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Ecology and Epidemiology

Movement of Zoospores of Phytophthora cryptogea in Soils of Various Textures and Matric Potentials. J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616; Phytopathology 66:877-882. Accepted for publication 29 January 1976. DOI: 10.1094/Phyto-66-877.

Active movement of Phytophthora cryptogea zoospores in soils was examined by placing susceptible safflower seedlings at various distances from a source of motile zoospores for 24 hours. The presence of P. cryptogea in seedlings was determined by symptom development and by isolation on a selective medium. Tension plates were used to control soil moisture at various matric potentials ( ψ m). The bottom of soil samples 1.5-cm thick was the reference point for ψ m values which are expressed in millibars (mb). Sporangia that formed in soil at ψ m = 300 mb and were used to inoculate soils at higher ψ m values only released zoospores at ψ m ≥ 10 mb. When either sporangia or motile zoospores were used to inoculate soil, zoospores readily swam 25 to 35 mm in the surface water over flooded soils or through a coarse textured U.C.-type soil mix at ψ m ≥ 1 mb. Active zoospore movement in the soil mix was reduced at ψ m = 10 mb and was only detected over a distance of 5 mm at ψ m = 50 mb. No detectable zoospore movement occurred at ψ m ≤ 100 mb. Active zoospore movement in sieved and reconstituted silt loam and fine sandy loam soils was limited to a distance of 5 mm at ψ m = 1 mb and was not detected at ψ m = 10 mb. Zoospores moved a distance of 5 mm in a clay loam soil at ψ m = 10 mb. Water flow down through 40 mm soil columns carried zoospores to depths of 8-12 mm in the loam soils and 40 mm in the coarse soil mix.

Additional keywords: Phytophthora root rot, Phytophthora drechsleri, Carthamus tinctorius, water potential.