It sounds like your Tamarillo is too dry. These plants have a tendancy to drop a lot of leaves anyway because they direct a lot of nourishment into their fruit. Try adding some evenly balanced liquid fertilizer with an MPK of about 12, 9 and 11.
My Tamarillo plant got off to a shaky start in an exposed part of the garden so moved it to a more sheltered and reasonably sunny spot. It looks much healthier with bright green leaves and what appears to be the beginning of fruit. However, the leaves eventually discolour and if I shake the tree (gently) some drop off. I've fed it with a general fertiliser...is this sufficient?
