Seasonal development of Dutch elm disease on white elms in central Ontario, Canada. II. Following feeding by the North American native elm bark beetle

Adults of the North American native elm bark beetle, Hylurgopinus rufipes, naturally infested with Dutch elm disease fungus, Ceratocystis ulmi, were caged on trunks of white elms (Ulmus americana) so that infection could be studied. In surveys made in 1971, expression of external symptoms of Dutch elm disease was observed on elms which had been caged with beetles during the period May 26 to July 9, 1971. Vascular discoloration (internal symptom expression), fungus spread, fungus contamination of beetles, and transmission of fungus to the host by beetles were pronounced on elms caged with beetles from June 3 to July 2. Ceratocystis ulmi was recovered from leaves of trees caged during the period May 26 to July 2. Until May 31, 1972, dieback and death resulting from infection were restricted to trees caged with beetles during June. Corresponding development of vascular discoloration and fungus spread in the host also occurred, mostly during the caging period in June. There was no appreciable change in beetle activity in xylem tissue during the period of experiment. These results suggest that the period of highest susceptibility of white elm to Dutch elm disease in 1971 in central Ontario was June 3 to July 2.Inoculation through feeding by caged beetles carrying C. ulmi is probably the method that most closely approximates natural infection.