Transmission of strains of raspberry ringspot and tomato black ring viruses by Longidorus elongatus (de Man)

Scottish isolates of raspberry ringspot (RRV) and tomato black ring viruses (TBRV) showing slight serological differences were associated in the field with the nematode Longidorus elongatus and were all transmitted equally efficiently by this species in laboratory experiments. Forms of both viruses from southern England and Germany that are distantly serologically related to the Scottish isolates were also transmitted experimentally by L. elongatus, although in the field they are associated with other Longidorus species. L. elongatus transmitted English isolates of RRV almost as efficiently as the Scottish isolates but it transmitted English and German isolates of TBRV only occasionally. Four isolates each of TBRV and RRV were detected by inoculating plants with extracts of nematodes; the results paralleled those of the vector studies.     

Seed-transmission in the ecology of nematode-borne viruses

Virus-free populations of vector nematodes can acquire tomato black ring (TBRV), raspberry ringspot (RRV) and arabis mosaic (AMV) viruses from weed seedlings grown from virus-carrying seed. When soils from fields where nematode-borne viruses occurred naturally were air-dried to kill vector nematodes and then moistened, TBRV and RRV occurred commonly in the weed seedlings that grew, but AMV occurred only rarely. Similar tests did not detect tobacco ringspot, grapevine fanleaf or tobacco rattle viruses in weed seeds in the single soil studied in each instance, although these three viruses are also seed-borne in some of their hosts. Many weed species, when infected experimentally, readily transmit TBRV and RRV to their seed, but the viruses were much commoner in naturally occurring seed of some of these species than of others. These discrepancies between the frequency of seed-transmission of viruses from experimentally infected plants and the extent of natural occurrence of infected seed seem largely to reflect the host preferences of the vectors. Infective Longidorus elongatus kept in fallow soil retained TBRV and RRV only up to 9 weeks. When weed seeds in the soil were then allowed to germinate, the nematodes reacquired virus from the infected seedlings. Some weed species were better than others as sources of virus. Persistence of these viruses in fields through periods of fallow or fasting of the vector therefore depends on a continuing supply of infected seedlings produced by virus-containing weed seeds. This is probably less true of viruses like AMV and grapevine fanleaf, which persist for 8 months or more in their vectors (Xiphinema spp.). A few seeds containing TBRV and RRV were found in soils free of vector nematodes, suggesting that the viruses are disseminated in weed seed. This probably explains how TBRV and RRV have reached a large proportion of L. elongatus populations in eastern Scotland.     

Nematode transmissibility of pseudo-recombinant isolates of tomato black ring virus

Pseudo-recombinant isolates of tomato black ring virus (TBRV), containing RNA-i of the potato bouquet serotype and RNA-2 of the beet ring-spot serotype, were transmitted by the nematode Longidorus elongatus, which also transmits the beet ringspot serotype but not the potato bouquet serotype. Transmissibility by L. elongatus was correlated with antigenic specificity of the virus particles, providing further evidence that nematode transmissibility depends on the structure of the virus coat protein. The distribution of genetic determinants for biological properties between the RNA-1 and RNA-2 of TBRV resembles that for raspberry ringspot virus.     

The association of Xiphinema diversicaudatum (Micoletsky) with strawberry latent ringspot and arabis mosaic viruses in a raspberry plantation

An outbreak of strawberry latent ringspot virus (SLRV) in a plantation of Mailing Jewel raspberry coincided with the greatest abundance of the nematode vector, Xiphinema diversicaudatum. Arabis mosaic virus (AMV) was not detected in the crop but was, together with SLRV, in many weed species present. AMV was transmitted through the seed of Poa annua, Capsella bursa-pastoris and Senecio vulgaris and SLRV through the seed of Mentha arvensis. X. diversicaudatum were more numerous within the rows than between them and vertical sampling showed that most occurred between 4 and 12 in depth in both locations. Monthly sampling showed that egg laying occurred from April to July; populations increased to a peak in late autumn but declined during the winter, resulting in about a twofold annual increase in numbers. Females, males and juveniles transmitted AMV and SLRV to cucumber seedlings, and in the absence of plants the nematode retained AMV for 112 days and SLRV for 84 days.     

