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I bought this Dieffenbachia about 3 months ago and have been able to keep it well. I watered it once a week misted the leaves once or twice a week. It sits next to a window and gets 5 to 8 hours of indirect light plus is sits directly (4 feet) under a large florescent fixture.
About a month ago, I noticed a leaf starting to die so I cut back on weekly watering and only watered when the soil completely dried based on readings from a moisture meter. I continued to mist the leaves once or twice a week though.
Two weeks ago, my brother came to visit. He noticed that almost all of the leaves were a light green instead of the dark green it should be. He said the soil was too dry and started watering it every other day or so. The last day he was here (9 days altogether), he also said the plant needed fertilizer so I put some Miracle Grow in the water the 9th day. I think I put too much MG in the water though. I also stuck a piece of banana peel in the soil. Within a week the plant started turning yellow and now looks like it does in the photos. I have removed dead leaves, let the soil completely dry-out then watered it with less Miracle Grow. It is steadily dying and has white spots on it.
Is there any hope for this plant?
If so, what can I do: more/less water, more/less fertilizer, more/less light?
I do see signs of mealybugs, but I don't think that's why the plant looks as sad as it does (you should still try to get rid of them though--they certainly won't help the plant recover. I use a q-tip dipped in rubbing alcohol to wipe them off).
Too much fertilizer certainly could have contributed, as could your brother's attempts to help you by overwatering your plant. I would definitely not fertilize any more if you're concerned you added too much already--and if you think that's a possibility you might try repotting in some fresh soil. While you're repotting, trim off any slimy/mushy rotten roots (from overwatering), then go back to your approach of letting the soil dry out a bit in between waterings.
Mealybugs are some of the most destructive plant pests. If they have gotten to the roots they can most certainly impact your plant the way you describe. I would TOSS the plant and don't reuse the soil. If you want to try to save - check out the roots - trim if necessary if water mushy -- toss if the bugs have eaten the roots. If you do try to save repot the plant in a different soil and clean that pot real good.
Oh no Cap'n Kirk, don't give up. Gardeners have big garbage cans, you know, and just because you failed with one plant - yes, this one is due for the can - chalk it up to experience, and try again. If you want to try another dieffenbachia, you need to know that they need to have the soil go almost dry all the way to the bottom of the pot between waterings. Your first instinct was right on, actually, though probably too late. Moisture meters are not very good for beginners, they are often wrong or broken. Much better to get a wooden dowel that will reach to the bottom of the pot, insert it all the way to the bottom, then pull it up and feel it. It should have just the barest trace of moisture, and only a few bits of soil clinging to it. When you water, allow the water to run out of the drainage holes, then don't water again until the soil has dried again.
So your plant was too wet, and the roots were damaged. Then the mealy bugs set in - they're always out there looking for ailing plants. There are a number of things you can do if you see them in the future, which you can research. But you had such a bad infestation that now they are everywhere, and you'll have to clean and disinfect the surfaces the pot was on, the window sills, floor, under rugs, and of course that pretty pot your unfortunate plant was in.
Another thing is that you never want to fertilize an obviously sick plant. Try to find out what the problem is, and don't fertilize until the problem has been corrected and the plant is looking healthy again.
Don't be angry with your brother - he didn't understand any more than you did, and he just delivered the coup-de-grace with all that water. But growing indoor plants can be immensely satisfying and take up very little time, if you just learn a few basics. So suck it up, and full speed ahead.
A case of bugs that bad didn't happen in 3 months. This plant was doomed when you got it, you just didn't know until it looked doomed. No reason to not try another plant, probably from another store. Look closely at it, under the leaves, in the growth tips. If you see anything unusual, pass.