Rust?

I was recently asked if I would take on watering a couple of newly planted apple trees in the allotment's communal orchard, as my plot was very close and I had a hosepipe that would reach. 

This week when watering them I noticed that both had developed rust or some similar problem, I let the committee member who looks after the orchard know, she will be going to look at them, but as she is in Germany at the moment she can't do it until she is back in the UK.