Canker attack by Hypoxylon pruinatum (Klotzsch) Cke. did not occur when inoculations were made on dormant cuttings of Populus trichocarpa Torrey and Gray that had been dipped in a 1/10,000 aqueous solution of Acti-dione concentrate, and provided with a continuous water supply. However, the antibiotic was not successful in disease control when water was withheld temporarily from the dipped and inoculated cuttings. Canker spread was arrested when water was supplied to these partially dried cuttings, although it was evident that the antibiotic had caused injury to the developing shoots and roots. The results demonstrated that successful control by Acti-dione was dependent on the maintenance of normal water relations in the experimental trees.The Acti-dione tests are discussed in relation to the excellent control obtained in earlier experiments by the application of extracts of bark saprophytes to susceptible host material.