Colonization of bean leaf fragments by Pythium nunn and Pythium ultimum in naturally and artificially infested soils was studied. In soil naturally infested with both P. ultimum and P. nunn (Ascalon sandy loam), the fungi colonized 20 and 0% of the fragments after 6 h, and 54 and 78% after 12 h, respectively. After 24 h, 73% of the fragments were co-colonized by both fungi, but after 72 h, less than 10% of the fragments were colonized by P. ultimum, and over 90% were colonized by P. nunn. In soil artificially infested with P. nunn (300 cfu/g), colonization by P. ultimum was significantly reduced after 18 h. Similar trends were observed in aerated–steamed and other soils artificially infested with P. nunn. When fragments were precolonized by P. ultimum for 24 h and then challenged with P. nunn, 77% were colonized by P. nunn after 24 h. In the reciprocal treatment, less than 5% of the fragments precolonized by P. nunn were colonized by P. ultimum. In bean leaf fragments placed in aerated–steamed soil, P. nunn reduced the density of sporangia of P. ultimum. This evidence suggests that P. nunn is both a primary and secondary colonizer capable of possession of substrates previously occupied by P. ultimum, and that P. nunn can displace P. ultimum from bean leaf fragments.
