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Methods and Criteria for Assessing the Transmission of Plant Viruses by Longidorid Nematodes

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Methods and Criteria for Assessing the Transmission of Plant Viruses by Longidorid Nematodes Tribune METHODS AND CRITERIA FOR ASSESSING THE TRANSMISSION OF PLANT VIRUSES BY LONGIDORID NEMATODE’S David L. TRUDGILL*, Derek J.F. BROWN* andDavid G. “NAMARA** * Scottish Crop Research Institute, Invergowrie, Dundee, Scotland and ** East Malling Research Station, Maidstone, Kent, England. Since Hewitt,Raski and Goheen (1958) first & Cadman, 1959). Similarlytransmission of rasp- showed that Xiphinema indexis a vector of grapevine berryringspot virus English strain (HRV-E ; fanleaf virus more than forty plant viruslnematode specific field vector Longidorus rnacrosorna; Harrison, vectorcombinations have been reported. Many of 1962 ; Debrot,1964) has been reported for seven these reports have not been confirmed, but among species of longidoridnematodes (Tab. 1). If al1 thosethat have beensubstantiated a pattern of these reports of transmission are true then it would specificity between the viruses and their longidorid seem that nematode species other than those with nematodevectors isapparent. Harrison, Mowat which these viruses are specificallyassociated with andTaylor (1961) observed thatthe degree. of in thefield can also act as vectors. In this conneciion, similaritybetween the different viruses seemed to Taylorand Robertson (1969)found unattached parallel the degree of systematic relationship between particles of AMV in the buccal capsuleof L. eloizqatus theirnematode vectors. This relatedness of speci- which suggested that some transmission may &,ult ficity maybe partly due to virus particles with from non-specific retention of virus. particular protein coat properties (Harrison, 1964 ; McNamara(1978) offered anotherexplanation Harrison et al., 1974)becoming attachedto the for apparent non-specific transmissions. He suggested lining of the feeding apparatusat specific sites that,inlaboratory experiments, contamination withintheir vectors. These are the inner surface of the outside of bait plant root systems may Occur of the odontophore and oesophagus in vector species either from virus coming from bodies of nematodes of Xiphinerna (Taylor & Robertson, 1970 ; McGuire, entangled in, or in nematode faeces adhering to the Kim & Douthit, 1970 ; Raslri,Maggenti & Jones, outside of, the roots. He reached these conclusions 1973) and the inner surface of the odontostyle and afterattempting tosubstantiate the claim by between the odontostyleand the guiding sheath Valdez (1972) that RRV-E could be transmitted by in Longidorus spp.(Taylor & Robertson,1969 X. diversicaudatum. McNamara(1978) found that, & 1973 ; Taylor, Robertson & Roca, 1976). Further although he could recover this virus from the roots evidence for the “narrowness” of specificity between of bait plants exposed to X. diversicaudatum, none virusesand their nematode vectors is providedin of theseplants was systemically infected, al1 the severalreports of nematodepopulations differing RRV-Edetected apparently coming from external in their ability to transmit isolates of a virus (Dal- contamination of the roots.In contrast, when he masso,Munck-Cardin & Legin,1972 ; VanHoof, used L. macrosomu, the natural vector of RRV-E, 1966 ; Brown & Taylor, 1981). In contrast to the manybait plants eventually became systemically above,the results of somelaboratory experiments infected. McNamara (1978) concluded that evidence haveindicated nematode transmission of viruses fornematode transmission “can only be fully ac- contrary to the pattern of specificityproposed by ceptable if virus is translocatedfrom the roots of Harrison(1964). For example, six species of longi- thebait plant after transmission and infection is doridnematode have been reported as vectors of shown to be present in the leaves, hypocotyl, or in arabis mosaic virus (AMV ; specific field vector X. otherregions to whichnematodes have not had diversicaudatum ; Jha & Posnette, 1959 ; Harrison direct access”. Revue Némalol. 6 (1) : 133-141 (1983) 133 D.L. Trudgill, D.J.F. Brown & D.G. McNamara Table 1 Longidorus, Paralongidorus and Xiphinema species reported as vectors for arabis mosaic virus (AMV) and the English strain of raspberry ringspot virus (RRV-E) V irus Field Virus vector vectors Other AMV X. diversicaudatum A'. coxi (Fritzsche, 1964) X. bakeri (Iwaki & Komuro, 1974) X. index (Fritzsche & Thiele, 1979) L. caespiticola (Valdez, 1972) P. maximus ' (McElroy et al. 1976) RRV-E L. macrosoma L. elongatus (Taylor & Murant, 1960) L. profundorum (Fritzsche & Kegler, 1968) L. caespiticola (Valdez,1972) L. leptocephalus (Valdez, 1972) X. diversicaudatum (Fritsche & Kegler, 1968) P. maximus (McElroy et al. 1976) More than two-thirds of the reports of longidorid 1) Thevirus and nematode must be