P. L. Guy

A review of the plant virus, viroid and mollicute records for New Zealand

Considering New Zealand’s small size and remoteness, a remarkable number of plant pathogens have been recorded. A complete review of the records of plant viruses, viroids and mollicutes in New Zealand found good documentary evidence for 186 viral diseases, 5 viroids and 6 mollicutes, of which 59 viruses, 3 viroids, and 4 mollicutes have been recorded since publication of the previous complete review in 1989. Based on a lack of sufficient documented evidence, a further 27 viruses, 5 viroid and 3 mollicute records were considered unconfirmed. The confirmed pathogens not only infect agricultural and horticultural crops but some also affect the native flora.

Introduced Plant Viruses and the Invasion of a Native Grass Flora

Weed and native grasses from the South Island of New Zealand were surveyed for virus infection. Cocksfoot mottle virus (CfMV) and Ryegrass mosaic virus (RgMV) were restricted to a few introduced species; however, Barley yellow dwarf viruses (BYDVs) have invaded native grasses in New Zealand. Virus incidence was significantly lower in the native species (2%) than in the introduced species (12%). Four different serotypes (RMV, RPV, PAV, MAV) were detected in the introduced grass flora but only two (RMV, PAV) were detected in native species. In experimental transmission tests the aphid vector Rhopalosiphum padis survival was variable on the 20 native species tested but this was not due to the presence or absence of endophytic fungi as none were detected in the New Zealand species. Aphid numbers increased and plants were killed when R. padi fed on Agrostis muelleriana and Festuca multinodis. R. padi transmitted a PAV isolate to these and six other native species. BYDVs infected 4/5 of the subfamilies tested. Virus incidence in native Arundinoideae and Pooideae was significantly lower than in introduced Pooideae and Panicoideae. One species of Bambusoideae collected from the field was not infected but was found susceptible in glasshouse tests. Agrostis capillaris, Dactylis glomerata and Lolium perenne were identified as the most likely reservoirs of infection for the native flora. Anthoxanthum odoratum was not infected but if the SGV serotype and its vector Schizaphis graminum were ever introduced, A. odoratum could form an effective reservoir from near sea level into alpine areas.

First record of ryegrass mosaic virus and its mite vector Abacarus hystrix (Nal.) in New Zealand

Abstract Ryegrass mosaic potyvirus was identified, using immune serum electron microscopy, at one site near Dunedin, New Zealand. A mite infesting the infected Lolium multiflorum Lam. plants was identified as Abacarus hystrix (Nal.), the vector of RyMV. A commercially available monoclonal antibody which detects a group-specific epitope on aphid-transmitted potyvirus capsids was used successfully to detect RyMV in immunoblots using polyvinylidene difluoride (but not nitrocellulose) membranes.

Metarhizium spore surface antigens are correlated with pathogenicity

An antiserum produced to intact spores of an isolate of Metarhizium anisopliae (DAT F-001), which is pathogenic to Adoryphorus couloni, was used to determine the ELISA-index (DAS-ELISA) of 49 isolates of M. anisopliae. The ELISA-index gave a measure of the level of spore surface antigens present in spore populations of each isolate. The virulence of the 49 isolates was determined by bioassay and ranged from non-pathogenic to highly virulent. The virulence of the isolates was correlated with the ELISA-index. We suggest that the spore surface antigens of M. anisopliae are the primary determinant of host-specificity.

Distribution of ryegrass mosaic virus in New Zealand perennial ryegrass pastures as determined by ELISA and RT‐PCR

Abstract Ryegrass mosaic virus (RgMV) is a mite‐transmitted member of the Potyviridae that can seriously affect yield and persistence of ryegrass in pastures. To determine the incidence and distribution of RgMV in New Zealand, perennial ryegrass pastures were surveyed by indirect ELISA. RgMV was detected in all seven mature (≥4 years) North Island pastures and 60% of 20 mature South Island pastures, indicating that RgMV is established throughout New Zealand. A reverse transcriptase‐polymerase chain reaction (RT‐PCR) assay was developed to confirm the identity of RgMV. This assay was 200 times more sensitive than the indirect ELISA. A possible correlation between numbers of the eriophyid mites and the incidence of RgMV was observed in South Island pastures. Our results suggest that mite transmission may be more significant than ryegrass cultivar, pasture age, endophyte infection, or mechanical transmission in defining the incidence and distribution of RgMV. Options for the control of RgMV in New Zealand pa...

