Melanins are darkly pigmented polymers that protect organisms against envir
onmental stress. Even when not directly involved in pathogenesis, fungal me
lanin is likely required by melanizing phytopathogens for survival in the e
nvironment. However, some phytopathogenic fungi that produce melanized appr
essoria for host invasion require appressorial melanogenesis for pathogenic
ity. Much less is known about the role melanins play in pathogenesis during
infection by other phytopathogens that do not rely on appressoria for host
penetration. Here we focus on one such phytopathogenic fungus, Gaeumannomy
ces graminis var. tritici, the etiologic agent of the devastating root dise
ase of cereals, take-all. This fungus is lightly pigmented in culture, but
requires melanin biosynthesis for pathogenesis, perhaps to produce melanize
d, ectotrophic macrohyphae on roots. However, the constitutively melanized,
asexual Phialophora anamorph of G. graminis var. tritici is nonpathogenic.
In addition, melanization of G. graminis var. graminis is not required to
produce root disease on its rice host. Explanations for these apparent cont
radictions are suggested, as are other functions for the melanins of phytop
athogenic fungi.