Plant pathogens are used in non-classical inundative releases of mycoherbicides, as discussed by Auld (1997a), and in classical biological control of weeds, as discussed in this section.

Of all pathogens, fungi have received most attention for the biological control of weeds because they are the most common class of pathogens attacking higher plants and are the easiest pathogens on which to work. Of the 80 or so species or strains of pathogens being studied as potential weed biological control agents in the early 1980s, about 70 were fungi and the remainder were nematodes, viruses or bacteria (Templeton 1982). Of the 67 projects involving fungi, 38 involved Deuteromycetes, mainly for inundative release, 18 involved the Basidiomycetes, seven involved Ascomycetes, and four involved Phycomycetes (Templeton 1982). The rust fungi (Basidiomycetes) have been the most common candidates for classical biological control, perhaps because of their high degree of host specificity. Pathogenic fungi deliberately introduced into various countries as classical biological control agents of weeds are listed in the Table below.