Question
I have a collection of different plants, and it all started when I bought a poinsettia for Christmas, and then, it's leaves went brown before it can finish growing!
It died, and then my rosemary has the same problem and then my mint. My bonsai have started, but I cut the dead leaves away and I don't know if it will die. Is it a disease or just some kind of insect eating them?

Answer
Hi Kate:
Your question is a big one.  There are a myriad of possibilities that can cause leaves to turn brown.  If it is a insect possibility, usually you can see something on the leaves or stems with something like a magnifying glass. Look for the classics like aphids or mites. Look for anything crawling.
60% of the time, it is an unfavorable growing environment rather than a disease or insect.  Check for too much or not enough water in the soil, is it in a place where it may be too hot or too cold, too much light or not enough.  Lots of things can go wrong.  Someone really needs to see these plants in person and up close to get a better idea.  A good place to start is your local garden center or nursery. You can carry a plant in or describe the problem in person to them, but again, someone really needs to evaluate the plants in person to see what is going on.  I have mentioned some things to check first before carry a plant in to the garden center.
Believe it or not, my thumbs aren't as green as I would like them to be.

Regards
Steve