I've spotted a flower that is dead:
It looks like most of the tissue has rotted away and only the veins remain.
(Sorry, I don't know the proper name for flower veins).
What is the technical term for this phenomenon?
Retting could be a valid term. But I think that the term "skeletonization" is more common. For instance here is a link to the University of Arizona showing how you would take a flower or a leaf and remove all components, leaving the veins: Skeletonization
I think that "retting" is confined to the industrial harvesting of plants like flax. I would use the term "skeletonization" to include acts of botanical science in which the veins alone were sought. For instance in the above link the botanist Frederick Ruysch published a method of chemical skeletonization in his book Adversariorum Anatomico - Medico - Chirurgicorum Decas Tertia in 1723.
It also became a form of art and craft in the late 1800's in the United States; "Skeleton Leaves & Phantom Flowers" was published by E.Parrish. I would assume that the end result as a craft would be to display the skeletonized flowers or leaves on a display board.
As a natural, observed event the term skeletonization could be used. But if the underlying causes such as fungus were considered then both rot, or fungal rot, and skeletonization might be used together in the same sentence. The wikipedia entry skeletonization considers animal decomposition. It's a reasonable and analogous step to use the same term for plants that decompose and leave fibres rather than bones.
I have heard the process that leads to this called retting.