Dracaena Marginata Problem
So I've searched the forums and elsewhere online to try and figure out what exactly is wrong with my plant, which I believe is a Dracaena Marginata. I'm new to having plants so this is way beyond my ability to figure out on my own. Any help would be greatly appreciated!
I bought the plant in January and it was very bright green and had a lot of leaves. A couple months ago some of the leaves on the bottom started to turn yellow, dry up, and fall off.
I've been watering only when the soil is dry (I put my finger in the soil and if I can't feel moisture, I water). About a month ago I started watering more frequently because I thought maybe the leaves were falling off because the plant needed water. I've also been putting in stick miracle grow fertilizer once a month.
The plant is in a room that gets a lot of natural light throughout the day. It's about 5 feet from a huge east facing window. I also tried moving it directly in front of a window a couple weeks ago, but out of direct sunlight. That hasn't helped as the leaves have continued to drop.
I cut off the brown (dead, I assume) leaves last weekend, and there were probably 20-30 of them. Another 5 or 6 are already down and look to be lost.
The leaves have some white stuff on them that I can wipe off with my finger. And there are some black dusty looking things on the bottom of the leaves.
Here are some photos that I hope will help show my problem. I have one photo from when I bought the plant showing how bright, green and leafy it was when I took it home.