Iris yellow spot virus
(iris yellow spot)

Pictures

Picture	Title	Caption	Copyright
Title Symptoms Caption Iris yellow spot virus (IYSV); symptoms on onion, showing extensive leaf lesions on heavily affected plant. USA. Copyright ©Whitney Cranshaw/Colorado State University/Bugwood.org - CC BY 3.0 US	Symptoms	Iris yellow spot virus (IYSV); symptoms on onion, showing extensive leaf lesions on heavily affected plant. USA.	©Whitney Cranshaw/Colorado State University/Bugwood.org - CC BY 3.0 US
Title Symptoms Caption Iris yellow spot virus (IYSV); close-up of symptoms on field onion leaf. USA. Copyright ©Howard F. Schwartz/Colorado State University/Bugwood.org - CC BY 3.0 US	Symptoms	Iris yellow spot virus (IYSV); close-up of symptoms on field onion leaf. USA.	©Howard F. Schwartz/Colorado State University/Bugwood.org - CC BY 3.0 US

Identity

Preferred Scientific Name

• Iris yellow spot virus Cortês et al. 1998

Preferred Common Name

• iris yellow spot

International Common Names

• English: Lisianthus leaf necrosis; straw bleaching on onion
• Portuguese: sepaca

English acronym

• IYSV

EPPO code

• IYSV00 (Iris yellow spot ?tospovirus)

Summary of Invasiveness

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).

Taxonomic Tree

• Domain: Virus
•     Unknown: "Positive sense ssRNA viruses"
•         Unknown: "RNA viruses"
•             Order: Mononegavirales
•                 Family: Bunyaviridae
•                     Genus: Tospovirus
•                         Species: Iris yellow spot virus

Notes on Taxonomy and Nomenclature

Iris yellow spot virus (IYSV) is a tospovirus which is closely related to two other serious viruses: Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV). Tospoviruses belong to a genus of enveloped viruses within the family Bunyaviridae. They are the only group of plant-infecting viruses in this family.

Description

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.

Distribution

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.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

There is no evidence of the history of introduction as such, but there are reports of the spread of the disease in several areas of the world. Gent et al. (2004) reported a rapid expansion of iris yellow spot in onion in Colorado, USA, with an increase from 6 to 73% of the surveyed fields being infected. Molecular studies point to multiple introductions of IYSV into the western USA (Gent et al., 2006).

Risk of Introduction

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.

Hosts/Species Affected

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.

Host Plants and Other Plants Affected

Symptoms

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).

List of Symptoms/Signs

Biology and Ecology

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).

Means of Movement and Dispersal

IYSV is only vectored by thrips, Thrips tabaci, so movement and dispersal is linked to both the movement of infected plants and the dispersal of the vector. The virus also perpetuates itself and overwinters in weed species in or near protected crops.

Frankliniella fusca can also transmit IYSV, but at alower efficiency than T. tabaci (Srinivasan et al., 2012).

Seedborne Aspects

IYSV is not seedborne.

Pathway Causes

Pathway Vectors

Plant Trade

Vectors and Intermediate Hosts

Economic Impact

Tospoviruses are agriculturally important because they cause severe economic damage to various vegetable and ornamental crops and are transmitted by thrips in a circulative and propagative manner. 

The economic impact of IYSV can be important in onion crops. The loss of an entire crop has been reported in Brazil (Pozzer et al., 1999), Israel (Kritzman et al., 2001), some states in the USA (e.g., Oregon, Idaho and Texas) (Mohan and Moyer, 2004; Crowe and Pappu, 2005; Miller et al., 2006), Spain (Córdoba-Sellés et al., 2005) and the Netherlands (Mavric and Ravnikar, 2001). The incidence of IYSV in onion often reaches up to 60% in Israel (Kritzman et al., 2001), and in Slovenia over 90% of an onion crop was infected by the virus but there was no record of yield loss (Mavric and Ravnikar, 2001). In Spain, the impact on onion production was considered potentially devastating by Córdoba-Sellés et al. (2005). In the Netherlands 50-90% of iris plants became infected with IYSV. The projected economic impact of IYSV in the western USA could reach 60-90 million dollars (for 10-15% yield loss), in addition to environmental and economic costs due to additional pesticide sprays for thrips control estimated at 7.5-12.5 million dollars (for three to five sprays on 48,500 hectares/year) (Gent et al., 2006).

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Highly adaptable to different environments
• Has propagules that can remain viable for more than one year

• Highly likely to be transported internationally illegally
• Difficult to identify/detect as a commodity contaminant
• Difficult/costly to control

Uses List

Human food and beverage

• Root crop
• Vegetable

Ornamental

• Cut flower

Diagnosis

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).

Detection and Inspection

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.

Similarities to Other Species/Conditions

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). 

Prevention and Control

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).

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Contributors

11/08/10 Original text by:

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

Distribution Maps