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

Abstract

Levels were highest in the cooler seasons and always lowest during midsummer (Aug.-early Sept.). Crop residues, especially surface litter, supported population increases in autumn in many fields providing moisture was available and the substrate suitable. A general seasonal fluctuation pattern was evident in 7 of 10 field sites; populations were uniformly low in the other 3. In the lab., P. ultimum did not develop or survive well on cotton leaves in soils held at >30 deg C for extended periods. High soil temps. (30-37 deg ) during July and Aug. apparently contribute to the uniformly low levels encountered during the summer in field sites. Soils possessing uniformly low populations did not support increases in P. ultimum when cotton leaves were incorporated under controlled conditions, but it developed well when the leaves were incorporated by soils supporting increases under field conditions. Certain soils appear to be conducive to P. ultimum development, others suppressive. It is concluded that the seasonal fluctuations depend partly on climatic conditions, the presence of organic residues and soil factor(s).