A comparison of the effectiveness of the four British virus vector species of Longidorus and Xiphinema

The frequency with which the four virus-vector species of longidoroid nematodes occurring in Britain transmitted their associated plant viruses were compared in a series of experiments using a standard procedure. In these tests Xiphinema diversicaudatum proved an effective vector of British isolates of arabis mosaic virus and strawberry latent ringspot virus and Longidorus attenuatus of an isolate of tomato black ring virus from England. In comparison, isolates of raspberry ringspot virus and tomato blackring virus from Scotland and of raspberry ringspot virus from England were transmitted much less readily by their respective vectors, L. elongatus and L. macrosoma. These differences in ability to transmit virus were not related to differences in feeding access on the virus source- or bait-plants, in the extent to which virus was retained within the nematode feeding apparatus or in the frequency with which virus was recovered from Longidorus in concurrent slash tests. Three Scottish isolates of raspberry ringspot and tomato black ring viruses were transmitted equally infrequently by two populations of L. elongatus and the frequency with which virus was transmitted was not greatly increased when the species of source or bait plants was changed.     

Ingestion,Retention, and Transmission of two Strains of Raspberry Ring-spot Virus by Longidorus macrosoma

The transmission of two strains of raspberry ringspot virus (RRV) by small numbers of nematodes was compared. A strain of RRV from Scotland (RRV-S), originally found in the field associated with Longidorus elongatus, was transmitted frequently by L. elongatus but only once by L. macrosoma. A strain from England (RRV-E) associated with L. macrosoma in the field was transmitted infrequently by each species of nematode. The reasons why L. macrosoma infected only a small proportion of bait plants with virus were investigated, and it was found that most of the nematodes tested had fed on the source plants and many had ingested virus. Most nematodes exposed to RRV-E or RRV-S had fed on the roots of the bait plants and, when thin sections were examined by electron microscope, had retained particles (thought to be those of the virus) in the region of the anterior odontostyle, Thus, most nematodes seem to have had ample opportunity to transmit virus, and the low frequency of transmission may have been due to a failure of the virus particles to be released from the site of retention or to a lack of infectivity of the virus when L. macrosoma was the vector and Petunia hybrida was the host.     

Populations of Longidorus elongatus (Nematoda: Longidoridae) in two New Zealand soils under grazed pasture

Abstract

Information on the “needle nematode” Longidorus elongatus in New Zealand pastures has hitherto been limited. Monthly sampling of Pukepuke black sand and Manawatu fine sandy loam yielded L. elongatus populations up to 87 500 m^?2 (February) and 21 600 m^?2 (August). First stage juveniles appeared when soil temperature exceeded 15°C; females dominated populations every month and males were rare. Differences in abundance between soils may reflect the pore space available to these relatively large nematodes. Specimens survived 24 weeks storage at 5 and 15°C. While abundance tended to decline with depth, at 30–40 cm depth in Pukepuke sand, numbers increased, perhaps through the impact of groundwater levels on rooting patterns. In Pukepuke sand, plant species were associated with significantly different populations of L. elongatus, with Trifolium repens and T. subter‐raneum supporting more than Lolium perenne. More L. elongatus were found in grazed pasture with lower plant available P.     

Effects of Pratylenchus penetrans on development of strawberry wilt caused by Verticillium dahliae

The nematode Pratylenchus penetrans or the closely related P. fallax occurred in three out of four strawberry plantations infested with Verticillium dahliae surveyed in Kent. When plants of cv. Cambridge Vigour were inoculated with P. penetrans and V. dahliae together in field ‘micro-plots’ the nematode increased the rate of wilt development except when the concentration of V. dahliae microsclerotia in the soil was very low. In a pot experiment, under conditions not conducive to symptom expression, the incidence of infection was increased by nematodes in Cambridge Favourite, Cambridge Vigour and Redgauntlet; the incidence was least in Red-gauntlet, the most resistant cultivar. The nematode appeared to cause local changes in the root cortex which aided hyphal penetration of the adjacent tissues. Growth of strawberry plants in the absence of V. dahliae was not significantly affected even by large populations of the nematode, but growth was reduced by V. dahliae at all rates of inoculation in the field. Infection by Verticillium did not affect the total numbers of nematode per plant at the end of the experiments, although the number per g fresh weight was greater on the small amount of white root on severely diseased plants than on the larger amount on healthy or mildly diseased plants.     

