Development of the Uredinial Thallus and Sorus in the Orange Coffee Rust Fungus,Hemileia vastatrix

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Abstract
Orange coffee rust is a critically important disease, but the development of the pathogen within the host had not been carefully studied. Infection of coffee [Coffea arabica] plants with urediniospores of H. vastatrix resulted in radially expanding lesions containing numerous sori. Lesions were sorted into zones of pioneer hyphae, 1st haustoria, nutritive hyphae, protosori (unemerged incipient sori), and immature, mature, and senescent sori. The zones of nutritive hyphae and protosori corresponded with a band of chlorosis on the leaf surface. Sori were formed in about 1/3 of host substomatal cavities. Protosori developed from a layer of isodiametric cells, distinct from the constituent hyphae. Some protosori, although they developed in mesophyll cavities not beneath stomata, apparently grew into the correct position for emergence. Two or 3, later 7 or more, sporogenous cells exited a stoma as a tight fascicle and bore spore buds in a spiral fashion. Spines were only on the upper surfaces of the urediniospores and appeared when the spores were 1/4 to 1/3 their full size. Developing sori were covered by a matrix that appeared mucilaginous. The pattern of expansion of the H. vastatrix mycelium through the leaf, with continual production of new sori, was the starting point for a new model of the continuum of thallus complexity levels in the rust fungi.