The forward-genetics screen using Barrel clover (Medicago truncatula) Tnt1 retrotransposon insertion lines followed by the authors found that the inhibitor of rust germ tube differentiation 1 (irg1) mutant failed to promote the pre-infection structural differentiation of two rust pathogens, Phakopsora pachyrhizi and Puccinia emaculata, on the abaxial leaf surface. The gene discussed is ACOD1; the disease is infection.