The modern apple was developed from an ancient stock, the scrab (it's Anglo-Saxon for scrub), or crab apple as it's now called. If you do want to let your tree fruit, try to get an 'ornamental' apple such as Malus (latin for Apple) 'Golden Hornet' as they carry a fruit of about 25mm.

In common with most fruit trees, Apple species are often grafted onto a more vigourous rootstock, or indeed a dwarfing rootstock as in the example shown. Grafting onto a dwarfing rootstock will keep the tree at a managable height, without affecting the amount of fruit.
The area of the graft has a tendency to swell and this can have unfortunate concequences for a bonsai.

Apples are prone to Scab, a disease commonly associated with fruit trees. It is often seen in trees continually fed with high nitrogen feeds. It's unlikely to be seen in a bonsai fed with a well balanced fertiliser.

High Nitrogen feeds promote rapid, soft growth and this, if damaged may allow scab to enter, so watch out.

Scab itself is typified by the shrinking and drying of an area of bark. If you are the victim of such an attack you will find a remedy, usually a spray in your garden center.

Assuming you've stopped the attack in time, you might consider applying a 'Dead wood' effect to the damaged area.

The tree to the right shows both Scab (the Diamond shape at the top) and its associate Canker.

The crab apple's natural distribution is vast,as far north as southern scandinavia, south as the mediteranian and west to asia minor.

However the 'Domestic' apple is now grown worldwide and is both a major economic and food resource (Let's not forget Cider!)

Windfall apples. Traditionally in the UK farmers would release their pigs into their orchards at this time. Great Bacon!

Allen. C. Roffey August 4, 2018 21:58