Thanks for writing. If the problem is crown rot, it's best to keep your gerberas on the dry side and encourage good air circulation to help fight back the fungus. If your gerberas are planted closely together, transplanting some so they're farther apart and there's better airflow can help.
Also: Watering with a soaker hose (versus a sprinkler) can also help prevent disease. And be sure they're in full sun -- if the plants are in conditions where they struggle a bit, they'll be more likely to suffer from insect or disease problems.
---Justin, Senior Garden Editor, BHG.com