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 bacteria, fungi, or mycoplasma-like organisms in either 'Brilliant Diamond' or 'C-1 Red' plants. Treatment of both cultivars with tetracycline-hydrochloride produced no differences in branching pattern or leaf morphology in either cultivar, indicating that the agent may not be a mycoplasma-like organism. Scions of a poinsettia mosaic virus indicator species (Euphorbia cyathophora) grafted onto 'Brilliant Diamond' and 'C-1 Red' stocks exhibited the mottling symptoms characteristic of poinsettia mosaic virus, while self-grafted E. cyathophora scions showed no mottling, indicating that poinsettia mosaic virus was not the agent. The agent was not transmitted by pin prick, carborundum, or dodder (Cuscuta sp.), and ribaviran did not eliminate expression of the branching agent from 'Brilliant Diamond' plants. No differences in double-stranded RNA banding patterns were found between extracts of free- and restricted-branching poinsettias by polyacrylamide gel electrophoresis. The double-stranded RNA was attributed to poinsettia mosaic virus and other unknown RNA viruses. Attempts to detect a specific DNA associated with free-branching were inconclusive.