I have had problems with some of my Tasmanian seedlings. Some of the leaves have turned yellow, and I have noticed some brown and black blotching.
I have lost some of them and have decided to destroy the plants that show those symptoms for fear of viral infection. Have also seen more aphids than ever before, so perhaps they serve as a vector.
I haven't experienced this before, so not sure of the origin since I grew them from seed sent in sterile media. The majority are growing wonderfully though.