Penetration and colonization of unwounded maize tissues by the maize anthracnose pathogen Colletotrichum graminicola and the related nonpathogen C. sublineolum.

The maize anthracnose stalk-rot fungus Colletotrichum graminicola infects its host primarily through wounds in the stalks that are caused by insects. However it also can cause stalk-rot disease without wounding. It is not known how the pathogen enters stalks in the absence of wounds. Studies have suggested that direct invasion through the highly lignified rind tissues is not a viable means of entry. A cytological approach was used to investigate the ability of C. graminicola to penetrate and colonize intact maize stalks. The pathogen had a significant capacity for direct penetration, but this mechanism of infection was much slower and less efficient than penetration through wounds. The fungus breached the lignified rind fibers by passing through small openings in the cell walls via narrow hyphal connections. Epidermal cells and rind fiber cells did not appear to become rotted. Rotting only occurred once the pathogen had penetrated into the pith parenchyma cells. To our surprise the closely related fungus C. sublineolum, which is not normally a pathogen of maize, also was capable of infecting intact maize stalks, although to a lesser degree than C. graminicola. The two species also were observed on intact roots and leaves, and C. sublineolum was incapable of infecting those tissues whereas C. graminicola efficiently colonized both. This suggests the interesting possibility that nonhost resistance to C. sublineolum is conditional and perhaps also tissue-specific.