  We consider large-N confining gauge theories with a Hagedorn density of
states. In such theories the potential between a pair of colour-singlet sources
may diverge at a critical distance r_c = 1/ T_H. We consider, in particular,
pure N=1 super Yang-Mills theory and argue that when a domain-wall and an anti
domain-wall are brought to a distance near r_c the interaction potential is
better described by an ``open QCD-string channel''. We interpret the divergence
of the potential in terms of a tachyonic mode and relate its mass to the
Hagedorn temperature. Finally we relate our result to a theorem of Kutasov and
Seiberg and argue that the presence of an open string tachyonic mode in the
annulus amplitude implies an exponential density of states in the UV of the
closed string channel.
