Seedling and Rootlet Diseases of Forage Alfalfa Caused by Pythium irregulare. J. G. Hancock, Professor and Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720. Plant Dis. 75:691-694. Accepted for publication 14 December 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0691.

Isolates of Pythium irregulare caused seedling diseases of alfalfa (Medicago sativa) over a range of temperatures (16–32 C); however, postemergence damping-off and root-forking were most severe at 16 and 21 C. Alfalfa shoot growth was retarded when seedlings were grown in either pasteurized or raw soils amended with culture-grown inocula adjusted to give inoculum densities of propagules within ranges measured in field soils. A negative correlation was found between infection rates of feeder roots by P. irregulare and shoot dry matter during early harvest cycles with greenhouse-grown plants. However, degrees of reduction in shoot dry matter differed significantly between cultivars of alfalfa.

Keyword(s): Pythium paroecandrum, Pythium ultimum.