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Postharvest Pathology and Mycotoxins

Pathogenesis in Aspergillus Ear Rot of Maize: Light Microscopy of Fungal Spread from Wounds. M. G. Smart, Fermentation Biochemistry Research Unit and Mycotoxin Research Unit, USDA-ARS, Northern Regional Research Center, 1815 North University Street, Peoria, IL 61604; D. T. Wicklow(2), and R. W. Caldwell(3). (2)Fermentation Biochemistry Research Unit and Mycotoxin Research Unit, USDA-ARS, Northern Regional Research Center, 1815 North University Street, Peoria, IL 61604; (3)Department of Plant Pathology, Russell Laboratories, 1630 Linden Drive, University of Wisconsin, Madison, WI 53706. Phytopathology 80:1287-1294. Accepted for publication 21 February 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/Phyto-80-1287.

We describe the histology of fungal development in maize ears wound inoculated with Aspergillus flavus. Plants were inoculated 21 days after style emergence; wounded grains and adjacent spikelets (with their rachis segments) were harvested at intervals up to 28 days later. Tissues were processed for plastic embedding and 1.5-?m thick sections were examined by bright field microscopy. The fungus spread from the wound sometime after 14 days postinoculation, and at 28 days postinoculation it could be found in small amounts throughout all rachis tissues except the pith and lignified fibers. The fungus entered the rachillae of adjacent spikelets from the rachis and also from the bracts at their insertion point. The fungus grew through the aerenchyma in the rachilla to the floral axis and innermost layers of the pericarp (the endocarp). Hyphae did not penetrate to the endocarp from the exterior of the pericarp. The hyphae were always intercellular in the rachis, rachilla, and pericarp. They were both inter- and intracellular in the floral axis and internal to the testa (i.e., inside the seed proper). From the endocarp, entry into the seed was not across the black layer; random tears in the testa over the embryo were the probable immediate pathway. Hyphae were vacuolate everywhere except in the seed. Host cells died (and even collapsed) ahead of the fungus, but no other structural alterations were seen.