Alternative means of transmission for Prunus necrotic ringspot virus and Prune dwarf virus

Samples taken from stool beds of cherry rootstock Prunus avium 'F12/1' propagated from nuclear stock material free of known viruses occasionally tested positive for Prunus necrotic ringspot virus (PNRSV). After a possible source of infection was found, means of transmission were discussed. Since all known possibilities for virus spread could be excluded (transmission via seed and pollen are not relevant, nematodes were absent), uptake of virus particles through the plant roots in the absence of a vector were considered as an alternative way of transmission. In order to test this hypothesis, the bait plant technique was used with plants grown in containers under field, greenhouse, and in vitro conditions. Various inocula containing either Prunus necrotic ringspot virus (PNRSV) and/or Prune dwarf virus (PDV) or Petunia asteroid mosaic virus (PAMV) were used. The bait plant tests with woody Prunus avium 'F12/1' and herbaceous indicators did not provide evidence for vector-less soil transmission of PNRSV and/or PDV. However, the uptake of purified PAMV particles from nutrient media solidified with agar was proven for the first time for Prunus avium 'F12/1' (herbaceous plantlets). With non-flowering Prunus plants also the hypothesis of extrafloral virus transmission by insects feeding on nectaries of virus infected individuals and transferring the virus particles to nectaries of healthy plants was proposed. This was investigated by caging bumblebees with infected Prunus trees and virusfree Prunus avirum 'Fl2/1'. After 18 months five out of 24 bait plants were found infected. It is concluded that whenever transmission by pollen or seed is to be excluded transmission by insects feeding on extra-floral nectaries rather than soil transmission without any vector can be assumed.