The turbulent advection of a reacting substance is considered. The existenc
e of two different states, one smooth and the other filamental, is discusse
d. The transition between these states is characterized in terms of the com
petition of the stretching and folding mechanism of the turbulent field and
the tendency of the scalar field to relax to the non-homogeneous steady st
ate. (C) 2001 Elsevier Science B.V. All rights reserved.