From your description, this sounds like a thrips infestation. Thrips are extremely small insects that multiply quickly in warm, humid conditions and feed by rasping the leaf surface and sucking out sap. This feeding damage often causes leaves to develop a bronze or silvery appearance, and the problem can spread to nearby plants.

Thrips are capable of building up quickly through summer, so good garden hygiene and plant management are important to reduce future outbreaks.

To help manage the problem:

• Remove and dispose of infected plant material. Prunings and heavily affected leaves should be removed from the garden rather than composted, as this helps reduce the pest population.
• When pruning your apple tree in winter, ensure there is adequate air movement through the tree - open up the canopy so light and air can move freely through the tree. Thrips tend to thrive in dense growth where air movement is poor and humidity stays high.
• Apply a winter clean-up spray of horticultural oil or lime sulphur over the dormant tree to help reduce overwintering pests hiding in bark crevices.
• Regular inspection during the growing season will also help catch any new infestations early before they spread through the tree or to other plants in the garden.