Heat sensitivity of mycelium and haustoria in bean leaves infected with bean rust

Bean leaves experimentally inoculated with bean rust were subjected to heat treatment at 50° for 20 s, at different times after inoculation, and the modifications induced by heating in the pathogen and host cells were studied by electron microscopy, at different times after the treatment. This induced no alterations either in healthy or infected leaf cells. On the contrary, it caused severe ultrastructural alterations in the pathogen. These alterations were already visible in the haustoria and hyphae at the end of the treatment, became progressively more severe with time, and led to the death of the pathogen two days after the treatment. From this time onwards the haustoria appeared shrunken, extremely electron-dense, and encased in a mass of granular material, seemingly secreted by the host cell. No encasement was ever found around haustoria apparently dead by senescence in untreated leaves.