The apple is Malus domestica, and the correspondence attached to it rests on something genuinely observable: cut an apple across its equator rather than through the stalk, and the seed chambers form a clear five-pointed star.
This is not a coincidence or a stretch. The Rosaceae, the rose family, are characteristically five-parted, five petals, five sepals, and the apple's carpels follow, giving five locules arranged in a regular pentagon. Any apple will do it. Renaissance writers read the figure as the pentagram of Venus, and the association of Venus with the number five has an independent astronomical basis: successive inferior conjunctions of Venus trace an approximate five-pointed figure against the zodiac over eight years, a pattern known to Babylonian astronomers.
So the chain here is unusually solid at both ends. The apple really does contain a pentagram; Venus really is associated with five; Culpeper places the apple under Venus and Taurus takes it as her earth sign. What does not follow is any pharmacological or medicinal consequence, and that is where the doctrine of signatures stops being observation and starts being inference. A visible five-fold symmetry is a fact about the flower's developmental genetics. Reading it as a message about the fruit's virtues requires a theory that the universe writes labels, which is the doctrine, and it is false.
It remains one of the better-motivated correspondences in the material, in the narrow sense that the thing it points at is actually there.