Biotrophic obligate pathogens are key agents for the clas they are highly host-specific and severely damage vigorous plants sical approach obtaining nutrients directly from living tissues. Agents of this type include those respon sible for the so-called 'rusts', 'smuts' and 'moulds. They rarely kill plants directly, but hav negative effects, comparable to those of herbivorous insects, resulting in wasted energy (Charudattan, 2005). Basically, the microorganism is uncontrollable once released i the environment. To be successful and a the same time not cause harm, the pathogenic