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.
