D. M. Persley

A new tospovirus serogroup IV species infecting capsicum and tomato in Queensland, Australia.

A previously undescribed tospovirus infecting tomato, capsicum and chilli in Queensland was characterised. The virus reacted with antibodies to serogroup IV tospoviruses, and the complete nucleotide sequence of its nucleocapsid protein gene indicated that it was a new tospovirus species. Sequence identities at the nucleotide and amino acid levels were <85% with recognised members of serogroup IV The virus infected test plants in five families and, importantly, overcame the Sw-5 resistance gene in tomato and TSWV resistance in Capsicum chinense accessions PI 152225, PI 159236 and AVRDC 00943. The name Capsicum chlorosis virus (CaCV) is proposed for this virus.

Tospoviruses—an Australian perspective

The detection, distribution, molecular and biological properties, vector relations and control of tospoviruses present in Australia, including Tomato spotted wilt virus (TSWV), Capsicum chlorosis virus (CaCV) and Iris yellow spot virus (IYSV), are reviewed. TSWV occurs throughout Australia where it has caused serious sporadic epidemics since itwas first described in the 1920s. The frequency and distribution of outbreaks has increased in the 1990s, with the arrival and dispersal of the western flower thrips (Frankliniella occidentalis) being one factor favouring this situation. The crops most frequently and severely affected are capsicum, lettuce, tomato, potato and several species of ornamentals. Minimal differences were found between the nucleocapsid (N) gene amino acid sequences of Australian isolates and these were most closely related to a clade of northern European isolates. CaCV was first detected in Australia in 1999 and is most closely related to Watermelon silver mottle virus, a serogroup IV tospovirus. The natural hosts include capsicum, tomato, peanut and Hoya spp. The virus also occurs in Thailand and Taiwan. IYSV was first found in Australia in 2003, infecting onion and leek, with the distribution in three States suggesting that the virus has been present for some time.

Distribution in Australia and Seed Transmission of Tobacco streak virus in Parthenium hysterophorus

Tobacco streak virus (TSV) was found to commonly occur in Parthenium hysterophorus, as symptomless infections, in central Queensland, Australia across a large area infested with this weed. Several isolates of TSV collected across the geographic range of P. hysterophorus were found to share identical coat protein sequence with each other and with TSV from crop plants in the same area. Seed transmission of TSV in P. hysterophorus was found to occur at rates of 6.8 to 48%. There was almost no change in the rate of TSV seed transmission when P. hysterophorus seed was stored for up to 24½ months. Implications of this relationship between TSV and P. hysterophorus for the development of virus disease epidemics in surrounding crops are discussed.

Iris yellow spot virus found infecting onions in three Australian states

Iris yellow spot virus (IYSV) was detected for the first time in Australia, infecting onions in three and leeks in one state. Identification was confirmed using sap transmission to Nicotiana benthamiana, two IYSV-specific antisera in ELISA, RT-PCR with IYSV-specific primers, and sequence comparison with published IYSV sequences. Spring onion, onion seed and onion bulb crops were all infected, with spring onion being the most severely affected. The virus was also detected in nursery-grown onion and leek seedlings.

An outbreak of Potato spindle tuber viroid in tomato is linked to imported seed

In 2011, an outbreak of the quarantine-regulated pathogen Potato spindle tuber viroid (PSTVd) occurred in a commercial glasshouse-grown tomato crop in Queensland, Australia. Phylogenetic studies showed that the genotype of this isolate grouped in a cluster of PSTVd genotypes from tomato and Physalis peruviana, and exhibited an interesting mutation (U257→A) that has previously been linked to lethal symptom expression in tomato. Transmission studies showed that the viroid could be mechanically transmitted from crushed fruit sap, but not from undamaged fruits. A low rate of asymptomatic infection was determined for plants in the affected glasshouse, demonstrating the efficacy of using symptoms to detect PSTVd infections in tomato. No PSTVd infections were detected in solanaceous weeds located outside of the infected glasshouse, excluding them from playing a role in the viroid epidemiology. Monitoring and subsequent testing of new tomato crops grown in the facility demonstrated successful eradication of the pathogen. A trace-back analysis linked the outbreak of PSTVd to an infected imported tomato seed-lot, indicating that PSTVd is transmitted internationally through contaminated seed.

