Iris yellow spot virus
(iris yellow spot)

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).

Iris yellow spot represents an immediate and serious threat to sustainable and productive onion cropping systems around the world, and the recent detection of this disease in numerous onion-producing countries demonstrates that the disease is spreading rapidly in a range of environments.

IYSV is on the EPPO Alert list (http://www.eppo.org/QUARANTINE/Alert_List/alert_list.htm).

As the symptoms of IYSV are now well described and rapid diagnostic protocols (both ELISA and RT-PCR) are available, it is likely that IYSV will be reported in many other parts of the world in the years to come.

Iris yellow spot represents an immediate and serious threat to sustainable and productive onion cropping systems around the world, and the recent detection of this disease in numerous onion-producing countries demonstrates that the disease is spreading rapidly in a range of environments.

IYSV has a relatively restricted host range. Edible Allium 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.

IYSV is a vectored virus, so two organisms are involved in the initiation and spread of the disease. This datasheet focuses on the virus. Tospoviruses are usually transmitted by a large number of thrips species; however, IYSV is only transmitted by onion thrips (Thrips tabaci). IYSV is transmitted by both larvae and adult thrips, but only the larvae can acquire the virus from infected plants. Virus transmission is persistent and once a thrips has acquired the virus, it can transmit it for the remainder of its lifetime. IYSV is likely to overwinter from one season to the next in volunteer onions or weeds found among or around crops. Emerging thrips spread the virus from infected to healthy hosts whilst feeding. The disease has the potential to spread rapidly in fields with large numbers of viruliferous thrips. The distribution of infected plants in the field is associated with feeding activity by the vector. In many cases, the damage is first noticed at the field edges, in areas of stressed plants, or in locations with thin plant stands. The virus is not seedborne nor does it survive in the soil (Gent et al., 2006; Pappu et al., 2009). 

Physiology and Phenology

Most tospoviruses are systemic in most of their hosts; however, IYSV tends to remain localized (Smith et al., 2006). The virus is distributed unevenly in the infected host: the highest titer is usually detected in the younger (inner) leaves at the centre of the plant. The virus might not be present in all leaves and is often detected only within 30-50 mm of visible lesions in onion plants (Kritzman et al., 2001; Gent et al., 2006). IYSV does not appear to be seedborne or seed transmitted in onion (Kritzman et al., 2001) but it has been shown to accumulate in some onion bulbs. Robène-Soustrade et al. (2006) reported that 27% of onion bulbs were infected in onion bulb- and seed-production fields in Reunion Island suggesting that there is the potential for spread of IYSV by the distribution of infected or culled bulbs. 

Associations

Tospoviruses are transmitted by several species of thrips in a circulative and propagative manner (Pappu et al., 2009). However, IYSV is only transmitted by one species, the onion thrips (Thrips tabaci). IYSV is thought to be acquired by the larvae of T. tabaci, with transmission occurring through second larval instars and adults only after circulation and replication in the vector (Gent et al., 2006). Studies in Israel have demonstrated a positive relationship between the incidence of T. tabaci in onion crops and the incidence of plants infected with IYSV (Kritzman et al., 2001).

IYSV is not seedborne.

Human food and beverage

• Root crop
• Vegetable

Ornamental

• Cut flower

Serological diagnostic techniques used for the detection of tospoviruses are based on ELISA using specific polyclonal antiserum against the nucleocapsid. ELISA testing is an established method in the routine diagnosis of plant viruses, including tospoviruses, but it can cross-react within the same serogroup due to common antigenic determinants on the nucleocapsid (Uga and Tsuda, 2005). PCR using a common primer pair to amplify conserved regions of the family Potyviridae has been successfully used for the detection of several species. Unfortunately, it did not discriminate between individual species (Gibbs and Mackenzie, 1997). Uga and Tsuda described a one-step RT-PCR that can simultaneously detect and identify multiple tospoviruses. For further information, see Pappu et al. (2008) and Smith et al. (2006). There is no known cross reactivity with INSV and TSWV (Pappu et al., 2008).

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.

The symptoms of IYSV can be confused with those of some other tospoviruses such as Tomato spotted wilt virus. IYSV symptoms can also be confused with those caused by thrips infestations, hailstorms, herbicide phytotoxicity, or early infections caused by various fungal diseases such as Cladosporium leaf spot.

The necrotic areas caused by IYSV infection can be colonized by secondary invaders such as Stemphylium spp. or Alternaria spp., leading to inaccurate diagnosis (Pappu et al., 2008). 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Control measures for tospoviruses need to be deployed within integrated pest management strategies which include phytosanitary, cultural, host-plant resistance, chemical and biological measures. Volunteer onions plants and weeds acting as reservoirs should be destroyed (either by tillage or herbicide) and weeds should be actively managed around cropping systems and within fields. Crop rotation should be implemented to reduce the build-up of thrips populations and transplants free of IYSV and thrips should be used. Studies have shown that thrips are less attracted to green hues than blue hues; green-leaved onion cultivars are less attractive to thrips and therefore less likely to be infected with IYSV (Kirk, 1997). 

Eradication

Eradication is possible if IYSV is detected in a glasshouse situation. However, an outbreak in field crops may be more difficult to eradicate, especially if there is an abundance of alternative hosts (including weeds) and a large outbreak of T. tabaci.

Cultural Control and Sanitary Measures

Volunteer onion plants, neighbouring infected weeds and contaminated transplants are the primary inoculum sources and provide an important early-season source of inoculum to initiate outbreaks in neighbouring onion crops. Measures should be put in place to kill volunteers and control weeds and only virus-free transplants should be used. As the vector (Thrips tabaci) has a relatively limited host range, rotations of host with non-host crops and spatial isolation of host crops limit spread of the virus within a region. It is recommended that onion bulb and seed crops should be isolated geographically (Gent et al., 2006). 

Movement Control

Movement of infected onion transplants facilitates the spread of new strains of IYSV and biotypes of T. tabaci within and among regions of onion production. Only IYSV-free onion transplants should be used. 

Chemical Control

The management of thrips is essential. The insecticidal management of T. tabaci as an indirect means of controlling iris yellow spot has been an area of study in recent years (Hammon, 2004; Cranshaw, 2006). For a list of chemical options, see Gent et al. (2006). 

Host-plant Resistance

The potential value of Systemic Acquired Resistance (SAR) compounds for control of iris yellow spot was demonstrated by Gent et al. (2004). The team demonstrated that the application of SAR including chemicals (acibenzolar-S-methyl) lead to a 34% reduction in the incidence of plants with symptoms of iris yellow spot, compared with non-treated controls in onion crops. 

Differences in the susceptibility of onion cultivars to iris yellow spot have been reported (Gent et al., 2004; du Toit and Pelter, 2005). Breeding for resistance to/tolerance of thrips damage and IYSV is a candidate for effective disease management. 

IPM

An effective integrated management programme has been developed for another tospovirus which causes important losses in tomato crops. Momol et al. (2004) developed a successful IPM strategy to control Tomato spotted wilt virus by combing UV-reflective mulch, induction of SAR with acibenzolar-S-methyl, and applications of ‘soft’ insecticides (to preserve the natural enemies of thrips). Similar strategies could be used in the control of IYSV in onion crops (Gent et al., 2006).

11/08/10 Original text by:

CRCNPB Australia, CRC for National Plant Biosecurity, Canberra, Australia