Methods for Screening and for the Rapid Selection of Elms for Resistance to Dutch Elm Disease

The purpose of this study was to shorten the time needed for selecting elms resistant to Ceratocystis ulmi (Buisman) C. Moreau, the causal factor of the Dutch elm disease. By a critical examination an attempt was made to improve the methods in use up to now. The fungus The strains of C. ulmi were found to vary greatly in pathogenicity. A correlation between virulence and the cultural types of the fungus was not found. Therefore virulence of the C. ulmi strains to be used for inoculation purposes in the field, has to be tested on susceptible elm material to be raised beforehand in the glasshouse. Virulence remains unchanged when strains are kept under mineral oil or in lyophilized condition. Perithecia are readily formed when an “A” and a “B” type of strain are grown together on a piece of sterilized elm twig. Some ascospore cultures proved to be more virulent than their parent strains. Probably specialization occurred, since some strains showed a high degree of virulence to one elm clone and a low degree to another one. Methods of inoculation Inoculations were performed with a spore suspension consisting of a mixture of five virulent C. ulmi strains grown in a nutrient solution of a special composition (p. 412). With a special knife a cut was made into the spring wood and a drop of spore suspension placed on the knife. The inoculum had to be sucked up by the tree. The spores induced maximal effect when about four days old. The number of spores per ml of inoculum was of less importance. Disease severity was evaluated in such a way that more stress was laid on severely diseased trees than on those that showed only slight external symptoms of disease (p. 413). The period of sensitivity of the elms Inoculated trees were able to show disease symptoms, only when they were inoculated during a short period which occurred about 40 to 50 days after bud break. The length of this period depended on weather conditions. Inoculations were most successful under the climatological conditions in the Netherlands when performed about mid-June. In that time the xylem vessels of the spring wood are fully developed. The physiological condition of the trees is also of the highest importance: unless growth is vigorous, susceptible backward trees do not show disease symptoms and may escape. This may happen after transplanting in the year of inoculation. Pathological anatomy The presence of internal symptoms may be a help in screening on resistance. The susceptible Ulmus hollandica ‘Belgica’ elm showed a “spreading” type of discoloration of the wood, whilst the resistant ‘Bea Schwarz’ elm and clone 296 showed a “spotted” type. Most elms showed, however, a pattern between the two. A strict correlation between the type of pattern and the degree of susceptibility was not obtained. Conditions which have to be fullfilled for effective testing These conditions are given on page 436, as the “ten commandments”. Selection of elms in the juvenile stage of development Contrary to the results from past experiments, it was shown that one-year-old seedlings and callus cuttings are able to show disease symptoms after inoculation. A comparison between the callus cuttings which show juvenile characters and oneyear-old grafts, which are in the adult stage, revealed that the disease indices of both groups of trees were similar. For testing of clones for resistance, callus cuttings are to be preferred to grafts, since clones of the former show greater uniformity. If they are resistant, their roots can not be affected by the fungus, but this is not so with resistant elms grafted on susceptible rootstocks. Procedure for a rapid selection When screening seedlings for resistance, damage of their roots must be avoided. The saplings should therefore be planted out directly after emergence in the nursery at distances of 25 X 30 cm. The following procedure has then proved satisfactory: A first screening was performed when the elms were one year old. Inoculations were repeated in the following years. In the fourth year only the fast growing, resistant trees with a good shape were selected. From the roots of these plants “callus cuttings” were grown, which were subjected to a preliminary screening in the glasshouse, after which the resistant clones were planted in a clonal test garden for further testing and observation (see scheme on page 441). If the suggestions given in the “ten commandments” and the proposed scheme of selection is followed, selection of a resistant elm can reasonably be accomplished within eleven years.