Blackening on yam in response to Colletotrichum gloeosporioides

A blackening confined to the cuticle and/or outer epidermal cell walls of mature leaves of Dioscorea alata was found to be associated with the appressoria of the fungus Colletotrichum gloeosporioides on plants subjected to high levels of inoculum in the field and to dense spore suspensions in controlled tests. The blackening occurred with little evidence of fungal penetration into the host cells. Preliminary analyses of the nature of the blackening detected melanin‐like compounds and suggest the involvement of phenolic compounds and water‐soluble chemicals, possibly enzymes, in the blackening process.