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Detection of Phytophthora lateralis in Soil Organic Matter and Factors That Affect its Survival. W. D. Ostrofsky, Research Assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331, Present address: Department of Forestry, University of Nebraska, Lincoln 68503; R. G. Pratt(2), and L. F. Roth(3). (2)(3)Research Associate, and Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331, (2)Present address: Texas A&M University Agricultural Research and Extension Center, P.O. Box FFF, Corpus Christi 78406. Phytopathology 67:79-84. Accepted for publication 27 May 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-79.

Phytophthora lateralis could not be detected in most naturally and artificially infested soils using baits of Port-Orford-cedar tissue floated over soil. Detection was much improved when baits were floated over particles of organic matter (OM) separated from soil by wet-sieving. Phytophthora lateralis was detected three and 15 times more frequently, respectively, from slurries of two soils with the OM present than from slurries of the same soils but with most OM particles removed by filtration. The fungus also was detected more efficiently by baiting from OM than by growing cedar seedlings in soil and observing them for disease symptoms. Phytophthora lateralis survived at very low levels for 16 weeks in frozen OM and in OM in dried soil (25 bars tension). Survival for 16 or 20 weeks was maximal at low temperatures (5-15 C) and in moist but not in saturated soil. Survival in OM added to soils was prolonged by the presence of both host and nonhost tree seedlings. Phytophthora lateralis was not detected at high frequencies in OM from soil collected under diseased trees on forest sites. These results suggest that P. lateralis survives in soil primarily within host debris, and that death of trees frequently occurs without most of the feeder roots becoming infected.

Additional keywords: Chamaecyparis lawsoniana.