Identification, distribution and comparative pathogenicity of Pythium spp associated with alfalfa seedlings

Root system colonization of alfalfa seedlings by Pythium spp was investigated during the first 4 wk after seedling emergence at two field locations in Missouri over 2 years. Pythium spp were isolated from most roots, but isolation was not necessarily associated with necrosis, lesions or overt evidence of disease. P. sylvaticum, P. irregulare, P. ultimum and P. torulosum were the most prevalent species isolated, comprising up to 65% of all Pythium spp isolated at a given site. Changes in the species composition of the Pythium community associated with alfalfa seedlings were monitored in relation to seedling age, root development stage and various environmental conditions. Root systems were classified by root orders using the morphometric system of analysis to define root segments of similar physiological age and function. P. ultimum, P. irregulare and P. sylvaticum caused severe pre- and post-emergence damping-off, necrosis and stunting of root and shoot growth in greenhouse pathogenicity tests. Several other species, including P. dissotocum, P. acanthicum, P. torulosum and P. rostratum reduced root system length, but elicited no other symptoms. Variations in the frequency of isolation of different species were most closely associated with environmental conditions, particularly temperature and moisture. P. sylvaticum was prevalent at higher temperatures and drier conditions, and several other species preferred cooler, wetter conditions. The stage of root development, as described by the morphometric system, and seedling age had no observable effects on the frequency of colonization or the composition of the community of Pythium spp colonizing the roots.