Choosing pest- and disease-resistant varieties and matching perennial species to the site -- sunlight, soil, hardiness zone etc. -- will help reduce plant health problems. Also:
- Choose only healthy plants when shopping at local nurseries and garden centers. Make sure they don't have any signs of insect damage or foliar diseases and inspect the roots to make sure they are white (not brown) and growing vigorously.
- Remove diseased plant tissue and rogue out chronically infected plants.
- Clean up and remove dead plant foliage at the end of the growing season.
- Reduce plant stress through proper watering and soil care. Improve drainage to reduce root diseases.
- Space plants farther apart to provide better air circulation if foliar diseases are a problem.
- Carefully monitor your plants for signs of insects or diseases, especially where you have large groupings of a single species.
See How to Grow Annuals for more information on some common insect pests.