Pollen transmission of Cherry leafroll virus in sweet cherry (Prunus avium L.)

Hui Hou

This project examines pollen-mediated horizontal transmission of Cherry leafroll virus (CLRV) in sweet cherry. In a commercial orchard, three 'Van' trees were tested and found to be free of CLRV at the beginning of the study; these trees were adjacent to infected 'Bing' trees. At shuck fall, CLRV was detected by RT-PCR in extracts from ovaries and pedicels of the 'Van' trees, whereas at pit hardening and commercial harvest, all tissues including exocarp/mesocarp, seed and pedicel yielded detectable CLRV. Three weeks after commercial harvest, extracts of spur and leaf tissue of fruit bearing branches contained detectable CLRV. These results suggest that a pathway exists to transport CLRV from pollen through the pedicel, into the main plant. Immunolocalization studies substantiated the RT-PCR results and revealed CLRV in ovary, endosperm, and pedicel tissues of developing fruit. Label was concentrated in and near vascular bundles of the ovary at shuck fall. When pedicels were examined at shuck fall and at pit hardening, label was primarily associated with the vascular bundles with additional label in sub-epidermal cells. At commercial harvest, label was only located within sub-epidermal cells. The occurrence of virus in vascular tissues of the pedicel before pit hardening presents an opportunity for movement of CLRV through the pedicel from fruiting structures to the mother tree. The mechanism(s) of pollen-mediated horizontal transmission of CLRV was also studied by hand pollination experiments at a Moxee research block. Four treatments were established to explore the role of fertilization and/or thrips involvement in horizontal transmission. At shuck fall, CLRV was detected by RT-PCR in ovary and pedicel samples from all treatments. The frequency with which virus was detected is not altered significantly by the presence or absence of added thrips, or whether the flower-infected pollen combination was a compatible or incompatible interaction. Immunolocalization revealed the presence of CLRV inside the ovary of flowers pollinated with incompatible infected pollen; the label was adjacent to cells of vascular tissue but not in the epidermal layer where thrips feed. These data suggest that CLRV is transported from pollen to pedicel without requiring fertilization or thrips activity.

Pollen transmission of Cherry leafroll virus in sweet cherry (Prunus avium L.)

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