As my nickname suggests, I'm a bit of a plant killer, not on purpose, but I can't seem to keep my indoor plants from dying. I'm hoping someone can help me with my latest sick plant. It's a yukka spp and it's not looking so good. History: The yucca is indoors sitting next to a north-east window so I think it gets adequate light. It gets about 500 mL (17 fl oz) of water once a week which is enough for the water to drain through to the saucer. My partner thinks it's being overwatered, I think it's being underwatered, but adjustments to watering fequency hasn't seem to improve the situation, just worsen it. Symptoms: There is a white, powdery coating on the leaves. When rubbed off, the white coating feels powdery, a little sticky and turns a kinda brownish color. The older leaves at the bottom of the plant have dried out and died and what leaves are left is severely droopy. Picture of the leaves: I have no idea what could be wrong and I'm deperately trying to save the plant before it gives up all will to live. Please help if you can, I'll be forever grateful of any advice!