Influence of Container Medium pH on Sporangium Formation, Zoospore Release, and Infection of Rhododendron by Phytophthora cinnamomi. N. S. Blaker, Graduate Student, Department of Plant Pathology, University of California, Davis 95616. J. D. MacDonald, Assistant Professor, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 67:259-263. Accepted for publication 9 August 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-259.

The effect of pH on the asexual life cycle of Phytophthora cinnamomi was studied in pH-adjusted U.C. mix (UCM) and UCM extracts. Most sporangia formed on colonized leaf disks in unadjusted UCM with a pH of 5.5–6.0 when maintained at the near-optimum matric potential of –10 mb. Sporangium formation was greatly reduced in UCM or Yolo fine sandy loam (YFSL) soil adjusted to pH 3.8–4.0 with H₂SO₄. The upper pH limit on sporangium formation in K₂CO_3- or CaCO_3- adjusted UCM and KOH-adjusted YFSL was 8.5. When colonized leaf disks bearing sporangia were removed from UCM and placed in extracts of known pH, the maximum release and motility of zoospores occurred at pH 5.6. Zoospore release and motility were not affected at pH 3.8 but were almost nil at pH 3.3. Although zoospores were not released at pH 3.3, they did form, encyst, and germinate within sporangia, and mycelial growth from colonized leaf disks was not significantly reduced. Susceptible rhododendrons grown in UCM infested with P. cinnamomi at pH 3.4–3.7 or 5.7–6.0 developed severe disease in the high-pH treatments but no detectable disease in the low-pH treatments after 44 days, although viable P. cinnamomi was still present in the medium. This work confirms an earlier report that low pH can control Phytophthora root and crown rot of rhododendron and indicates that it may do so by severely reducing sporangium formation, zoospore release, and zoospore motility.