Seed-transmission of nematode-borne viruses

Transmission through seed of crop and weed plants seems to be characteristic of nematode-borne viruses. It occurred with tomato black ring virus (TBRV) in nineteen species (thirteen botanical families), with arabis mosaic virus (AMV) in thirteen species (eleven families), with raspberry ringspot virus (RRV) in six species (five families), and also, in more limited tests, with tomato ringspot, cherry leaf roll and tobacco rattle viruses. A remarkable feature was that infected seedlings, except those containing tobacco rattle virus, often appeared healthy. The occurrence and extent of seed-transmission depended on both the virus and the host plant. In many progenies more than 10%, and in some 100%, of seedlings were infected. The viruses were transmitted through at least two or three generations of seed of those host species tested. After 6 years' storage, TBRV- and RRV-containing seed of Capsella bursa-pastoris and Stellaria media germinated to give infected seedlings. In controlled crossing experiments with strawberry and raspberry, virus was transmitted to seed from both male and female parents but, at least in raspberry, the presence of competing virus-free pollen much decreased the ability of pollen from infected plants to set seed. There was no evidence that healthy mother plants became infected when their flowers were pollinated with infected pollen.     

Infectibility of some new raspberry cultivars with arabismosaic and raspberry ringspot viruses and further evidence for variation in British isolates of these two nepoviruses

In field trials at sites of an outbreak of arabis mosaic nepovirus (AMV) in England and of raspberry ringspot nepovirus (RRV) in Scotland, the results of exposure of some new raspberry cultivars to natural infection with these viruses showed discrepancies from those obtained in graft inoculation tests using AMV-Lib and RRV-S, the Scottish type isolates. In particular, cv. Glen Prosen, which is immune to AMV-Lib and RRV-S, was infected with AMV and RRV in the field trials. Studies on these and other field isolates of AMV and RRV showed that they differed from the type isolates in Rubus host range and in symptomatology in herbaceous hosts. However, whereas four isolates of RRV found infecting Rubus were distinguishable by spur formation in gel double-diffusion serological tests, six AMV isolates were indistinguishable by this method. Immunoelectrophoresis of virus particles did not distinguish the six AMV isolates, but isolates RRV-MX and RRV-T were distinguishable from RRV-S and the English type isolate, RRV-E. Like the two RRV type isolates, RRV-MX contained a single electrophoretic component, but it migrated must faster whereas RRV-T contained two components, one with a migration rate similar to that of RRV-MX and the other similar to that of the type isolates. Polyacrylamide gel electrophoresis of protein preparations from highly purified virus particles of RRV isolates E, S and MX detected a single polypeptide of estimated mol. wt 54 × 10³, 54 × 10³ and 50 × 10³ respectively but that of isolate T contained two polypeptides of estimated mol. wt 54 × 10³ and 50 × 10³. These data suggest that RRV-T is a mixture of two isolates. In laboratory tests the nematode Xiphinema diversicaudatum transmitted four isolates of AMV efficiently whereas two populations of the nematode Longidorus elongatus were less efficient vectors of four RRV isolates. Neither vector species transmitted virus to any of nine raspberry cultivars. The results are discussed in relation to the control of nepoviruses in raspberry and to the biology of these viruses.     

Lead phytoextraction from printed circuit computer boards by Lolium perenne L. and Medicago sativa L.

This work assessed the ability of Lolium perenne and Medicago sativa for extracting lead (Pb) from particulate printed circuit computer boards (PCB) mixed in sand with the following concentrations: 0.5, 1.0 and 1.5 g of PCB, and including a control treatment without PCB. The PCB were obtained from computers, and grinded in two particle sizes: 0.0594 mm (PCB1) and 0.0706 mm (PCB2). The PCB particle sizes at their corresponding concentrations were applied to L. perenne and M. sativa by using three experimental assays. In assay II, PCB2 affected the biomass production for both plants. For assay III, the PCB1 increased the biomass of M. sativa (236.5%) and L. perenne (142.2%) when applying either 0.5 or 1.0 g, respectively. In regards to phytoextraction, assay I showed the highest Pb-extraction by roots of L. perenne (4.7%) when exposed to 1.5 g of PCB1. At assay I, L. perenne showed a Pb-bioconcentration factor higher than 1.0 when growing at 0.5 g of PCB1, and when HNO₃ was used as digestion solution; moreover, in assay III both plants showed a Pb-translocation factor higher than 1.0. Therefore, Lolium perenne and Medicago sativa are able to recover Pb from electronic wastes (PCB).     