Tomato yellow leaf curl virus in Australia: distribution, detection and discovery of naturally occurring defective DNA molecules

Tomato yellow leaf curl virus (TYLCV) was detected for the first time in Australia in March 2006 in field-grown tomatoes in Brisbane, Queensland. Surveys showed that the virus was confined to south-east Queensland. Virus transmission studies carried out using Bemisia tabaci (B biotype) verified that resistant tomato lines containing the Ty-1 or Ty-5 genes displayed tolerance to infection by TYLCV isolates from Australia. A PCR assay specific for TYLCV was designed and optimised to confirm the presence of the virus in samples that tested positive in begomovirus-specific double-antibody sandwich enzyme-linked immunosorbent assay. Eight isolates of TYLCV from various sites were cloned and sequenced, and were shown to have near-identical sequences and a high nucleotide sequence similarity (98%) to the monopartite Tomato yellow leaf curl virus-Israel (TYLCV-IL). No DNA-B, DNA-1 nor DNA-b satellite molecules were detected using degenerate PCR assays. Phylogenetic analysis revealed that Australian isolates of TYLCV separated into two sequence groups, TYLCV-IL[Au:Bri:06] and TYLCV-IL[Au:Bun:06], that showed a defined geographic segregation. Naturally occurring defective DNA molecules containing partial, rearranged segments of the native DNA-A, were present in one isolate. To our knowledge, this is the first report of an incursion of a begomovirus into Australia, and the first report of the characterisation of naturally occurring defective DNAs of TYLCV. Additional keywords: diagnostics, Geminiviridae, rolling-circle amplification, virus host range.

First report of Tomato torrado virus on tomato from Australia.

Since 2005, a disease of greenhouse tomatoes has been observed in the North Adelaide Plains of South Australia. Symptoms include chlorosis and necrotic lesions on the leaves, stunted plants, and leaves that are frequently distorted. Necrotic lesions typically had a light green or yellow margin and affected areas often fell out, leaving small holes. The onset of disease was generally associated with an increase in population of the greenhouse whitefly (Trialeurodes vaporariorum). Isometric virions (25 to 30 nm) were observed by electron microscopy in partially purified extracts, but no other virion types were observed. In 2008, reverse transcription (RT)-PCR products were obtained using primers specific for RNA-1 (TR1F/TR1R) and RNA-2 (TR2F/TR2R) of Tomato torrado virus (ToTV) (1) using a fresh diseased sample (isolate 2136) and a 2005 archived, lyophilized sample (isolate 1883). Cucumber mosaic virus was not detected in these samples by RT-PCR (data not shown). The RT-PCR products were cloned and sequenced. The partial 892-bp RNA-1 consensus nucleotide (nt) sequences of isolates 1883 (GenBank Accession No. GQ844869) and 2136 (GenBank No. GQ844871) were 100% identical. The partial 573-bp RNA-2 consensus nt sequences of isolates 1883 (GenBank No. GQ844870) and 2143 (GenBank No. GQ844872) were 99.8% identical. The partial RNA-1 and -2 nt sequences of isolate 1883 were 99.1% and 98.5% identical to the published ToTV sequences (RNA-1, GenBank No. DQ388879; RNA-2, GenBank No. DQ88880), respectively (2). For isolate 2136, the identities were 98.1 and 98.7%, respectively. The putative amino acid sequences were all 100% identical to the published sequences. The virus was graft transmitted to tomato cv. Grosse Lisse and Datura stramonium, in which typical disease symptoms and leaf chlorosis, respectively, were induced, and by mechanical inoculation to Nicotiana benthamiana, which displayed leaf chlorosis. All inoculated plant species indexed positive by RT-PCR for ToTV. Seeds were collected from known ToTV-infected plants and the seedlings were grown in isolation in an insect-proof glasshouse. None of the seedlings exhibited symptoms of virus infection, and ToTV was not detected in 97 seedlings from cv. Beatrice, 368 seedlings from cv. Ediez, or from 286 seedlings from cv. Loretto with the virus-specific RT-PCR assays. The pathway for entry of ToTV into Australia is unknown, and although seed transmission was suspected, no evidence for this could be found. To our knowledge, this represents the first report of ToTV from Australia. References: (1) H. Pospieszny et al. Plant Dis. 91:1364, 2007. (2) M. Verbeek et al. Arch. Virol. 152:881, 2007.

