The grapevine (Vitis spp.) undoubtedly represents one of the woody crops most widely grown in temperate climates, and a highly valuable agricultural commodity. As most of the vegetatively propagated crops, grapevines are exposed to the attacks of a variety of pests and pathogens among which infectious intracellular agents (viruses, viroids, phloem‑ and xylem‑limited prokaryotes) play a major role, causing heavy losses, shortening the productive life of vineyars, and endangering the survival itself of affected vines. The importance of the grapevine industry and the magnitude of the problems caused by these pathogens has generated wide interest which, in turn, has fostered intensive research which has been especially active at the international scale from the late 1950’s onwards.

Directory of virus and virus-like diseases of the grapevine and their agents

The term virulence has a conflicting history among plant pathologists. Here we define virulence as the degree of damage caused to a host by parasite infection, assumed to be negatively correlated with host fitness, and pathogenicity the qualitative capacity of a parasite to infect and cause disease on a host. Selection may act on both virulence and pathogenicity, and their change in parasite populations can drive parasite evolution and host-parasite co-evolution. Extensive theoretical analyses of the factors that shape the evolution of pathogenicity and virulence have been reported in last three decades. Experimental work has not followed the path of theoretical analyses. Plant pathologists have shown greater interest in pathogenicity than in virulence, and our understanding of the molecular basis of pathogenicity has increased enormously. However, little is known regarding the molecular basis of virulence. It has been proposed that the mechanisms of recognition of parasites by hosts will have consequences for the evolution of pathogenicity, but much experimental work is still needed to test these hypotheses. Much theoretical work has been based on evidence from cellular plant pathogens. We review here the current experimental and observational evidence on which to test theoretical hypotheses or conjectures. We compare evidence from viruses and cellular pathogens, mostly fungi and oomycetes, which differ widely in genomic complexity and in parasitism. Data on the evolution of pathogenicity and virulence from viruses and fungi show important differences, and their comparison is necessary to establish the generality of hypotheses on pathogenicity and virulence evolution.

https://bsppjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1364-3703.2007.00460.x

The evolution of virulence and pathogenicity in plant pathogen populations

Genetic resistance often fails because a resistance-breaking (RB) pathogen genotype increases in frequency. On the basis of an analysis of cellular plant pathogens, it was recently proposed that the evolutionary potential of a pathogen is a major determinant of the durability of resistance. We test this hypothesis for plant viruses, which differ substantially from cellular pathogens in the nature, size, and expression of their genomes. Our analysis was based on 29 plant virus species that provide a good representation of the genetic and biological diversity of plant viruses. These 29 viruses were involved in 35 pathosystems, and 50 resistance factors deployed against them were analyzed. Resistance was found to be durable more often than not, in contrast with resistance to cellular plant pathogens. In a third of the analyzed pathosystems RB strains have not been reported, and in another third RB strains have been reported but have not become prevalent in the virus population. The evolutionary pote...

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2003.93.8.941

An Analysis of the Durability of Resistance to Plant Viruses

SUMMARY Grapevine fanleaf virus (GFLV) is responsible for fanleaf degeneration, which is one of the most severe virus diseases of grapevines worldwide. GFLV causes substantial crop losses, reduces fruit quality and shortens the longevity of grapevines in the vineyard. GFLV is transmitted specifically from grapevine to grapevine by the ectoparasitic nematode Xiphinema index, and belongs to the genus Nepovirus in the family Comoviridae. Since the discovery of the nematode vector in the late 1950’s and the identification of GFLV as the agent responsible for fanleaf degeneration in the early 1960’s, a wealth of information has been accumulated on its transmission, biological properties and serological characteristics, as well as on the structure and expression of the GFLV genome. Although dissemination of the virus through propagation material has been drastically reduced over the past two decades by implementing rigorous certification schemes and establishing quarantine facilities, effective strategies are still needed to control GFLV in naturally infected vineyards. Recently, significant progress has been made on the elucidation of the function(s) of most GFLV proteins, in particular those involved in critical steps of the virus multiplication cycle, including RNA replication, cell-to-cell movement, and transmission by X. index. New insights have also been gained into the population structure and genomic variability among isolates from naturally infected vineyards, which have opened new avenues for designing alternative strategies to control this destructive virus. This review article offers a comprehensive overview of the most significant advances made over the past 15 years on GFLV and discusses novel control strategies for one of the major threats to the grapevine industry worldwide.