The effect of resistance to Amphorophora rubi in raspberry (Rubus idaeus) on the spread of aphid-borne viruses

The effectiveness of resistance to the aphid Amphorophora rubi in restricting the spread of aphid-borne viruses was assessed in a field experiment using six genotypes of red raspberry. In one block of the experiment, the genotypes alternated with rows of virus-infected Mailing Jewel raspberry, and in the other they alternated with virus-free Mailing Jewel. During 4 years, the numbers of A. rubi and the amount of 52V virus spread in the two blocks were similar, suggesting that this virus was mostly introduced from outside the plots. Lloyd George and Mailing Jewel raspberry became heavily infested with A. rubi and were rapidly infected with raspberry leaf mottle, raspberry leaf spot and 52V viruses. Glen Clova and Norfolk Giant raspberry, which contain minor genes for resistance to A. rubi, were infested with fewer A. rubi and virus spread more slowly in these cultivars. A. rubi were rare on Mailing Orion and an East Mailing raspberry selection (888/49) which have genes A₁ and A₁₀ respectively for resistance to A.rubi, and these plants remained largely free of virus. The role of minor and major gene resistance to A. rubi in restricting virus spread is discussed. A few Macrosiphum euphorbiae and Myzus ornatus were recorded on several of the raspberry genotypes.     

In vitro: Régénération de plants de fraisier à partir de fragments de pétioles et de bourgeons floraux

Strawberry plants have been regenerated from petioles and flower buds Petiole segments derived from strawberry plants(Fragaria vesca L. andFragaria xananassa Duch. cvs. Gorella and Redgauntlet) maintainedin vitro have been cultured on the Gambobg’s. agar medium supplemented with 2,4-D (2 mg 1?¹) either alone or with kinetin or BAP (0.1 mg I?¹ or 1 mg I?¹). The calli thus obtained have been transferred to media differing from the first ones only by the nature and the concentration of the growth regulators used. One or two months later, shoots appeared on these calli. The influence of the media hormonal content on plant regeneration has been observed. Moreover, genotypic variations have also been investigated: the wild strawberry exhibited the greatest organogenic capacity. Shoots have also been obtained from flower buds(Fragaria xananassa Duch. cvs. Fern, Hummi Gento, Gorella and Redgauntlet). In this case, shoots appeared directly on small calli derived from the peduncular base of the flower buds. These regenerations have been observed, without changes of media, on the Lee and de Fossabd’s agar medium.     

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different resistance genes

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different A. idaei resistance genes were studied under infestation tunnel, glasshouse and laboratory conditions. Each test consistently ranked raspberry cultivars, with increasing levels of resistance in the order, non-resistant (cv. Mailing Jewel), resistant cultivars containing minor genes, major gene A₁, gene A₁₀ (red raspberry) and gene A₁₀ (black raspberry) respectively. Resistance was expressed in three different ways; decreased alatae settling and feeding, decreased apterae settling and decreased aphid fecundity and rate of nymphal development. Following exposure to a large population of alatae, significantly fewer aphids settled on A₁, A₁₀ and to a lesser extent, minor gene-containing cultivars, compared to cv. Mailing Jewel. More alatae settled on the top than the bottom zone of non-resistant, minor gene resistant and two of four A₁-containing cultivars; alatae settling was low on all zones of A₁₀-containing raspberries. Aphid fecundity and nymph development patterns on different cultivars and resistance classes were similar to those found for alatae settling. After 7 days reproduction more than 30% of the nymphs developed to third or fourth instar on cv. Mailing Jewel, whilst on minor gene and major gene-containing cultivars the total number of nymphs and the proportion of later instars decreased. On resistant cultivars the nymphs were found mainly on the middle and bottom leaf zones, compared to cv. Mailing Jewel. A rapid (48 h) screening test using floating leaflets was developed and, on the basis of apterae settling, ranked cultivars reliably; it was particularly effective in distinguishing moderate and strong resistance to strain 1 A. idaei. Gene A₁-containing cultivars bred in England were much more resistant to strain 1 A. idaei than those bred in Scotland. Possible reasons for this difference are discussed, in relation to modified screening procedures and to the control of the viruses transmitted by this aphid vector.     

Microsatellite markers for Fragaria from ‘Strawberry Festival’ expressed sequence tags

We present 37 microsatellite primer pairs developed from a cDNA library of Fragaria ^xananassa Duch. cv. Strawberry Festival. Polymorphism was high and the number of presumptive alleles of 13 expressed sequence tag–simple sequence repeats (EST–SSRs) in 70 strawberry cultivars ranged from five to 32 per primer pairs, averaging 16.1. Cross‐species amplification was also high and ranged from 89% in Fragaria vesca L. to 100% in the progenitor species of octoploid strawberry, Fragaria chiloensis (L.) Duch. and Fragaria virginiana Duch.     

Seed germination of Fragaria vesca L. From atypical ecotopes of West Siberia

Seed formation and seed germination rate were established to be essentially higher in Fragaria vesca L. populations growing in the ecotopes which are atypical for wood strawberry than in the standard ecotopes for the species. Having equal capabilities for cross-pollination and self-pollination, the plants exhibit higher level of xenogamy than autogamy when growing in atypical ecotopes. Xenogamy predomination promotes the gene diversity of the population, thus ultimately providing realization of the ecological plasticity of the species.     

