Infection of winter wheat by a beta-glucuronidase-transformed isolate of Cephalosporium gramineum

Field-grown winter wheat was inoculated with a beta -glucuronidase-transfor med isolate of Cephalosporium gramineum in two field seasons to elucidate t he mode of infection in resistant and susceptible cultivars. Colonization o f viable root epidermis and cortical cells occurred as soon as 15 days post inoculation and the pathogen was found in the vascular tissues by 20 days p ostinoculation, well before freezing soil temperatures occurred. Penetratio n occurred directly through the root epidermis and through wounds adjacent to emerging secondary roots. The pathogen also penetrated through root cap cells and colonized meristematic tissues near root tips to gain access to t he vascular system. Lower stem base colonization was observed where the pat hogen penetrated directly through the epidermis, wounds, or senescent tissu es. Appressorium-like structures, which appeared to aid penetration of cell walls, were often found within cells of both roots and stems after initial colonization. The mechanisms of resistance were not apparent, but less col onization occurred in resistant than in susceptible cultivars.