The clue is the web. This sounds like spider mite and this tiny insect makes a web and loves to attack marigolds especially if they are stressed from drying out.  They suck the juice from the foliage turning them a pale color.  You can try using soapy water or a mitecide spray on the marigolds but a more practical solution is to add them to the compost pile and replace with a more mite resistant annual flower. Petunias, geraniums and lantana are a few sun lovers that are less likely to be attacked by mites.  My sympathies. Keep growing, Marianne Binetti Editor's note: here's a link to learn more about marigolds