Your yukka is kept in a conservatory and gets lots of light The tops of a lot of the leaves have started to go brown but you dont know why you water about once a week but spray the leaves every day?

1. Maybe you should water it more often. 2. If the plant is standing in the sun, you should not spray it: The tiny drops act like prisms and the leafs are burned. 3. If the plant is not standing in direct sunlight, maybe the water in the sprayer is too fresh. You should let it settle for at least 24 hours before you spray, else all those minerals and chemical elements in the water will settle down on the leafs instead of settling down in the water-bottle, and the leafs cannot breath anymore. 4. Maybe the humidity of the air is too low.