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Histopathology of Colonization in Leaf Tissue of Castilleja, Pedicularis, Phaseolus, and Ribes Species by Cronartium ribicola. Robert F. Patton, Professor, Department of Plant Pathology, University of Wisconsin-Madison 53706; