University of California

Factors affecting soil populations of Pythium ultimum in the San Joaquin Valley of California


Joseph G. Hancock

Author Affiliations

Joseph G. Hancock was Professor of Plant Pathology, Department of Plant Pathology, and Plant Pathologist in the Experiment Station, Berkeley.

Publication Information

Hilgardia 45(4):107-122. DOI:10.3733/hilg.v45n04p107. May 1977.

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Pythium ultimum soil population levels were highest in the cooler seasons in the San Joaquin Valley, and always lowest during midsummer (August or early September). Crop residues, especially surface litter, supported population increases in the autumn (late September through November) in many fields, providing moisture was available and the substrate was suitable. A general seasonal pattern of P. ultimum fluctuations was evident in 7 of 10 field sites; populations were uniformly low in the remaining sites.

In the laboratory, Pythium ultimum did not develop or survive well on cotton leaves in soils held above 30 C for extended periods. High soil temperatures (30 to 37 C during July and August) apparently contribute to the uniformly low levels of P. ultimum encountered during the summer in field sites in the San Joaquin Valley. Soils that possessed uniformly low populations in the field during the course of this study did not support increases in P. ultimum when cotton leaves were incorporated under controlled conditions. Pythium ultimum developed well when cotton leaves were incorporated into soils which supported population increases under field conditions. It appears that certain soils are conducive to P. ultimum development, whereas others are suppressive. It is concluded that seasonal fluctuations in P. ultimum populations depend, in part, upon climatic conditions, the presence of organic residues, and soil faetor(s).

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Hancock J. 1977. Factors affecting soil populations of Pythium ultimum in the San Joaquin Valley of California. Hilgardia 45(4):107-122. DOI:10.3733/hilg.v45n04p107
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