I have some varieties of malus in trainig, most grown from seed since 1999.

Since last year the zumi calocarpa has presented a yellowing in some leaves, beggining mostly in the central region, that turns a bright orange brown and spreads. It specially happens on the grown leaves.

The trees appear healthy, keep growing.

The floribunda and sargentii appear imune.

We are in summer now and last year, the first this lesion appeared, it was also summer.

I took some pictures that I can send.

Any idea of what this is ?

Thank you.