Observations of Colletotrichum dematium f. circinans on Allium cepa: halo formation and penetration of epidermal walls

Abstract

Spores of Colletotrichum dematium f. circinans (Berk.) cv. Arx, the causal agent of onion smudge, were inoculated on to epidermal bridges of white, yellow and red onions (Allium cepa L.). Penetration and halo formation were observed using microscopy and cytochemistry. Penetration occurred by 24 h and halos were visible with interference and phase-contrast microscopy, but not with bright-field or polarized light microscopy. Halos appeared as sunken areas with scanning electron microscopy (SEM). Interference microscopy indicated that the dry mass of halos was less than comparably sized areas of non-infected cell wall. Cytological staining and SEM indicated that the cuticle was not disrupted. However, pectin was removed from the walls, and aldehydes were accumulated in halos. Cellulose stains accumulated in halos, but there was no indication of degradation of cellulose. Deposits were found beneath initial penetration sites. Penetration of the wall is thought to involve enzymes.