COLONIZATION OF ROOTLETS OF ALFALFA BY SPECIES OF PYTHIUM IN RELATION TO SOIL-MOISTURE

Colonization of feeder roots of alfalfa by Pythium irregulare and P. ultimum was either unaffected or affected only slightly by soil moisture status (water potential range: > -0.02- -0.5 MPa) in a well-drained, sandy-loam soil in a sprinkler-irrigated field plot in the San Joaquin Valley of California or over a range of constant soil moisture regimes (0.0- -2.2 MPa) in a greenhouse and lathhouse. Seasonal differences in root colonization were evident, with species of P. irregulare most active in winter and early spring and P. ultimum most active in summer. Root-length densities in surface soil in plots in the San Joaquin Valley were inversely related to soil moisture potential during the growing season. However, there was no evidence that differences in root-length densities in plots were related to root colonization by Pythium.