Outbreaks of the green peach aphid in spring are usually associated with good growing conditions.
Green peach aphid
Aphid populations can be reduced by hot, dry winds and conditions which harden tree growth.
A number of biological control agents can reduce aphid populations and should be encouraged.
Avoid using pesticides which will kill these biological control agents.
Avoid excessive amounts of nitrogen fertilisers which promote growth of soft plant tissue, and prune out water shoots.
Control weeds around the orchard as these act as reservoirs for migrating aphids.
See this article in Tree Fruit Sept 2019