Pythium root rot of white bean in Ontario.
P. spp. were isolated from 35% of stem and 15% of root lesions on bean (Phaseolus vulgaris) plants in southern Ont. The most abundant type of isolate (P. 'spherical') which produced spherical vegetative bodies 24 µm diam. but no oospores on V-8 juice agar, constituted 69% of stem and 50% of root isolates. Other spp. commonly isolated were P. paroecandrum (stem 5%, root 32%). P. salpingophorum (23%, 12%) and P. rostratum (2%, 4%). P. ultimum, P. oligandrum and P. arrhenomanes were rare (1% or less of isolates). In pot tests, all tested isolates of the common spp. caused rotting of fine roots similar to that observed in the field, but few caused stem rotting. Over 2 trials with each isolate, root weight was significantly reduced by 11 of 16 isolates of P. 'spherical' and 8 of 10 isolates of P. paroecandrum. Emergence and shoot weight were also reduced, but by fewer isolates. None of 10 P. rostratum or 15 P. salpingophorum isolates reduced emergence, root weight or shoot weight. Because of their abundance, occurrence in root lesions in the field and pathogenic abilities, it is concluded that P. 'spherical' and P. paroecandrum are important agents of the disease in Ont.