Disease Note.

Seedling Blight of Castor Bean in Hawaii Caused by Phytophthora palmivora. J. Y. Uchida, Department of Plant Pathology, Hawaii Institute of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu 96822. M. Aragaki, Department of Plant Pathology, Hawaii Institute of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu 96822. Plant Dis. 72:994. Accepted for publication 31 August 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0994C.

Phytophthora palmivora (Butl.) Butl. was isolated from blighted seedlings of castor bean (Ricinus communis L.) from fields in Puna, Hawaii, previously grown to papaya (Carica papaya L.). Over 30% of the seedlings in an experimantal planting were blighted after several weeks of heavy rains. Isolations also yielded Pythium helicoides Drechs. and binucleate Rhizoctonia solani-like (BRS) fungi. Pythium helicoides and four isolates of BRS fungi were not pathogenic to castor bean in controlled inoculations. The pathogenicity of single-zoospore isolates of P. palmivora (H647 from castor bean and H463 from papaya) was tested by spraying zoospore suspensions onto young castor bean and papaya seedlings. On castor bean, both organisms produced irregular, dark, restricted lesions on young leaves, blighted cotyledons, and induced limited necrosis of roots. Foliage and stems of papaya were severely blighted by both organisms, and roots were severely rotted. Propagules of P. palmivora surviving from previous papaya plantings were probably the source of the seedling blight outbreak on castor bean.