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Many isolates of the virus induce no conspicuous symptoms in many potato cultivars, pepino (Solanum muricatum) and weed species. However, some isolates cause undulation of leaf margins and some rugosity of leaf surfaces in susceptible potato cultivars and caused leaf bronzing, especially in very sensitive cultivars infected with virulent isolates (Beemster and de Bokx, 1987).
PVS-induced yield losses are best minimized by the production, large-scale propagation and distribution of virus-free nuclear stocks. Such stocks are obtainable by meristem tip culture (for example, Sampson and Stephens, 1981; Cassells and Long, 1982; Faccioli and Colombarini, 1996), especially if preceded by thermotherapy (Stace-Smith and Mellor, 1968; Sip, 1972; Aruta and Fuentealba, 1977; Sampson and Stephens, 1981; Brown et al., 1988), or if antiviral reagents such as Virazole or Ribivirin are included in the culture medium (Cassells and Long, 1982; Klein and Livingston, 1983).
The use of elite virus-tested stocks in production schemes is now well established in many countries (Shepard and Claflin, 1975; Wright et al., 1977; Horio, 1981; Salazar, 1996).
The use of resistant cultivars is also a standard procedure to minimise virus-induced losses (for example, Goth and Webb, 1985a; Salazar, 1996). Non-conventional resistance using transgenic plants has also been shown to have a great potential value (MacKenzie and Tremaine, 1990), but has not yet been used on a field scale.
The local importance of the virus is mainly dependent upon its incidence, the virulence of the virus isolate, tolerance of the potato cultivar and environmental conditions. However, most isolates reduce tuber yield by 3-20% (Wetter, 1971; Gladysiak and Wieckowski, 1977; Wright et al., 1977; Manzer et al., 1978; Beukema and van der Zaag, 1979; Wenzl, 1980).