Impossible to say without knowing previous symptoms, I'm afraid. Its even possible that indiscriminate spraying with insecticide has affected the trees. Its always best to diagnose what the problem is before treatment. There are a number of problems which affect fruit trees, some of which are fungal or bacterial, as well as insect infestation, but a 'multi bug' treatment will only deal with the latter.

The leaves may have fallen because of the hot weather, particularly if the ground has dried out and they needed watering. In my part of London, you'd think we'd not had rain for months, the top few inches of soil are so dry.