SEEDLING AND ROOTLET DISEASES OF FORAGE ALFALFA CAUSED BY PYTHIUM-IRREGULARE

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.