Hello. On the stem you can see the presence of a caterpillar, most likely belonging to the genus Iphiclides (possibly the species Iphiclides podalirius). It is recommended to remove and destroy the damaged leaves, and then to carry out 2 treatments with insecticides such as (in a dose of 5 ml/ 7 L water) and (in a dose of 5 ml/ 10 l water), alternating the products at intervals of 7-10 days between treatments, according to the instructions on the product label. Spray the solutions as finely as possible so as to achieve a more even distribution on the plant. More information on how to apply phytosanitary treatments in optimal conditions can be found in our article: Phytosanitary treatments - environmental and labor protection.
In order to stimulate the overcoming of this stressful period, it is recommended to fertilize the fruit tree. More information on this topic can be found in our articles: Peach tree, planting, growing and harvesting and Peach tree treatments, pest and disease control.