Most often, yellowing leaves and spots on Prayer Plants are caused by a water quality issue. There may be too much fluoride, salt or chlorine in your tap water–especially if you use a water softener. Water your Prayer Plant with distilled water or leave a pitcher of tap water uncovered overnight so the minerals can evaporate before watering.
When you water your plant, make sure the water does not splash onto the leaves. Use lukewarm water and allow it to flow freely from the bottom of the pot into your sink or the plant’s saucer. Never allow any water to accumulate in the saucer; Prayer plants do not like “wet feet.” Water in the morning so the plant has the day hours to dry down a bit. Your plant should be placed in an area with bright indirect light until it recovers.
If the yellow spots spread and become tan with yellow halos, fungus may have set in. An application of a general houseplant fungicide can kill active fungal disease and prevent future outbreaks. Follow the directions on the bottle carefully.
You can also make your own homemade fungicide with 1-2 tablespoons or of baking soda and 1-2 teaspoons of mineral oil in a spray bottle of water. Shake the solution well and then spray all areas of the plant that are infected. Repeat daily until no new spots appear.