Diversity of Entomopathogenic Fungi

More than 90 genera and 700 species of fungi have been reported to be etiological agents in insect diseases. The term "fungus" was redefined when molecular tools indicated that some "fungi" were no longer considered within this group. This was the case for Oomycets, now associated to brown algae (Chromista or Stramenopila) rather than to fungi,97 making the chitin (and not the cellulose) content of their cell walls a definitive feature of true fungi, which belong to the kingdom Mycota. This new classification somewhat hampers the definition of entomopathogenic fungi, as a few of them belong to the Class Oomycetes.98 So, for practical purposes only, we will consider insect-infecting oomycetes within the entomopathogenic fungi, and for the same reason, the recent integration of Microsporidia within the fungi will not be included in this review,99 as their diversity and mode of action is totally different to what "used to be" fungi.

Figure 8.7 White grub (Coleoptera: Scarabaeidae) covered by a mat of mycelia, sporangia, and conidia of the green muscardine, Metarhizium anisopliae. Under appropriate conditions, the fungus inside the mummified cadaver grows out through the insect cuticle and sporulates to form a mat of infecting conidia.

Figure 8.7 White grub (Coleoptera: Scarabaeidae) covered by a mat of mycelia, sporangia, and conidia of the green muscardine, Metarhizium anisopliae. Under appropriate conditions, the fungus inside the mummified cadaver grows out through the insect cuticle and sporulates to form a mat of infecting conidia.

Entomopathogenic fungi have been organized in four major classes: Chytridiomycetes, Oomycetes, Zygomycetes and Ascomycetes. There are two genera of Basidiomycetes infecting insects; however, rather than a pathogenic relationship, these show a commensalistic relationship with the insect, causing no serious deleterious effect. One additional consideration is that the class Deuteromycota is now considered within the class Ascomycota. Although most Deuteromycota (Hyphomycetes) only show the asexual (anamorphic) reproductive phase and lack the typical ascospores, asci, and asocarps of Ascomy-cota, their molecular characterization has shown an unambiguous relationship between both groups.

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