Effects of Didymella applanata and Botrytis cinerea on axillary buds, lateral shoots and yield of red raspberry

The viability of axillary buds and the growth and potential yield of lateral shoots at nodes of red raspberry (Rubus idaeus) infected naturally by Didymella applanata or Botrytis cinerea were measured on excised nodes, decapitated nursery canes or on canes from fruiting plantations. In comparison with lesion-free nodes, buds at infected nodes were smaller and fewer of them were capable of growth when excised and ‘forced’, although the difference in growth between infected and uninfected nodes decreased during late winter. After February, those buds at infected nodes which were capable of forced growth did so as early and with a similar growth rate as those at lesion-free nodes. In April, 70% of buds at infected nodes were capable of growth compared with 94% of those at lesion-free nodes. When nursery canes of cv. Mailing Delight were decapitated above infected nodes the emergence of lateral shoots from the terminal infected node did not differ significantly from that at lesion-free nodes. On a range of farm sites in Scotland the emergence of shoots at infected nodes in the cropping region of canes was significantly poorer than from uninfected nodes but substantially better at infected nodes of cv. Glen Clova than at those of cvs Mailing Jewel and Mailing Orion. It is suggested that cv. Glen Clova is relatively tolerant of spur blight and cane botrytis. The length and potential yield of laterals which developed at infected nodes in the cropping region of canes in these three cultivars did not differ significantly from those at lesion-free nodes. In all tests there was no significant difference in growth at nodes infected by D. applanata and B. cinerea which may indicate a common mechanism for suppression of buds.     

Properties, relationships and transmission of a strain of raspberry ringspot virus infecting raspberry cultivars immune to the common Scottish strain*

Plants of Lloyd George and Seedling M raspberry (Rubus idaeus L.) were found in eastern Scotland infected with raspberry ringspot (RRV), a virus to which these varieties were previously considered immune. Most RRV isolates from affected plants caused milder symptoms in herbaceous test plants than did the type isolates of the common Scottish and English strains. In graft-transmission tests the Lloyd George strain of RRV infected all the raspberry cultivars tested, including those immune to the common Scottish strain. No consistent differences were found between isolates of the two strains in in vitro properties or serological behaviour. Both strains were transmitted in seed of Stellaria media and in soil containing Longidorus elongatus. Possible reasons why the new strain is uncommon in Scotland are discussed.     

Identification of flowering genes in strawberry, a perennial SD plant

Background  

We are studying the regulation of flowering in perennial plants by using diploid wild strawberry (Fragaria vesca L.) as a model. Wild strawberry is a facultative short-day plant with an obligatory short-day requirement at temperatures above 15°C. At lower temperatures, however, flowering induction occurs irrespective of photoperiod. In addition to short-day genotypes, everbearing forms of wild strawberry are known. In 'Baron Solemacher' recessive alleles of an unknown repressor, SEASONAL FLOWERING LOCUS (SFL), are responsible for continuous flowering habit. Although flower induction has a central effect on the cropping potential, the molecular control of flowering in strawberries has not been studied and the genetic flowering pathways are still poorly understood. The comparison of everbearing and short-day genotypes of wild strawberry could facilitate our understanding of fundamental molecular mechanisms regulating perennial growth cycle in plants.     

Association of different kinds of bacilliform particle with vein chlorosis and mosaic diseases of raspberry (Rubus idaeus)

Thin sections of diseased raspberry (Rubus idaeus) were examined by electron microscopy. Plants of the cv. Baumforth's B and of an aphid (Amphorophora rubi)-resistant breeding selection (6820/54), both infected with raspberry vein chlorosis virus (RVCV) but not with other detectable viruses, contained large bacilliform particles c. 430 × 65 nm. Particles occurred in the cytoplasm and perinuclear space of a small proportion of xylem parenchyma cells. They had an inner core c. 25–30 nm in diameter with cross-banding of periodicity 4·5 nm, and were bounded by an outer membrane. They are probably the particles of RVCV. Plants of cv. Mailing Jewel and of a selection (M14) both showing symptoms of raspberry mosaic (veinbanding) disease contained smaller bacilliform particles c. 125 × 30 nm, which occurred singly or in clusters in the cytoplasm of a small proportion of vascular parenchyma cells. It is not known which, if any, of the viruses associated with raspberry mosaic are represented by the particles.