Basal Stem Rot of Cultivated Poinsettia Caused by Amphobotrys ricini. G. E. Holcomb, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. W. L. Brown, Hammond Research Station, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Plant Dis. 74:828. Accepted for publication 23 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0828E.
A basal stem rot was observed on poinsettia (Euphorbia pulcherrima
Willd. ex Klotzsch) growing at the Hammond Research Station in
October 1988 and 1989. Scattered plants (<1% of 400 in 1989 test)
showed shoot wilt symptoms. Examination of these plants revealed
a soft, watery rot of cortical stem tissue that extended 10 cm below
and above the soil line. There was no evidence of additional root
or shoot infections, and no fungal fruiting structures were present
on the rotting stems. The use of Exotherm Termil (vaporized chorothalonil)
prevented infection of shoot tips and leaves but did not control
basal stem rot. Pathogen isolations from rotted tissue plated on 2%
water agar yielded the fungus Amphobotrys ricini (Buchw.) Hennebert
(1), recently reported to cause shoot tip infection of poinsettia (2).
Pathogenicity tests were positive when A. ricini spores were misted
on wounded and nonwounded healthy, succulent poinsettia stems,
and the pathogen was reisolated. Basal stem rot is a new disease manifestation
by A. ricini on cultivated poinsettia.