Reduced Anthracnose Stalk Rot in Resistant Maize is Associated with Restricted Development of Colletotrichum graminicola in Pith Tissues

Resistance to anthracnose stalk rot (ASR) in maize was investigated for its effects on the development of Colletotrichum graminicola. ASR and fungal presence in pith tissues of resistant and susceptible genotypes, inoculated at time intervals after wounding in the first internodes, were assessed by rating tissue discoloration and by quantifying ergosterol production using high performance liquid chromatography (HPLC) and fungal recovery from tissues, respectively. Slices (30 mu m thick) of pith cores (2 mm diam) of first internodes at late-whorl and kernel blister stages were also inoculated with a suspension of fungal conidia immediately, 2 or 6 h after slicing. Fungal development was observed in tissues by light microscopy. ASR was markedly reduced in resistant genotypes when compared to susceptible genotypes and when inoculation was delayed after stalk wounding. Ergosterol content in tissues was associated with extent of discoloration due to ASR and fungal recovery. Conidial germination, germ tube elongation, appressorium formation and penetration of cortical cells were all markedly delayed in resistant genotypes, in both resistant and susceptible maize at vegetative stages, and by wound healing. C. graminicola macerated more rapidly and to a greater extent pith tissues of susceptible than resistant genotypes. Resistance mediated by maize genotype and ontogeny, and wound healing is expressed at early stages and subsequent events of host-pathogen interaction. Fungal structural development in detached pith tissues and the rapidity and extent of pith maceration in susceptible when compared to resistant genotypes was indicative of genotypic reaction to ASR in maize in the field. Laboratory inoculation and observation of detached pith tissues could be a useful and accurate tool for rapid screening of maize germplasm to identify ASR resistant genotypes that will function well in the field even where pathogen ingress occurs via wounds.