I live in Massachusetts, growing zone 6a, and have a lot of experience with marigolds, grown from seeds, seedlings, or mature plants. I like them for the wide variety of colors and sizes, and because the chipmunks and rabbits don't eat them! Best of all, I find them very forgiving, willing to tolerate all kinds of conditions, including large changes in temperature and humidity. In our area, they have a very long growing season. Mine bloom from late spring, about 60° Fahrenheit, through the summer, which can go as high as 100°, all the way until after the first frost. They're the easiest plants I have, and although I don't know your weather conditions, I hope that once yours perk up a bit, you'll find them fun and easy too!
I don't believe your plants are dying, but you've asked the question at a perfect time in order to prevent that. They're suffering a bit, but I've had some look a lot worse for many weeks, only to come back stronger and last longer after making a few adjustments to their care.
Pick off the large flowers. The plant is using energy to keep them blooming. Once it feels better, it will reward you with a new and more profuse crop. Using a sharp knife or garden shears, cut them all the way back to where the stem meets the stalk. That's called the joint, and the new flowers emerge from there, so be careful not to go too far. Don't throw them out! They'll keep their color and pretty scent for ten days or so in in a small vase or cup of water, either outside or in. Leave the plump unopened buds alone, I see healthy flowers inside. If, after a week or two, they completely shrivel up, gently remove them as you did with the flowers.
I completely agree with Bamboo. You're providing the right amount of sun, but they can dry out quickly in the heat. That's probably what's going on, as one of the first symptoms is those curling leaves. As she said, they can be finicky about water, needing regular watering, but not liking wet roots. Make sure your pots have good drainage, and water them small amounts frequently. You can put saucers under the pots to catch runoff, just empty them if they start to overflow. Don't worry if marigolds end up in a big rain, though. They'd rather be too wet than too dry.
If you're going to keep them in pots, try to avoid clay or ceramic, as those dry out extremely quickly, especially in the sun, and require a light watering pretty much every day. As for a growing medium, I use a Moisture Control Potting Soil made specifically to keep all of your annuals from being too wet or too dry. If you have access to a similar type of product, I recommend it. It also contains food which can increase the size. If not, any potting soil will do.
Marigolds do like some space for the roots, deeper being a little better than wider. Lack of room can impede both drainage and growth. To check to see if the pot's big enough, look at the bottom. If there are any roots pushing out through the drain holes, or if you can see tangles of roots, that means it's root bound, and needs a larger home. They're extremely easy to transplant, from pot to pot, or pot to ground.
Unfortunately, I don't know anything about fungus, so I'll leave that to the experts. I do hope they're just going through a rough patch, and will recover very soon and thrive for a long time!