What has caused this gelatinous substance to ooze from a developing peach?
This gelatinous substance is often called “gummosis” and can be caused by a number of things, such as disease, mechanical damage, cold damage, or insect damage. In this case, the most likely cause is stink bug feeding damage. When stink bugs feed on developing peaches right after bloom, the fruit are likely to drop from the tree. If feeding occurs while fruit are growing (likely what happened here) scarring, known as cat-facing, appears around the wound site. Feeding damage that occurs on near-ripe peaches may just appear as a sunken area. Damage symptoms often do not show up for a week or more after feeding, making timing insecticide applications difficult. Damage is usually most common on the outside rows of an orchard, especially near wooded areas. Read more about stink bugs in peaches here.