  We study the effect on the zeros of generating functions of sequences under
certain non-linear transformations. Characterizations of P\'olya--Schur type
are given of the transformations that preserve the property of having only real
and non-positive zeros. In particular, if a polynomial $a_0+a_1z
+\cdots+a_nz^n$ has only real and non-positive zeros, then so does the
polynomial $a_0^2+ (a_1^2-a_0a_2)z+...+
(a_{n-1}^2-a_{n-2}a_n)z^{n-1}+a_n^2z^n$. This confirms a conjecture of Fisk,
McNamara-Sagan and Stanley, respectively. A consequence is that if a polynomial
has only real and non-positive zeros, then its Taylor coefficients form an
infinitely log-concave sequence. We extend the results to transcendental entire
functions in the Laguerre-P\'olya class, and discuss the consequences to
problems on iterated Tur\'an inequalities, studied by Craven and Csordas.
Finally, we propose a new approach to a conjecture of Boros and Moll.