Grapevine fanleaf virus: still a major threat to the grapevine industry

The efficiency of the different methods used to collect and extract virus-vector nematodes (Longidorus, Paratrichodorus, Trichodorus and Xiphinema) were examined as part of a collaborative assessment by staff at the principal Nematology laboratories in Scotland, of methods for field sampling, handling and excracting these nematodes. Vehicular transportation of soil samples from field to laboratory did not reduce the numbers of nematodes recovered but several short drops did, by up to 39%, largely by soil compaction. The two-flask method was found to be much slower and was less effective than the decanting and sieving method for recovering longidorid nematodes: When using the decanting and sieving method a 200 g soil sample stirred into suspension in a 5 I pail, given a 15 s settling time and the supernatant poured through a sieve with a 150 /-1m aperture mesh held over an empty 5 I pail was found to be satisfactory. This collects the large longidorid nematodes. The residue on the sieve is washed onto a 95 /-1m aperture mesh plastic sieve which is then placed in a water-filled Baermann funnel. Smaller trichodorid nematodes and juvenile longidorids were collected by passing the supernatant collected in the second pail through individual 75 /-1m and 53 /-1m aperture mesh sieves. The residue on both sieves is washed onto a 2-ply tissue supported in a plastic sieve which is placed in a second water filled Baermann funnel. Most longidorids had fallen to the bottom of the funnel after 24 h, but if required for biological studies were collected after 4 to 6 h. Trichodorids took longer and were recovered after 48 h or, 24 h if required for biological studies. Water temperature in the funnels should be maintained at 15 to 20°C.

/pdf/an-examination-of-methods-used-to-extract-virus-vector-4506y2y0ym.pdf

An examination of methods used to extract virus-vector nematodes (nematoda: longidoridae and trichodoridae) from soil samples

SUMMARY

Electron microscopy of thin sections of Xiphinema diversicaudatum and X. index fed on plants infected respectively with arabis mosaic and grapevine fanleaf viruses showed that the viruses are retained as a monolayer of particles adsorbed on to the cuticle lining the lumina of the odontophore (stylet extension), anterior oesophagus and oesophageal bulb. During the moult of the nematode the cuticular lining is shed and together with the detached virus particles is ingested into the intestine through the oesophago-intestinal valve; this supports the limited experimental evidence that viruses transmitted by X. diversicaudatum and X. index are not retained through the moult.

Sites of virus retention in the alimentary tract of the nematode vectors, Xiphinema diversicaudatum (Micol.) and X. index (Thorne and Allen)

SUMMARY

Electron microscopy of thin sections of Longidorus elongatus (de Man) fed on plants infected with raspberry ringspot and tomato black ring viruses showed virus-like particles in the lumen of the buccal capsule and in the space between the stylet and the guiding sheath. In sections of L. elongatus fed on plants infected with arabis mosaic virus, which it does not normally transmit, a few virus particles were seen in the buccal capsule but none was associated with the stylet guiding sheath. It is suggested that the association of virus particles with the cuticular guiding sheath is an explanation of the specificity of virus transmission evident in L. elongatus.

The location of raspberry ringspot and tomato black ring viruses in the nematode vector, Longidorus elongatus (de Man)

SUMMARY
The distribution of Xiphinema and Longidorus species in the British Isles and Ireland were mapped by means of a computer programme from 5527 records. Longidorus caespiticola, L. elongatus, L. goodeyi and L. leptocephalus were found to be widespread among a wide range of vegetation types, particularly in grassland and arable crops; L. macrosoma and L. profundorum were recorded frequently in southern and central England, and L. attenuatus in East Anglia; L. vineacola was recorded once only in Eire. Paralongidorus maximus was recorded from two localities in eastern Scotland and one in southern England, on each occasion from intensively cultivated ground. Four species of Xiphinema were recorded but only X. diversicaudatum is widespread although apparently with a northerly limit in central Scotland; X. diversicaudatum was equally prevalent in arable crops, grassland and deciduous woodland; X. coxi and X. vuittenezi were recorded from a few scattered localities in England, and the few records for X. mediterraneum were all from Kent, south east England. All species were found in a wide range of soil types but mostly in light soils with a large sand fraction. Many of the species occurred together in mixed populations; L. caespiticola was most frequently associated with X. diversicaudatum and with L. leptocephalus and L. elongatus. X. diversicaudatum also was commonly found with L. goodeyi. Viruses were transmitted in laboratory bait tests in only eighteen of 325 soil samples containing X. diversicaudatum and four of 265 containing L. elongatus.

The geographical distribution of Xiphinema and Longidorus nematodes in the British Isles and Ireland.

SUMMARY
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.

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

SUMMARY

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.

C. E. Taylor

Papers

Sites of virus retention in the alimentary tract of the nematode vectors, Xiphinema diversicaudatum (Micol.) and X. index (Thorne and Allen)

The location of raspberry ringspot and tomato black ring viruses in the nematode vector, Longidorus elongatus (de Man)

The geographical distribution of Xiphinema and Longidorus nematodes in the British Isles and Ireland.

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

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