So I would advise:

Correct pruning practises that remove infected shoots and leaves to maintain good airflow inside your tree, reducing the damp microclimate beloved by scab.  Scab will also harbour in fallen leaves over winter only to infect next year’s new growth, so it’s a good idea to either rake up fallen leaves in the autumn or collect them with a mower.

Powdery Mildew seems to hit my apple bonsai every year when the sun begins to shine.  Unlike most fungal infections, apple powdery mildew spores do not require moisture to germinate.  Therefore, this infection is known as the “dry weather disease.”

The leaves and blossoms become covered with the fungal spores as they emerge from their buds.  The spores look like a light gray or white powder, and the infected leaves curl upward and eventually die.  These spores are easily blown by the wind and cause secondary infections on new shoots, leaves, and fruit.  As long as the shoots continue growing, the leaves and shoots can continue to become infected.

So I would advise:

You should prune any shoots that appear white in the early spring, so they won’t spread spores.  Regular use of an appropriate fungicide is crucial – especially early spring and then almost weekly through the growing season.  After leaf fall in autumn critically examine the trunk and branches for signs of infected growth…remove anything that concerns you.