Hi, 

This looks like it could be any number of sucking insect: thrips, mites, leaf hoppers, etc. It is common this time of year with the heat and dry weather. Sometimes they will go after marigold foliage and in other years they won't. Are you providing irrigation since it has been rather dry? That will at least help the plants be less stressed. If haven't recently you can also give them a little general fertilizer, either a slow release granular or a water soluble kind. That could help produce some more blooms. Be sure to dead head or clip off the spent blooms as well. 

You can use any type of contact insecticide to help but you will need to thoroughly coat the underside of the leaves. Insecticidal soap or horticultural oil will work but be careful with the oil as it can burn the foliage if you apply in high temperatures. Just read the label for either of them before using. 

Emily