INVESTIGATIONS ON THE NATURE OF A GRAFT-TRANSMISSIBLE AGENT IN POINSETTIA

The free-branching poinsettia (Euphorbia pulcherrima) cultivar Annette Hegg Brilliant Diamond contains a free-branching agent that is graft- transmissible to the restricted-branching cultivar Eckespoint C-1 Red. Transmission electron microscopy failed to reveal evidence of bacteri a, fungi, or mycoplasma-like organisms in either 'Brilliant Diamond' o r 'C-1 Red' plants. Treatment of both cultivars with tetracycline-hydr ochloride produced no differences in branching pattern or leaf morphol ogy in either cultivar, indicating that the agent may not be a mycopla sma-like organism. Scions of a poinsettia mosaic virus indicator speci es (Euphorbia cyathophora) grafted onto 'Brilliant Diamond' and 'C-1 R ed' stocks exhibited the mottling symptoms characteristic of poinsetti a mosaic virus, while self-grafted E. cyathophora scions showed no mot tling, indicating that poinsettia mosaic virus was not the agent. The agent was not transmitted by pin prick, carborundum, or dodder (Cuscut a sp.), and ribaviran did not eliminate expression of the branching ag ent from 'Brilliant Diamond' plants. No differences in double-stranded RNA banding patterns were found between extracts of free- and restric ted-branching poinsettias by polyacrylamide gel electrophoresis. The d ouble-stranded RNA was attributed to poinsettia mosaic virus and other unknown RNA viruses. Attempts to detect a specific DNA associated wit h free-branching were inconclusive.