VIEW ARTICLE

Ecology and Epidemiology

Production of Sporangia by Phytophthora cinnamomi and P. palmivora in Soils at Different Matric Potentials. U. Gisi, Research associate, Department of Plant Pathology, University of California, Riverside 92521, Present address of senior author: Institute of Botany, University of Basel, Schoenbeinstrasse 6, CH - 4056 Basel, Switzerland; G. A. Zentmyer(2), and L. J. Klure(3). (2)(3)Professor, and staff research associate, respectively, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 70:301-306. Accepted for publication 25 September 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-301.

The production of sporangia by Phytophthora cinnamomi and P. palmivora was studied in a sandy loam and a clay soil at matric potentials between 0 and –15 bar. There was a strong correlation between the number of sporangia produced and matric potential, but there was no direct relationship between sporangium production and the water content of the soil. For each species, numbers of sporangia formed at the same matric potential were similar for both soil types. When mycelial inoculum was buried under 5 mm of natural soil, P. cinnamomi produced the most sporangia at –160 millibar (mb), with upper and lower matric potential limits of about –10 mb and –2,500 mb, respectively. The number of P. palmivora sporangia formed on inoculum buried in flooded soil was about 20%, and at –15 bar about 5% of the maximum observed, with buried inoculum yielding the maximum numbers of sporangia in both soils at –10 mb and inoculum on the soil surface yielding the maximum at –5 mb. Maximum numbers of sporangia were produced by P. cinnamomi on the soil surface only under flooded (+ 1 mb) and saturated (0 mb) conditions. Daily drying and rewetting of the sandy loam soil did not induce significantly more sporangia than were produced by P. cinnamomi at constant matric potentials. The ratio between the numbers of sporangia produced by P. cinnamomi and P. palmivora was about 1:200.