Vegetative Propagation of Elms by means of Shoots cut from Callused Roots

A method is described for the propagation of elms from cuttings, raised from root-callus in the glasshouse. The technique applied is a modification of that described by Doran and McKenzie (1949). Shoots of adequate size, developed from the callus, were excised and planted. In early summer, they could be hardened in a cold frame. Callus cuttings were raised in different months of the year. The best results were obtained with root segments cut during the dormant period of the elms. Shoots excised in early spring rooted readily under improving light conditions. From the results of investigations since 1959 it is apparent that seedlings, as well as clones, can be successfully propagated in this way. Different clones of elm have shown differences in ease of propagation. The Dutch elm, Ulmus hollandica “Belgica”, produced less callus and shoots on root segments than other clones. Clones No. 1 and No. 148 were good producers of cuttings: from one elm of 2 to 4 years old about 80 genetically equal individuals on their own roots could be obtained, representing homogeneous material. The callus cuttings can be used in the study of parasitism of Ceratocystis ulmi, e.g. for testing the virulence of the fungus. They may also be used for selection purposes. From a seedling apparently resistant to the Dutch elm disease, a clone on own roots can be grown for further testing and thus any influence of the stock on the scion is obviated. Comparison of young callus cuttings and stem cuttings of the same age made it clear that the former represent the juvenile and the latter the adult stage of the tree. Both are sensitive to the fungus from the second year of growth on. The hypothesis of insensitivity of elms during the juvenile stage, the so-called youth-resistance, seems to be improbable.