A virus isolated from white clover (Trifolium repens L.), dahlia, and tomato from Ontario, dahlia from Manitoba, and tomato and dahlia from British Columbia was identified as tomato spotted wilt virus (TSWV). Seven isolates of the virus examined had similar host reactions; four of them, investigated in detail, were similar in their localization in tomato leaf cells and physical properties also. The virus particles, 78–97 nm in diameter, were abundant in tomato leaf mesophyll cells and occurred in clusters or arrays surrounded by a membrane.The virus was purified by a method modified from a previously reported one and a specific antiserum was prepared. TSWV appears to be weakly immunogenic, as a high titer antiserum could not be obtained.At least three species of thrips, i.e. Thrips tabaci Lind., Frankliniella fusca Hinds, and F. occidentalis Perg., known as vectors of TSWV elsewhere, occur in Canada. F. fusca, which occurs in eastern Canada, efficiently transmitted all isolates of the virus. T. tabaci, a vector in several countries and widespread in Canada, failed to transmit the virus.

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Two isolates of tomato spotted wilt virus (TSWV) from British Columbia were transmitted by Frankliniella occidentalis, which is common in British Columbia but does not occur in eastern Canada. Frankliniella occidentalis from B.C. transmitted the virus to a smaller proportion of Emilia plants as compared with Frankliniella fusca, a known vector that occurs in eastern Canada, after a 48-h acquisition feeding as nymphs on infected Emilia leaves. Neither isolate of the virus was transmitted by Thrips tabaci, which is a vector of TSWV in some other countries. Males and females and macropterous and brachypterous forms of F. fusca did not differ significantly in their vector ability. TSWV was detected serologically in homogenates of 50 ‘exposed' F. fusca (first-generation adults reared on newly infected plants) but was difficult to detect in homogenates prepared from these thrips 2 weeks later. Thrip transmissibility of the virus declined considerably when propagated for long periods without passage through the thrip vector.

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