Interactions between Pythium nunn and Pythium ultimum on bean leaves

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.

Hyphal interactions between a mycoparasite, Pythium nunn, and several soil fungi

The modes of hyphal interaction between a mycoparasite, Pythium nunn, and several soil fungi were studied by both phase-contrast and scanning-electron microscopy. In the zone of interaction, Pyth. nunn massively coiled around and subsequently lysed hyphae of Pyth. ultimum and Pyth. vexans. In contrast, Pyth. nunn penetrated and eventually parasitized hyphae of Rhizoctonia solani, Pyth. aphanidermatum, Phytophthora parasitica, and Phyto. cinnamomi by forming appressoriumlike structures. However, Pyth. nunn was not mycoparasitic against Fusarium oxysporum f.sp. cucumerinum or Trichoderma koningnii and was destroyed by T. harzianum and T. viride. These observations demonstrated that Pyth. nunn was a necrotrophic mycoparasite, with a limited host range and differential modes of action among suscepts.