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.