Clay soil is very slow draining which can lead to saturated soils which can then lead to limited air exchange. Plant roots need both air and water. The tricky thing about overwatered plants is that they often exhibit the same look you would expect from a drought stressed plant such as wilting and leaf loss. Naturally, your instinct is to water which compounds the problem.
The other over-watering mistake is good love gone bad. Although there are a very few exceptions to the rule, watering a little bit every day is not what most plants (especially trees) need. Keeping the soil moist with frequent watering is important when starting seeds, establishing transplants, and sometimes in container gardens. Watering daily keeps the roots near the surface of the soil which can result in fried roots in a scorching summer. You are also promoting shallow rooting which then relies on daily water. In contrast, we want the roots of the plants to go deeper into the soil to create more stability and stronger, healthier and more resilient plants. If you have an irrigation system, it is especially important to do periodic checkups. Your system needs to be adjusted seasonally and it's always a good idea to make sure you don't have a midnight geyser.
Limiting the stress of your plants gives them a much better chance at shrugging off insects and diseases. Make sure the care you are giving your plants isn't causing undue stress.
If your rosemary bush looks a bit wilted is it too dry or too wet? Rosemary is particularly sensitive to wet feet and it is important to make sure it isn't overwatered.