Hmm, I beleive conjecture is used when you are speaking about a potential theorem (i.e. something than can be mathematically proven to be right given certain axioms). This is a biological/chemical/natural theory, and is believed to be true given certain findings in some observed phenomena (i.e. statistics are involved in a certain way).
A conjecture is an assertion made on the basis of incomplete evidence. I'm not familiar with Turing's work in this area but if (as others have commented) he studied PDEs and that appear to mimic biological processes, yet did not find a definitive link, then the conclusion that a connection between the two may exist can only be described as conjecture.
