Shoot, there are things one should do to prevent powdery mildew...such as fans blowing 24/7, spraying with 9:1 water/milk solution. You've got more going on than powdery mildew. Looks like too high of salts in your soil from tap water, fertilizers and perhaps a virus. The leaves are distorted and crinkly.
What a gorgeous plant for a Bonsai. I am seeing that there should have been a few tip prunings that have not happened.
Stop fertilizing. Cut this fertilizer stuff by half or even 3/4. Hopefully this soil is potting soil, yes? The branch from the main stem that goes to the right should be the top of this plant. Cut back to just above that branch without leaving a stub. Cut off these distorted leaves and their branches. Get a fan going. You should be working on root pruning by now and that doesn't mean just wacking off roots.
Your plant still looks very viable, hopefully this isn't a virus, just fertilizer out of balance. Too much fertilizer is worse than too little. Always remember that. This looks like a good product but I always cut fertilizer directions in half because I can always tell what is too much, what is too little and the plant tells me these things.
Growing and cultivating a Bonsai is like the top of all gardening skills. Completely artificial. You are completely in charge of this plant's life. Very rewarding and educational if one can keep that plant healthy or very discouraging if the plant dies.
Stop using tap water. A huge source of toxins and salts. Get bottled or distilled water without fluoride. Use rain water if you are able to collect. Not saying our rain water doesn't have problems but better than tap water.
And most importantly, sign up for a hands on Bonsai class by someone who knows this art! No book or internet can teach you these things. This is such a beautiful start for a Bonsai! If you fail, try not to be so bummed out that you resign from the plant world! The Japanese took thousands of years to perfect this art so don't think you could ever be successful without classes by a master.
All of our landscapes, gardens, potted plants are completely artificial. That puts the onus on us to learn the entire bunch of systems involved to include chemistry, hydrology, pruning, basic botany, light energy, photosynthesis, soils...on and on and on.
Look up a class by a real master of Bonsai. That is the only way you'll ever be able to keep a Bonsai alive as well as appreciate what that Bonsai is teaching you.
