It sounds like you are describing powdery mildew.

If that is indeed what we are discussing you will want to examine your plant selection and cultural conditions. First, some plants are highly susceptible to it. What plants are you growing? Often there are varities bred to have some resistance to it.

Lack of air flow, high moisture and shade all contribute to it's presense. A dense planting like you describe is ideal for it.

To control it clean up the garden each fall. The spores overwinter in the garden debris. There are fungicides available to control PM, but read the labels to make sure the one you choose is safe for the plants you will be treating.

Also, if you can switch your watering practice and avoid getting the leaves wet this may help.