Phytophthora cinnamomiin Hawaiian Forest Soils: Seasonal Variations in Population Levels

Abstract

Soils from 3 ohia [Metrosideros collina ssp. polymorpha] forest sites with trees showing varying degrees of decline, and from adjacent healthy and declining sites, were sampled bi-weekly for population levels of P. cinnamomi over an 8-13-mo. period. Population levels varied considerably among sites and within the same site throughout the course of the study. At the 3 sites, the fungus generally was undetectable or its population levels were lowest in the winter months when minimum soil temperatures were near 10.degree. C and then they increased as soil temperatures increased. Population levels decreased after extended periods of heavy rain and measurements at 6 of the 8 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 2984-3489 mm/yr apparently were favorable for sporangium formation. Zoospores were prevalent at certain times of the year.