IYSV is a tospovirus, similar to the type species of the genus, Tomato spotted wilt virus (TWSV). The virus particles of IYSV are protein-enveloped RNAs and consist of three genomic RNA segments: Large (L), Medium (M) and Small (S). The entire genome codes for six essential proteins via five different open reading frames. The L RNA is negative-sense coding for a polymerase, the M RNA codes for two glycoproteins (GN and GC) and a non-structural protein (NSm), and S RNAs are ambisense and code for the nucleocapsid (N) and the non-structural (NSs) proteins (Pappu et al., 2008). The three RNAs are tightly linked with the N protein to form ribonucleoproteins (RNPs). The RNPs are encased within a lipid envelope (Pappu et al., 2009). Serological divergence exists among tospoviruses, and little cross reaction among antisera is observed (Pozzer et al., 1999). PCR based detection is possible and is used for diagnostics.
Where IYSV infection is suspected, samples should be sent to a diagnostic laboratory for ELISA and PCR testing. The distribution of IYSV within an infected plant is uneven and samples should be taken in close proximity to the lesion (Gent et al., 2006;Pappu et al., 2008).
There is evidence to suggest that iris yellow spot (or a disease causing similar symptoms) may also be caused by Tomato spotted wilt virus (TSWV) or co-infection of TSWV and IYSV (Gent et al., 2006). Mullis et al. (2004) showed that a small proportion of onion plants displaying iris yellow spot-like symptoms were infected with both TSWV and IYSV. This is not surprising as thrips can transmit both viruses (and many others). This phenomenon has not been reported elsewhere and co-infection (TWSV and IYSV) on onion remains speculative.
Related invasive species
- Iris yellow spot virus
Related Farm Practice
- Pests
- Damage
- Light
- Feeding
- Host plants
- Incidence
- Breeding
- Control
- Hosts
- Development
In 1981, de Avila et al. (1981) described a disease characterized by chlorotic and necrotic, eye-like or diamond-shaped lesions on onion scapes (referred to as ‘sapeca’) in southern Brazil. In 1989, Hall et al. (1993) observed a very similar disease in onion in the USA and detected a tospovirus, which was later shown by Moyer et al. (1993) to be Iris yellow spot virus on the basis of molecular and serological data. In 1998, a new tospovirus was isolated and characterized in the Netherlands from infected iris and leek and named Iris yellow spot virus (IYSV) (Cortês et al., 1998). This virus was subsequently found naturally infecting onion in several major onion-producing states of the USA and around the world (for reviews, see Gent et al., 2006 and Pappu et al., 2009). Gera et al. (1998b) reported that IYSV was responsible for a ‘straw bleaching’ disease on onion in Israel. In 1999, a ‘sapeca’ isolate from Brazil was identified as IYSV on the basis of biological, serological and molecular data (Pozzer et al., 1999). In Israel, Kritzman et al. (2000) reported natural IYSV infection of lisianthus grown in the field. IYSV has now been endemic in south-western Idaho and eastern Oregon in onion, leek and chive seed production fields for over 10 years. Losses caused by IYSV can reach 100% in onion crops, for example, in Brazil (Pappu et al., 2009). However, studies in the Netherlands in 2008 showed that latent infections of IYSV were common in onion crops but did not cause economic damage (NPPO of the Netherlands, 2008).
Symptoms of IYSV consist of eyespot to diamond-shaped, yellow, light-green or straw-coloured lesions (sometimes necrotic) on the leaves, scape and bulb leaves of onion and other Allium host species. In the early stages of infection, lesions appear as oval, concentric rings. Some green islands can be observed within the necrotic lesions. They usually originate around a thrips feeding point. Infected leaves eventually fall over at the point of infection during the latter part of the growing season. Infection at early stages of crop growth results in yield losses. Infection at later stages of development can still cause significant losses due to reduced quality: severely infected fields will senescence prematurely and entire areas will turn brown before they collapse. Symptom severity is dependent on host cultivar, timing of infection, overall health of the host at the time of infection, and environmental conditions (Gent et al., 2004). Du Toit (2005) reported that out of 46 onion cultivars tested, all but 3 had a significant yield decrease and reduced bulb size. The incidence of symptomatic plants generally increases after bulb formation (Gent et al., 2006). IYSV does not always kill its host(s);however, the virus reduces plant vigour, disturbs photosynthesis and reduces bulb size. IYSV infection weakens the plants making them more susceptible to other diseases and pests. IYSV-infected onions grown for seed have reduced seed yield and quality (Evans and Frank, 2009;Pappu et al., 2009).
IYSV has a relatively restricted host range. Edible A llium crops including onion (bulb and seed crops), garlic, chive, shallots, leeks and some cut flower/potted ornamental species including Alstroemeria, chrysanthemum, iris and lisianthus are the most economically important crops affected by IYSV. Wild Allium species and ornamental alliums are also potentially at risk. A range of weed species (Datura stramonium, Nicotiana spp. and Amaranthus retroflexus) can also act as reservoirs.
Six species have been mechanically inoculated in experimental host range trials (Chenopodium amaranticolor, C. quinoa, Datura stramonium, Nicotiana benthamiana, N. rustica and Gomphrena globosa). There is no evidence that these species are infected in the wild. Ben Moussa et al. (2005) reported infection of another three members of the Solanaceae (capsicums, potatoes and tomatoes) but it is unclear if these are natural hosts or were artificially inoculated.