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Morphological Development of Physopella zeae on Corn. M. R. Bonde, Research plant pathologist, United States Department of Agriculture, P.O. Box 1209, Frederick, MD 21701; K. R. Bromfield(2), and J. S. Melching(3). (2)(3)Research plant pathologists, Agricultural Research Service, Plant Disease Research Laboratory, P.O. Box 1209, Frederick, MD 21701. Phytopathology 72:1489-1491. Accepted for publication 27 April 1982. 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, 1982.. DOI: 10.1094/Phyto-72-1489.

Free moisture on the surface of leaves for 12 hr at 22 C induced uredospore germination of Physopella zeae. By 5 hr, numerous large appressoria had developed, usually over anticlinal walls of leaf epidermal cells. Leaf penetration was directly through the leaf cuticle and was first observed at 12 hr following inoculation and placement of plants into dew chambers. A primary hypha (average maximum width 9.8 μm) grew from the penetration peg and by 28 hr averaged 18 μm in length. Secondary, narrow (avg 4.3 μm wide) hyphae grew from primary hyphae by 24 hr. The pathogen colonized the epidermis and mesophyll tissues as secondary intracellular hyphae. Although penetration was never observed before 12 hr (ie, absent in 5- to 10-hr samples), plants removed from dew at 56 hr and placed in a greenhouse still developed pustules. Apparently, once appressoria reach a certain maturity, visible free moisture is not required for subsequent penetration.

Additional keywords: Zea mays, corn rust, Uredinales.