Dry foliage or leaves with brown lower edges.
A sign of low humidity and a result of under watering. To speed up recovery increase watering and misting. Ensure fine mister is used, since sitting water on the foliage causes fungal and bacterial growth.
Droopy, curling leaves, with limp stems.
Draughts and cold cause the plant to go into shock. Check for any cold air currents and move the plant out of the way.
Grey fine mould on the leaves and stems.
This is known as Botrytis cinerea, which is a necrotrophic fungus caused by too much humidity. It is unlikely that Calathea will suffer with this in a home setting, however should this happen, ensure there is a good air circulation around the plant, which will allow for the excess moisture to evaporate.
Leaves are turning pale green suddenly.
The plant is too hot. Move it out of direct sunlight and check the leaves for scorch marks. Check the soil isn't too dry and if it is give it a good drink.
