Question
For the past two seasons the leaves on my marigolds look like something is sucking the green out of them.  They turn this sickly light gray, but the edges of the leaves are green.  It happens to all my different kinds of marigolds.  Any suggestions?  Thanks

Answer
The only thing I can think off is spider mites. Marigolds have natural repellants which makes them unattractive to the vast majority of garden bugs.

Here is an article on spider mites and pictures of the foliage (you can compare to your own plants)

http://www.floridagardener.com/critters/Insect_Pests/SpiderMites.htm

Insecticidal soap is very effective, but so are a powerful spraying with a garden hose (place thumb half way in front of opening to create a great pressure).

The most important preventiative meassure is to keep your plants well hydrated. do not expose them to draught stress. Mulch well around the plants and water frequently. When you water, try to water deeply so that the soil profile is uniformly wet and not just wetting the surface.

Do not use insecticides (see article outline).

Kenneth