I noticed some white fungi growing around the base of my bonsai, now there is more along with brown fungi as well. A lot of the leaves are dying and it’s makimg me quite sad the plant hasn’t even gotten to flower for the firs time. I used a fertilizer stick when I first got my plant could this have caused it?
Could what looks like white fungus actually be an accumulation of fertilizer salts? I don't know enough about bonsai to say how best to fertilize them but I would assume they need very little. A fertilizer stick is a very blunt instrument to use in this situation - not knowing what, if any, nutrients were necessary when you planted the tree. Also, can you say what type of tree it is?
I agree with the white being just salts, maybe calcium, too. Leaves are not forever. Yours may last more than a year, but not much longer. Sooner, or later, they are going to die and their death reflects the patterns of death in every species: we go downhill with age, our corpus gets less efficient, eventually we turn brown. The eldest leaves of the plant are those closest to the trunk on any branch, and lower branches are older than upper branches, and those will die before the leaves closer to the ends of the branches. As long as this is true, your plant is living life as normal. Also, as light is reduced, as in autumn, the process is accelerated. Generally speaking it is poor practice to fertilize when the plant is not in a high growth period, like now. Start in ~April, stop in August. Think about a football player who eats in March like he does in September... he'd just get fat, not stronger and not healthier. See this page for more info. I have added a couple bonsai instruction .pdf files that may be helpful.
Hm, yes looks like a build up of salts. Is that a bit of roots I see at the base? I'd be adding more soil to cover those roots up nicely. Assure the plants roots are covered entirely. Could the plant stick be the source of these salts, possibly calcium build up? Hard to say but I suppose is possible. Watering well helps leach those sort of build ups. Nice lush growth, wow. I think the browning of the leaf there is possibly due to the roots being exposed. Cover 'em up good. I do not see this as a mold problem.