Don't know if it would have much effect, but it wouldn't hurt to try strengthening the plants' immune system by inoculating bushes/topiary with beneficial fungal strains. Commercial Trichodermas might help or a good compost tea on a weekly or bi-weekly basis sprayed on the ground and the entire plant - apply at dusk to give beneficial organisms time to establish during the night.
Kentepoz, it's interesting that you mentioned those beneficial fungal strains. I was just reading a Monrovia (horticultural company) tag tonight, and that their soil blends are inoculated with mycorrhizae, a natural fungus that strengthens root growth, helps with absorption of nutrients, and helps fight transplant shock. They also include compost in their commercial soils.