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Etiology

Association of Phytophthora cinnamomi with Ohia Decline on the Island of Hawaii. J. T. Kliejunas, Assistant Professor, Department of Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720; W. H. Ko, Associate Professor, Department of Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720. Phytopathology 66:116-121. Accepted for publication 1 August 1975. DOI: 10.1094/Phyto-66-116.

Ohia (Metrosideros collina subsp. polymorpha) seedlings transplanted to soils collected from declining forests became infected with Phytophthora cinnamomi; inoculation of healthy ohia trees in the field with soils and roots from declining forests resulted in root necrosis and P. cinnamomi was isolated. Phytophthora cinnamomi was recovered from 96% of the decline areas and from 24% of the apparently healthy areas sampled throughout the island of Hawaii. Lupine baiting and the use of soil dilution plates indicated little difference between population of the fungus in soil of declining and healthy forests. Addition of nutrients to ohia trees in the greenhouse had little effect on subsequent root infection by P. cinnamomi, and the fungus was recovered from soil in areas where declining ohia trees produced new growth following application of complete fertilizer. Fungicides reduced infection of ohia trees and lupine by P. cinnamomi in greenhouse and laboratory experiments. Broadcast applications of fungicides plus complete fertilizer to declining ohia trees resulted in a greater growth response than application of complete fertilizer only.