Methods and criteria for assessing the transmission of plant viruses by longidorid nematodes.

Abstract

Since Hewitt, Raski and Goheen (1958) first showed tha t X i p h i n e m a i n d e x is a vector of grapevine fanleaf virus more than forty plant viruslnematode vector combinations have been reported. Many of these reports have not been confirmed, but among those that have been substantiated a pattern of specificity between the viruses and their longidorid nematode vectors is apparent. Harrison, Mowat and Taylor (1961) observed tha t he degree. of similarity between the different viruses seemed to parallel the degree of systematic relationship between their nematode vectors. This relatedness of specificity may be partly due to virus particles with particular protein coat properties (Harrison, 1964 ; Harrison et al., 1974) becoming attached to the lining of the feeding apparatus at specific sites within their vectors. These are the inner surface of the odontophore and oesophagus in vector species of Xiph inerna (Taylor & Robertson, 1970 ; McGuire, Kim & Douthit, 1970 ; Raslri, Maggenti & Jones, 1973) and the inner surface of the odontostyle and between the odontostyle and the guiding sheath in Longidorus spp. (Taylor & Robertson, 1969 & 1973 ; Taylor, Robertson & Roca, 1976). Further evidence for the “narrowness” of specificity between viruses and their nematode vectors is provided in several reports of nematode populations differing in their ability to transmit isolates of a virus (Dalmasso, Munck-Cardin & Legin, 1972 ; Van Hoof, 1966 ; Brown & Taylor, 1981). In contrast to the above, the results of some laboratory experiments have indicated nematode transmission of viruses contrary to the pattern of specificity proposed by Harrison (1964). For example, six species of longidorid nematode have been reported as vectors of arabis mosaic virus (AMV ; specific field vector X . diversicaudatum ; Jha & Posnette, 1959 ; Harrison & Cadman, 1959). Similarly transmission of raspberry ringspot virus English strain (HRV-E ; specific field vector Longidorus rnacrosorna; Harrison, 1962 ; Debrot, 1964) has been reported for seven species of longidorid nematodes (Tab. 1). If al1 these reports of transmission are true then it would seem that nematode species other than those with which these viruses are specifically associated with in the field can also act as vectors. In this conneciion, Taylor and Robertson (1969) found unattached particles of AMV in the buccal capsule of L. eloizqatus which suggested tha t some transmission may &,ult from non-specific retention of virus. McNamara (1978) offered another explanation for apparent non-specific transmissions. He suggested that, in laboratory experiments, contamination of the outside of bait plant root systems may Occur either from virus coming from bodies of nematodes entangled in, or in nematode faeces adhering to the outside of, the roots. He reached these conclusions after attempting to substantiate the claim by Valdez (1972) that RRV-E could be transmitted by X . diversicaudatum. McNamara (1978) found that , although he could recover this virus from the roots of bait plants exposed to X . diversicaudatum, none of these plants was systemically infected, al1 the RRV-E detected apparently coming from external contamination of the roots. In contrast, when he used L. macrosomu, the natural vector of RRV-E, many bait plants eventually became systemically infected. McNamara (1978) concluded tha t evidence for nematode transmission “can only be fully acceptable if virus is translocated from the roots of the bait plant after transmission and infection is shown to be present in the leaves, hypocotyl, or in other egions to which nematodes have not had direct access”.