IXth International Symposium on Thysanoptera and Tospoviruses, 31st August-4th September 2009, Sea World Resort, Queensland, Australia.

This proceedings contains abstracts of 108 papers focusing on the different Tospovirus diseases of various crops and their thysanopteran vectors. The genetics of these pests and pathogens, the different methods used in their control and their geographical distribution are also highlighted.

Plant hosts of the phytoplasmas and rickettsia-like-organisms associated with strawberry lethal yellows and green petal diseases

Candidatus Phytoplasma australiense (Ca. P. australiense) is associated with the plant diseases strawberry lethal yellows (SLY), strawberry green petal (SGP), papaya dieback (PDB), Australian grapevine yellows (AGY) and Phormium yellow leaf (PYL; New Zealand). Strawberry lethal yellows disease is also associated with a rickettsia-like-organism (RLO) or infrequently with the tomato big bud (TBB) phytoplasma, the latter being associated with a wide range of plant diseases throughout Australia. In contrast, the RLO has been identified only in association with SLY disease, and Ca. P. australiense has been detected only in a limited number of plant host species. The aim of this study was to identify plant hosts that are possible reservoirs of Ca. P. australiense and the SLY RLO. Thirty-one plant species from south-east Queensland were observed with disease between 2001 and 2003 and, of these, 18 species tested positive using phytoplasma-specific primers. The RLO was detected in diseased Jacksonia scoparia and Modiola caroliniana samples collected at Stanthorpe. The TBB phytoplasma was detected in 16 different plant species and Ca. P. australiense Australian grapevine yellows strain was detected in six species. The TBB phytoplasma was detected in plants collected at Nambour, Stanthorpe, Warwick and Brisbane. Ca. P. australiense was detected in plants collected at Nambour, Stanthorpe, Gatton and Allora. All four phytoplasmas were detected in diseased Gomphocarpus physocarpus plants collected at Toowoomba, Allora, Nambour and Gatton. These results indicated that the vector(s) of Ca. P. australiense are distributed throughout south-east Queensland and the diversity of phytoplasmas detected in G. physocarpus suggests it is a feeding source for phytoplasma insect vectors or it has a broad susceptibility to a range of phytoplasmas.

Rickettsia-like-organisms and phytoplasmas associated with diseases in Australian strawberries

Strawberry lethal yellows (SLY) disease in Australia is associated with the phytoplasmas Candidatus Phytoplasma australiense and tomato big bud, and a rickettsia-like-organism (RLO). Ca. P. australiense is also associated with strawberry green petal (SGP) disease. This study investigated the strength of the association of the different agents with SLY disease. We also documented the location of SLY or SGP plants, and measured whether they were RLO or phytoplasma positive. Symptomatic strawberry plants collected from south-east Queensland (Australia) between January 2000 and October 2002 were screened by PCR for both phytoplasmas and the RLO. Two previously unreported disease symptoms termed severe fruit distortion (SFD) and strawberry leaves from fruit (SLF) were observed during this study but there was no clear association between these symptoms and phytoplasmas or the RLO. Only two SGP diseased plants were observed and collected, compared with 363 plants with SLY disease symptoms. Of the 363 SLY samples, 117 tested positive for the RLO, 67 tested positive for Ca. P. australiense AGY strain and 11 plants tested positive for Ca. P. australiense PYL variant strain. On runner production farms at Stanthorpe, Queensland the RLO was detected in SLY diseased plants more frequently than for the phytoplasmas. On fruit production farms on the Sunshine Coast, Queensland, Ca. P. australiense was detected in SLY disease plants more frequently than the RLO.