Histopathology of infection of bean with Pythium myriotylum compared with infection with other Pythium species

Abstract

Clusters of abundant appressoria formed from branching hyphae of mycelial inoculum of Pythium myriotylum on the surface of bean hypocotyls and roots. Pythium aphanidermatum usually produced single appressoria, but sometimes small clusters of appressoria. Pythium ultimum produced only single appressoria. Early pathogenesis of all species was characterized by rapid radial growth of hyphae in the epidermal cells, which was more rapid than in the cortex. These hyphae were constricted at the host cell walls. Invaded tissue and adjacent cells stained differently from healthy cells with all isolates. Aerial hyphae were produced from vesicles below the cuticle or within epidermal cells soon after infection was established. Safranin-staining materials were observed in the xylem, phloem, and tannin sacs. Zoospores of P. myriotylum and P. aphanidermatum germinated and produced long prepenetration hyphae, which branched and formed single appressoria. Oospores of P. myriotylum, P. aphanidermatum, and P. ultimum, after germination, produced branched hyphae and single appressoria. Penetration, rapid advance through epidermal cells, and ramification of cortical and vascular tissue were identical with those of mycelial inoculum. Sporangia of all three species formed intercellularly and intracellulary within 48 h and predominantly in the epidermis and upper cortex. Oogonia were produced intercellularly and intracellularly in 4 day s throughout the tissue, but mainly in the inner two thirds of the cortex. Sporangia of P. myriotylum and P. aphanidermatum in infected bean hypocotyls germinated within 3 h when flooded with tap water and produced zoospores within 6 h. When similar tissue was air-dried for 3 days or rapidly dried for 3 h. there was neither germination nor zoospore production.