VIEW ARTICLE

Ecology and Epidemiology

Production of Sporangia and Release of Zoospores by Phytophthora megasperma in Soil. W. F. Pfender, Graduate Research Assistant, Department of Plant Pathology, University of Arizona, Tucson, AZ 85721; R. B. Hine(2), and M. E. Stanghellini(3). (2)(3)Professors, Department of Plant Pathology, University of Arizona, Tucson, AZ 85721. Phytopathology 67:657-663. Accepted for publication 3 December 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-657.

Segments of alfalfa radicles colonized by Phytophthora megasperma were buried in field soil and subjected to various moisture and temperature conditions in the laboratory. In flooded soil, sporangia were produced at 8-24 C, but not at 28 C; optimum temperatures for production of sporangia were 12-16 C. At 16 C sporangium production was initiated after 4 hr and reached a peak at 12 hr. Maximum production of sporangia occurred in flooded soil, with decreased production in soil at –0.05 and –0.1 bar matric water potential. Very few sporangia were produced at –0.6 bar and none at –2.8 bars. Sporangia germinated indirectly (released zoospores) in flooded soil at 8-24 C; optimum temperature for indirect germination was 16 C. At 16 C indirect germination began 6-8 hr after flooding and was 95% complete after 72 hr. In soil at –0.05 bar both direct (via germ tube) and indirect (via zoospores) germination were observed; sporangia in soil at –0.1 bar germinated directly. Zoospores originating from sporangia, produced at various depths in two flooded soils, were detected in surface water using alfalfa seedlings as bait. Zoospores were able to migrate upward through 65 mm of a sandy loam soil, but rarely moved more than 24 mm upward through a silt loam soil. In the flooded silt loam, probability of zoospores reaching surface water depended on number of sporangia initially present and their depth.