Looks like its just the older, lowest leaves. Maybe they are just getting ready to drop to make way for the new leaves that are coming. It is summer now so hopefully you'll get lots of new growth.

How long has the plant been in that container?

From the photo, it doesn't look like the soil has good drainage. Do you have plenty of perlite or pumice mix in to the soil?

Hello

The plant has been in the container for a quite a while. The plant is in a pot with normal potting mix. But with my other apple custards, their leaves are not turning brown.

Thanks
Cara

I did a check of mine but don't see anything like that happening. If it's only on the underside of the leaves and the upper surface looks normal I wouldn't worry. Just keep an eye on it to see if it changes or spreads a lot further.

Did you grow them from seed, from fruit you bought?

Hi

I did grow it from seed. It hasn't spread that quickly, but it is turning the upper surface of the leaf yellowy, browny, but still a bit green.

Could it be when I moved the apple custard from the greenhouse to direct sunlight?

Thank You

I notice they're the lower leaves, so just might be normal senescence. Damage from sun normally looks a bit more 'dramatic' than that. Usually the youngest leaves are the worst affected. But it isn't a good idea to move any plant suddenly from shade to full sun. Are you in the North Island?

When you say 'Apple Custard', you mean

Annona squamosa (Sugar Apple in the USA), right?
That is what it appears to be from the leaves.

Here is a link that might be useful: Sugar Apple (Annona squamosa)

Hi

I do live in the north island. I think it may also be called cherimoya. Could the leaves turning brown be caused by the pot not being big enough or heats changes when I moved it outside from the greenhouse?