annually into New York through imported onion transplants but it also persists in volunteer onion plants and selected weed species.

Temporal dynamics of iris yellow spot virus and its vector, Thrips tabaci (Thysanoptera: Thripidae), in seeded and transplanted onion fields

Onion thrips, Thrips tabaci (Lindeman) (Thysanoptera: Thripidae), can reduce onion bulb yield and transmit iris yellow spot virus (IYSV) (Bunyaviridae: Tospovirus), which can cause additional yield losses. In New York, onions are planted using seeds and imported transplants. IYSV is not seed transmitted, but infected transplants have been found in other U.S. states. Transplants are also larger than seeded onions early in the season, and thrips, some of which may be viruliferous, may preferentially colonize larger plants. Limited information is available on the temporal dynamics of IYSV and its vector in onion fields. In 2007 and 2008, T. tabaci and IYSV levels were monitored in six seeded and six transplanted fields. We found significantly more thrips in transplanted fields early in the season, but by the end of the season seeded fields had higher levels of IYSV. The percentage of sample sites with IYSV-infected plants remained low (<12%) until August, when infection levels increased dramatically in some fields. The densities of adult and larval thrips in August and September were better predictors of final IYSV levels than early season thrips densities. For 2007 and 2008, the time onions were harvested may have been more important in determining IYSV levels than whether the onions were seeded or transplanted. Viruliferous thrips emigrating from harvested onion fields into nonharvested ones may be increasing the primary spread of IYSV in late-harvested onions. Managing T. tabaci populations before harvest, and manipulating the spatial arrangement of fields based on harvest date could mitigate the spread of IYSV.