Production of Sporangia by Phytophthora cinnamomi in Forest Soils

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Abstract
Sporangial production by Phytophthora cinnamomi was investigated during a 3-year period for three types of conducive forest soils. A pilot survey conducted during the first year included garden soil and demonstrated that all forest soils tested stimulated sporangial production, that the stimulus was reduced by soil sterilization, and that soils without P. cinnamomi provided a greater stimulus than soils from diseased sites. For the final 2 years of the investigation soil and root samples were collected from diseased plants at 3- monthly intervals, and soil filtrates were tested for their ability to stimulate sporangial production. Soil matric water potentials and soil temperatures were recorded concurrently. Three factors which influenced the soil and indirectly affected sporangial formation were soil temperature, soil matric water potential and the presence of a stimulus produced by certain living microorganisms. Sporangial production showed highly significant seasonal variation with maxima in spring. No sporangia were produced in summer. Differences in the numbers of sporangia produced in response to the different soil filtrates were also highly significant, the highest numbers being formed in response to sandy soils of Wilson's Promontory. These results, particularly the large release of zoospores during spring, are of great importance in the spread of disease, and should be considered in relation to roadmaking and forestry in diseased areas.