VIEW ARTICLE

Ecology and Epidemiology

Phytophthora cinnamomi in Hawaiian Forest Soils: Seasonal Variations in Population Levels. J. T. Kliejunas, Assistant professor, Department of Plant Pathology, University of Hawaii, Hawaii Agricultural Experiment Station, Hilo 96720; J. T. Nagata, research technician, Department of Plant Pathology, University of Hawaii, Hawaii Agricultural Experiment Station, Hilo 96720. Phytopathology 69:1268-1272. Accepted for publication 4 June 1979. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-1268.

Soils from three ohia forest sites with trees showing varying degrees of decline, and from adjacent healthy and declining sites, were sampled biweekly for population levels of Phytophthora cinnamomi over an 8- to 13-mo period. Population levels varied considerably among sites and within the same site throughout the course of the study. At the three sites, the fungus generally was undetectable or its population levels were lowest in the winter months when minimum soil temperatures were near 10 C and then they increased as soil temperatures increased. Population levels decreased after extended periods of heavy rain and measurements at six of the eight monthly sampling times were significantly higher at the decline site than at the adjacent healthy site. Significant correlations between P. cinnamomi population levels and maximum soil temperature, minimum soil temperature, and rainfall were found at some sites but not at others. Soil matric potentials at all sites ranged from 0 to –30 mb and were seldom less than –25 mb. High water holding capacity of the soils combined with rainfall averages ranging 2,984–3,489 mm/yr apparently were favorable for sporangium formation. Zoospores were prevalent at certain times of the year.