ABSTRACT
Bark from the graft union of tomato ringspot virus (ToRSV) infected plum, symptomatic for brown line disease, showed anatomical changes characteristic of the wound response process. The wound tissue consisted of necrotic cells demarcated by pinkish purple necrophylactic periderm, whose function is to protect living tissues from detrimental effects associated with necrosing cells. However, formation of gray exophylactic periderm led to the sloughing off of the wound tissue and the necrophylactic periderm, resulting in discontinuity of the exophylactic periderm and secondary virus invasion into the wound site. The changes seen in the bark suggest that the hypersensitive response in plum rootstock bark to ToRSV is slow, allowing a systemic movement of the virus and development of a brown line (BL) along the scion and rootstock union. Necrophylactic periderm was not seen in the bark from the graft union of a healthy plum tree. In the graft union of a plum tree without a BL, but testing ToRSV-positive in the roots, localized areas of wound tissue with pinkish purple necrophylactic periderm developed only in the rootstock portion of the tree. Silver-enhanced protein A-colloidal gold immunolabeling was seen on the cell wall and in the cytoplasm of bark tissue from the BL region of scion and rootstock and leaves from the rootstock suckers.
