Gerbera Daisy Leaves Shriveled?

Salt burns the leaves and if the salt is carried into the soil by the water it will kill the plant. If it is in a pot put the pot in a bucket of water. Shower the leaves as well. Let it stay in the water for 30' then take it out to wash off the salt from the soil and let it dry. Do not try any other treatment until it recovers. The white grey stuff you saw could be oïdium.

A plant will better resist Oïdium or powdery mildew if it is not over watered especialy when temperatures are between 60 and 80 degrees Fahrenheit and if when watered it is only at ground level and the leaves are kept dry.

To treat Oïdium you can spray the leaves once a week with a mixture of one tablespoon of neem oil extract concentrate with half a gallon of plain water. To prevent Oidium apply the same treatment once in the spring and once in the summer. Hope this can help your Gerbera.