Micrograph through a pustule of Albugo candida (White Rust) on the leaf of a Shepherd's Purse.
The sporangiophores form a tight layer at the open surface of the pustule under the epidermis of the host. Eventually weathering, and internal pressure caused by growth of the pustule, causes the epidermis of the host to break open and expose the fertile layer. Each sporangiophore blips off a linear series of spherical, hyaline, sporangia to form a white powdery pustules. This is the aesxual reproductive stage whose purpose is to spread the disease throughout the season whewn opportunity permits.
At first glance the sporangiophore might appear like a conidiophore producing a chain of conidia. However, when the wind blown sporangia reach another suitable host they germinate (under suitable conditions of moisture and temperature) to release a number of biflagellate zoospores that swim over the surface of the host leaf before encysting and penetrating to begin a new disease cycle.
Ass seenin the micrograph above, the Oogonium and Antheridium are produuced deep in the host tissues. Fertilization results in a solitary, thick walled oospore that is only released after the host has died and the host tissues have degenerated to the point where the thick walled resting spore can be released back into the environment where it can persist for long periods until stimulated to produce new infections.
(Image from MycoAlbum CD by George Barron see www.mycographics.com)