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Cytology and Histology

Histology of the Suscept-Pathogen Relationship Between Glycine max and Phakopsora pachyrhizi, the Cause of Soybean Rust. M. R. Bonde, Research Plant Pathologist, Plant Disease Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 1209, Frederick, Maryland 21701; J. S. Melching(2), and K. R. Bromfield(3). (2)(3)Research Plant Pathologists, Plant Disease Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 1209, Frederick, Maryland 21701. Phytopathology 66:1290-1294. Accepted for publication 28 May 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1290.

Uredospores of Phakopsora pachyrhizi germinated on soybean plants 1-2 hours after inoculated plants were placed into a dew chamber at 20 C in the dark. Appressoria began developing within 2 hours, and within 5 hours many had grown to nearly the size of their parent spores. Many appressoria were sessile to their parent spores; measured germ tubes varied from a few µm to at least 320 µm in length. Eighty-five percent of the appressoria developed over anticlinal walls of epidermal cells. Penetration into the leaf epidermis always occurred directly through the cuticle; the earliest that penetration was observed was 7 hours after plants were placed into the dew chamber. Penetration was by means of a structure, the transepidermal vesicle, which transversed the lumen of the invaded epidermal cell. Transepidermal vesicles initially were 3 µm in diameter and eventually grew to form cylindrical structures with an average maximum diameter of 8 µm. By 22 hours a hypha often had grown from the distal end of the transepidermal vesicle, had emerged from the invaded epidermal cell, and had formed an intercellular primary hypha in the leaf mesophyll tissue. From the primary hyphae, intercellular secondary hyphae grew, branched, and within 8 days many had extended to at least 400 µm.