Incidence and spread of viruses in white-clover pastures of the South Island, New Zealand

Sixty-two white-clover pastures were tested by enzyme linked immunosorbent assay (ELISA) for five viruses (White clover mosaic virus (WClMV), Alfalfa mosaic virus (AMV), Red clover necrotic mosaic virus (RCNMV), Soybean dwarf virus (SbDV), beet western yellows virus (BWYV)) and also for members of the potyvirus group. Only one pasture was found to be virus-free. WClMV andAMVoccurred, often at high incidences, on farms in every region. RCNMV and SbDV occurred in more than half the pastures whereas BWYV and potyvirus infection was less common. There was a general reduction in virus diversity with increasing latitude, with the most northern farms having greater species richness (all 6 viruses present) than those in Southland (1–3 viruses present). There was a significant relationship between the presence of AMV, BWYV, RCNMV, SbDV and irrigation. Dairy farming also had a positive relationship with BWYV, RCNMV and SbDV. WClMV increase was monitored in six pastures and incidence was observed to increase geometrically in young pastures. The relatively high incidence of RCNMV is in contrast to the findings of previous studies on white-clover pastures and indicates that assessing this virus’s effects on white clover is of high priority.

Influence of white clover mosaic potexvirus infection on the endogenous levels of jasmonic acid and related compounds in Phaseolus vulgaris L. seedlings

Summary Primary leaves of 16-day-old Phaseolus vulgaris L. seedlings were inoculated with either white clover mosaic potexvirus (WClMV) or water. The concentrations of jasmonic acid JA), dihydrojasmonic acid (DHJA) and cucurbic acid (CA), and virus titre were measured over a ten day period following inoculation. A transient increase in the concentration of JA occurred immediately following inoculation with water or WClMV, which we attribute to wounding. A second increase in JA occurred only in virus-infected leaves. Concentrations of cucurbic and dihydrojasmonic acids were not affected by wounding, but virus-infection resulted in a decline in the concentration of CA and an increase in DHJA. Reasons for the changes in jasmonate concentrations after virus infection are discussed.

Distribution and diversity of New Zealand isolates of ryegrass mosaic virus

Summary. Sequences from the coat protein cistron of five ryegrass mosaic virus (RgMV) isolates indicated the presence of two distinct strains in New Zealand. The nucleotide differences between the strains, and their distribution, suggested that both strains were introduced recently, either as a mixed infection or as two independent introductions. The relationship between these New Zealand strains and other strains and isolates of RgMV, and their potential severity is discussed.

First record of Geminiviruses in New Zealand: Abutilon mosaic virus and Honeysuckle yellow vein virus

Two begomoviruses, Abutilon mosaic virus (AbMV) and Honeysuckle yellow vein virus (HYVV), are reported from New Zealand for the first time. AbMV was detected in variegated Abutilon spp. using a begomovirus-specific ELISA and a PCR designed to amplify a core coat protein fragment (≈575 bp) of begomoviruses. HYW was not detected using these assays but a 118 bp fragment was amplified from Lonicera japonica, showing yellow vein symptoms, using primers specific to the HYW coat protein gene.

Cocksfoot mottle virus spreads to the South Island of New Zealand

In previous surveys of New Zealand cocksfoot (Dactylis glomerata L.) pastures, Cocksfoot mottle virus (CoMV) was detected at only one site near Taupo on the North Island. However, we have recently found CoMV at two sites near Dunedin on the South Island. Isolates from these sites, associated with mild and severe symptoms in cocksfoot (CoMV-Wa and CoMV-Ok, respectively), were partially characterised. In agarose gels, the virions of the two isolates had slightly different mobilities when subjected to an electric field, indicating a difference in net surface charge. However, both isolates produced similar leaf mottle symptoms in wheat, their capsid proteins had similar molecular weights (Mr = ca. 31 kDa) as determined by SDS-polyacrylamide gel electrophoresis and they were serologically indistinguishable in double-diffusion tests using 17 antisera to 10 sobemoviruses. CoMV-Ok and CoMV-Wa were serologically related to Lucerne transient streak virus but not to eight other sobemoviruses. Further surveys using DAS-ELISA showed variable incidences of CoMV (range 0–80%) at nine South Island sites. The beetle vectors of CoMV have not been introduced into New Zealand. At two sites where CoMV was found, we observed severe feeding damage from armyworms (Persectania aversa Walker). However, in growth-cabinet experiments this species failed to transmit the virus. Incidences were significantly higher at mown sites (39%) than at unmown sites (11%), suggesting that mechanical transmission is responsible for spread. In view of the increased use of cocksfoot on New Zealand farms, the impact of CoMV on cocksfoot productivity now needs to be assessed and current levels of resistance to CoMV in commercial cultivars need to be evaluated.