What happened to this apple? The small hole tells us more than the rot
You describe a hole and crumbly frass. The photograph shows one apple that has turned brown and shrivelled while nearby fruit looks normal. That combination points more towards damage within the fruit than a disease affecting the whole tree.
This is damage to the fruit itself, rather than something that appears to have affected the whole tree. The hole and crumbly material around it fit a caterpillar entering the apple, often followed by brown internal rot. Codling moth is the most common explanation for this type of apple damage.
As you say only one apple is affected and the others look good, this suggests isolated fruit damage rather than a major outbreak. Remove the damaged apple from the tree and collect any affected windfalls beneath the crown to reduce the chance of caterpillars surviving.
In September, the most important action is clearance rather than treatment. Check fruit at harvest and immediately remove apples with holes, brown sunken patches or crumbly brown waste. If many more show the same damage next season, traps or trunk bands at the appropriate time next summer may be worth considering, but this one apple does not call for major action now.
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