Summary

Some types of elm hybrids resistent to the Dutch elm disease turned out to be susceptible toNectria cinnabarina(Tode) Fr. This fungus may behave as a parasite, killing branches. Virulent and non-virulent strains are known. A method is described for cultivatingNectria-strains in vitro on living desinfected branches ofAcer and ofUlmus. Three weeks after inoculation with a virulent strain the branches were killed. They were covered all over by a thick layer of mycelium. The mycelium of non-virulent strains could not kill the branches. It developed only sparsely. By this method it will be possible to select the most virulent strains out of a collection of isolates in order to use them for testing elm hybrids on their resistence toNectria cinnabarina.