Infection of Nicotiana species by the anthracnose fungus, Colletotrichum orbiculare

Colletotrichum gloeosporioides f. sp. malvae, isolate Biomal(R), ATCC 20767 , was originally isolated from round-leaved mallow (Malva pusilla) and deve loped as a weed biocontrol agent. Ribosomal DNA sequence analysis was recen tly used to re-classify this fungus as C. orbiculare, which is an aggregate species with a number of formae speciales. Several morphological features of ATCC 20767 were examined that were consistent with those described for C . orbiculare, and inoculation of a number of Nicotiana species and several cultivars of N. tabacum showed that this fungus was pathogenic to many of t hese previously undescribed hosts. Spore germination and appressorium forma tion were higher on tobacco than previously observed on round-leaved mallow . The pathogen produced melanized appressoria on N. tabacum leaves that for med preferentially at the anticlinal epidermal cell wall. A symptomless pha se of infection persisted for 72-96 h postinoculation, during which time th e fungus first produced a spherical infection vesicle from an infection peg , and then large primary hyphae which grew through the epidermal cells. The large primary hyphae were highly constricted at the points of penetration of the host cell walls. Thin secondary hyphae appeared at 96-120 h postinoc ulation coinciding with the appearance of light green, water-soaked spots a nd the formation of acervuli. The infection of tobacco by C. orbiculare ATC C 20767 is not a non-specific interaction but appears to follow an intracel lular hemibiotrophic infection process that is very similar to that establi shed for the C. orbiculare infection of round-leaved mallow, cucurbits and beans.