Overhead watering will cause problems if the foliage stays wet on a regular basis. If you have to water, water in the early morning in the spring. Not mid-day in the summer. Knowing your horticulture gets rid of most of the diseases. We have found we do not have to spray.
Native plants are for the most part unhybridized.
They still have their natural protective abilities if you treat them right. If you overwater them you remove those protections. Water for survival, not fast growth, and after a year or so the plant will give you faster growth. Fertilizer, insecticides, fungicides and bactericides also can remove this protection. I think it is funny that you can buy fertilizer with insecticide in it.