Counting curves of any genus on P^2_7

We compute Gromov-Witten invariants of any genus for del Pezzo surfaces of degree $\ge2$. The genus zero invariants have been computed a long ago, Gromov-Witten invariants of any genus for del Pezzo surfaces of degree $\ge3$ have been found by Vakil. We solve the problem in two steps: (1) we consider the plane blown up at 6 points on a conic and one more point outside this conic (weak Fano surface), and, using techniques of tropical geometry, obtain a Caporaso-Harris type formula counting curves of any divisor class and genus subject to arbitrary tangency conditions with respect to the blown up conic, (2) then we express the Gromov-Witten invariants of the plane blown up at 7 points via enumerative invariants of the weak Fano surface, using Vakil's version of Abramovich-Bertram formula.