Figure 1. Life cycle.

Thick walled dormant sexual oospores (A) are produced when plants are infected with both the A1 and A2 mating types. Asexual reproduction following infection rapidly creates very large populations of sporangia on the surface of the infected plants (B–D, G). Sporangia can detach and cause infection directly or, in the presence of free water, release 20–40 bi-flagellate swimming zoospores (E) that swim to plants and cause infection indirectly (F).