Reactions of Elm Wood to Attacks of Ophiostoma Ulmi (Buism.) Nannf.

The reactions of the xylem of Ulmus hybrids were studied after injecting branches with a spore suspension of Ophiostoma ulmi (Buism.) Nannf. Discolouration of the walls of the vessels and formation of gum droplets in the pit cavities, were followed by development of tyloses. The expanding living cells always pierced through those pits where gummosis was absent. The toxin produced by the spores will be the causal agent of these anomalities. At places where many spores adhere to the walls of the vessels, gummosis may occur. At other places, where toxin is less concentrated, tyloses may appear.