fully and nematodestransmitting virus fail to satisfy this correctly identified. requirement. Ako, many of thesereports give 2) Bait plant tissues must be shown to be infected inadequatedescriptions of themethods used or with the virus under test. describetests that did not have adequate controls 3) Thenematode under test must be shown to or in which thenematode and/or virus used were be the only possible vector in that experiment. not adequately identified. In this paperwe consider the criteria bywhich the In the followingsection each of these criteria is results of a virus transmission test with longidorid considered in more detail. nematodesshould be judged and describe atest procedurewhich, modified wherenecessary to suit the nematode/virus combination being t*ested, should IDENTIFICATIONOF THE VIRUS AND THE NEMATODE give results which satisfy those criteria. Our experi- ence has been limited to European species of virus Onlyone nematode species shouldbe tested at vector nematodes but we believe that the procedure any one time, unless controls with other nematodes wedescribe can also give information about the are required. The virus to be transmitted should be emciency withwhich a virus is transmitted and, characterised by serology and,where possible, its withinthe limits of thenumbers of nematodes relationship wit.h other viruses be established. Any tested, useful evidence of the inabilityof a nematode virustransmitted must beidentified serologically to transmit a virus. and,where appropriate, shown to be serologically identical to that in the source plant. The nematodes testedshould be from one population and, except wherethey are from naturally infected field soil, shown to be initially virus-free by bait-testing with Thecriteria €or evaluatingthe results, of appropriate contra1 plants. Details of the source of transmission tests nematodesshould be given, as it isknown that populations of a species may differ in their ability totransmit strains of a virus,and those trans- To demonstrate that a particular virus is trans- mittingvirus must beidentified andpermanent mittedby agiven species of longidoridnematode mountskept. Where necessary they should be weconsider thatthe following criteriamust be compared with paratypes or similar specimens and satisfied : any deviations from these noted. 134 Revue Nimatol. 6 (1) : 133-141 (1983) Assessing transmission of plant viruses EVIDENCETHAT THE BAIT PLANT IS INFECTED WITH exposed to viruliferous X. americanum S. la€. (l) and VIRUS found thatin those plant species to whichvirus was transmittedthe frequency of infectionvaried This evidence is best provided by demonstrating greatly.Similarly Trudgill and Brown (1979) and thatthe bait plant is systemically infected: This Trudgill,Brown and Robertson (1981) foundthat is mostreadily accomplished by recovering virus L. macrosoma transmittedRRV-E to Petunia from thehypocotyl or aerialparts but, in some hybrida and Fragariaananassa but not to Cheno- instances, production of leaf symptoms may sufice. podiumquinoa whereas X. index couldacquire If testing of the root system is unavoidable, infection grapevine fanleaf virus from, but not transmit it to must be unequivocallydemonstrated. In al1 tests Gomphrena globosa. infection of either tlle bait or assay plants due to contamination or byalternative vectors must be excluded. UNSUITABLEVIRUS ISOLATE OR STRAIN THENEMATODE MUST BE SHOWN TO BE THE VECTOR Differences in ability to transmit virus have been shown to occur within a nematode species, different In preliminarytests inhibition of transmission populationstransmitting different isolates of the bynematicides or byair-drying the soil may be samevirus (Van Hoof, 1966 ; Dalmasso,Munck- used to indicate that a nematode maybe the vector. Cardin & Legin, 1972 ; Brown & Taylor, 1981). Indefinitive tests nematodes must be extracted Viruses also differ in their ability to become system- afterexposure to the virus infected sourceplants ically distributed in source plants and in manually and individually transferred to the roots of the bait inoculated plants and may therefore Vary in their plantswhich should be grown in containers and a availability to feeding nematodes. mediumwhich have previously been partially sterilized to ensure freedom from potential vectors. The sievings from which the nematodes have been UNFAVOURABLEENVIRONMENT removed must be tested for alternative vectors and appropriatecontrols should be used to check for An unfavourable distribution of soil particle sizes, possible transmission by stray arthropods or wind- or toohigh, too low, or rapidlyfluctuating soil borne fungal spores. Where possible, these organisms moistures or temperatures may be unsuitable to the should also berigorously excluded. The control nematodesand prevent their moving and feeding. plants should be numerous, and be an integral part Toxins,pathogens, rough handling
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