Certified GKH Gardening Expert
This can be either underwatering or a fungus.
If the plants are growing in the ground, they need at least 2 inches of water a week. If they are in a container, they need to be watered when the soil is dry to the touch on the top. If the weather is warm where you live, this can mean daily or even twice daily.
If you feel the plants are being watered properly, it may be a fungus. Treat the plants with a fungicide.