Given a function field K and φ ∈ K[x], we study two finiteness questions related to iteration of φ: whether all but finitely many terms of an orbit of φ must possess a primitive prime divisor, and whether the Galois groups of iterates of φ must have finite index in their natural overgroup Aut(T d ), where T d is the infinite tree of iterated preimages of 0 under φ. We focus particularly on the case where K has characteristic p, where far less is known. We resolve the first question in the affirmative under relatively weak hypotheses; interestingly, the main step in our proof is to rule out "Riccati differential equations" in backwards orbits. We then apply our result on primitive prime divisors and adapt a method of Looper to produce a family of polynomials for which the second question has an affirmative answer; these are the first non-isotrivial examples of such polynomials. We also prove that almost all quadratic polynomials over Q(t) have iterates whose Galois group is all of Aut(T d ).v(φ n (b) − a) > 0 and v(φ m (b) − a) = 0 for all 1 ≤ m ≤ n − 1.Of course, one expects that most terms in the sequence φ n (b) − a have primitive prime divisors, and to measure the failure of this heuristic, we define the Zsigmondy set of φ and the pair (a, b) to be Z(φ, a, b) := {n : φ n (b) − a has no primitive prime divisors}.Evidence suggests that Z(φ, a, b) is finite, unless the tuple (φ, a, b) is special in some way; for instance, if b has finite orbit under φ, then Z(φ, a, b) is infinite for arbitrary a ∈ K, and the same conclusion holds for a = 0 and any b in the case φ(x) = x d . If K has characterisitc zero, then Z(φ, a, b) is known to be finite in many cases [5,13,14,22], with the strongest results coming over function fields [7].On the other hand, there are very few results known for fields of positive characteristic. Nonetheless, in [9, 12] the first author was able to prove the finiteness of Zsigmondy sets for 2010 Mathematics Subject Classification: Primary: 11R32, 37P15. Secondary: 14G05.