Australian Cool-Season Pulse Seed-Borne Virus Research: 1. Alfalfa and Cucumber Mosaic Viruses and Less Important Viruses

Here, we review the research undertaken since the 1950s in Australia's grain cropping regions on seed-borne virus diseases of cool-season pulses caused by alfalfa mosaic virus (AMV) and cucumber mosaic virus (CMV). We present brief background information about the continent's pulse industry, virus epidemiology, management principles and future threats to virus disease management. We then take a historical approach towards all past investigations with these two seed-borne pulse viruses in the principal cool-season pulse crops grown: chickpea, faba bean, field pea, lentil, narrow-leafed lupin and white lupin. With each pathosystem, the main focus is on its biology, epidemiology and management, placing particular emphasis on describing field and glasshouse experimentation that enabled the development of effective phytosanitary, cultural and host resistance control strategies. Past Australian cool-season pulse investigations with AMV and CMV in the less commonly grown species (vetches, narbon bean, fenugreek, yellow and pearl lupin, grass pea and other Lathyrus species) and those with the five less important seed-borne pulse viruses also present (broad bean stain virus, broad bean true mosaic virus, broad bean wilt virus, cowpea mild mottle virus and peanut mottle virus) are also summarized. The need for future research is emphasized, and recommendations are made regarding what is required.

Co-infection patterns and geographic distribution of a complex pathosystem targeted by pathogen-resistant plants

Increasingly, pathogen-resistant (PR) plants are being developed to reduce the agricultural impacts of disease. However PR plants also have the potential to result in increased invasiveness of nontarget host populations and so pose a potential threat to nontarget ecosystems. In this paper we use a new framework to investigate geographical variation in the potential risk associated with unintended release of genetically modified alfalfa mosaic virus (AMV)-resistant Trifolium repens (white clover) into nontarget host populations containing AMV, clover yellow vein virus (ClYVV), and white clover mosaic virus (WCIMV) in southeastern Australia. Surveys of 213 sites in 37 habitat types over a 300 000-km2 study region showed that T. repens is a significant weed of many high-conservation-value habitats in southeastern Australia and that AMV, ClYVV, and WClMV occur in 15-97% of nontarget host populations. However, T. repens abundance varied with site disturbance, habitat conservation value, and proximity to cropping, and all viral pathogens had distinct geographic distributions and infection patterns. Virus species frequently co-infected host plants and displayed nonindependent distributions within host populations, although co-infection patterns varied across the study region. Our results clearly illustrate the complexity of conducting environmental risk assessments that involve geographically widespread, invasive pasture species and demonstrate the general need for targeted, habitat- and pathosystem-specific studies prior to the process of tiered risk assessment.

Further studies on Pea seed-borne mosaic virus in cool-season crop legumes: responses to infection and seed quality defects

Field and glasshouse experiments (3 of each) were done during 2003–06 to determine the responses of a range of genotypes belonging to 13 species of cool-season crop legumes to infection with Pea seed-borne mosaic virus (PSbMV). Seed quality defects were determined and genotypes of some species were also tested for seed transmission of the virus. In field experiments, of 39 genotypes of field pea (Pisum sativum) evaluated, 15 were ranked as highly susceptible, 10 susceptible, 9 moderately resistant, and 5 resistant, while all 7 lupin species (Lupinus spp.) tested were resistant. In glasshouse sap and graft inoculations with PSbMV to genotypes not found infected in the field and 2 additional lupin species, no virus was detected in any of the 9 lupin species or in 5 field pea genotypes tested. Thus, the lupins all appeared to be non-hosts and the 5 field pea genotypes had resistance to the 2 PSbMV isolates used to inoculate them. All 14 genotypes of faba bean (Vicia faba) evaluated in the field were ranked highly susceptible, while 12 out of 16 lentil (Lens culinaris) genotypes were ranked as highly susceptible and 4 as susceptible. Chickpea (Cicer arietinum) genotypes were moderately resistant (50) or susceptible (7). Once infected, plant sensitivities (symptom severities) ranged from low in some field pea and most lentil genotypes to high in most faba bean genotypes. Chickpea genotypes all were ranked as moderately sensitive. Seed lots harvested from PSbMV-infected plants of field pea, faba bean, and chickpea all showed severe seed quality defects, but lentil was usually less affected. The predominant seed symptoms were necrotic rings and line markings on the seed coat, malformation, reduced size, and splitting. Kabuli chickpea types also showed darkening of the seed coat. Seed transmission of PSbMV was detected in faba bean (0.2%) and field pea (5–30%). When PSbMV infection foci were introduced into plots of lentil cv. Nugget, the virus spread to the lentil plants and decreased shoot dry weight by 23%, seed yield by 96%, and individual seed weight by 58%. Seed transmission of PSbMV (6%) was detected in seed from the infected lentil plants. In a survey for possible viral seed symptoms, all seed lots of kabuli chickpea (5) and field pea (70), and 10 of 18 of faba bean were affected, but none of the 23 of lentil. When seedlings from 16 faba bean and 7 field pea seed lots were tested for 3 viruses, neither Broad bean stain virus nor Broad bean true mosaic virus was detected, but PSbMV was found in 5 field pea seed lots at incidences of <1–14%. PSbMV was detected in commercial field pea seed stocks of cvv. Kaspa (33) and Parafield (12) at incidences of 0.5–47% and 0.3–30%, respectively. The implications of these findings in terms of genotype susceptiblility and sensitivity to PSbMV infection and their importance for the management of PSbMV in legume crops are discussed.

Occurrence of virus infection in seed stocks and 3-year-old pastures of lucerne (Medicago sativa)

In tests on seed samples from 26 commercial seed stocks of lucerne (Medicago sativa) to be sown in south-western Australia in 2001, infection with Alfalfa mosaic virus (AMV) was found in 21 and Cucumber mosaic virus (CMV) in 3 of them. Bean yellow mosaic virus (BYMV) and Pea seed-borne mosaic virus (PSbMV) were not detected in any. Incidences of infection within individual affected seed samples were 0.1–4% (AMV) and 0.1–0.3% (CMV), and the infected seed stocks were from 3 (CMV) and at least 11 (AMV) different lucerne cultivars. In a survey of 31 three-year-old lucerne pastures in the same region in 2001, in randomly collected samples, AMV was found in 30 and luteovirus infection in 11 pastures. Pastures in high, medium, and low rainfall zones were all infected. Incidences of AMV within individual infected pastures were high, with 50–98% of plants infected in 20 of them and only 3 having <10% infection, but luteovirus incidences were only 1–5%. In addition to various cultivar mixtures, at least 8 (AMV) and 3 (luteoviruses) different individual lucerne cultivars were infected. When the species of luteovirus present were identified, they were Bean leaf roll virus, Beet western yellows virus ( = Turnip yellows virus), or Subterranean clover red leaf virus ( = Soybean dwarf virus). CMV and legume-infecting potyviruses (BYMV, PSbMV, and Clover yellow vein virus) were not detected in any of the lucerne samples. Acyrthosiphon kondoi infestation was common in the samples collected, and A. pisum and Aphis craccivora were also found. Widespread infection in lucerne stands, and their frequent colonisation by aphid vectors, are cause for concern not only because of virus-induced production losses in lucerne itself but also because they provide virus infection reservoirs for spread to nearby grain legume crops and annual legume pastures.