Association of Pythium spp. with Carrot Root Dieback in the San Joaquin Valley of California. C. M. Liddell, Department of Plant Pathology, University of California, Davis 95616. R. M. Davis, and J. J. Nuñez, and J. P. Guerard. Department of Plant Pathology, University of California, Davis 95616, and University of California, Cooperative Extension, Bakersfield 93303. Plant Dis. 73:246-249. Accepted for publication 31 October 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0246.

Pythium irregulare and P. ultimum were frequently isolated from California carrot-producing soils with histories of carrot root dieback. In growth chamber studies, both species caused dieback symptoms and up to 80% mortality of carrot seedlings 7 days after sowing in infested soil. High soil temperatures (27–35 C) aggravated the disease. Saturated soils exacerbated root dieback caused by P. ultimum, although soils at – 30 k Pa matric suction (approximately 11% w/w moisture content) did not limit infection by the fungus. A survey of 39 fields in the San Joaquin Valley in 1987 showed no correlation between incidence of dieback (1–20%) and population densities of P. irregulare and P. ultimum (0–308 total cfu/g of soil) or between incidence of dieback and a number of soil factors (pH, electrical conductivity, moisture-holding capacity, organic matter, total and exchangeable calcium, and particle size distribution).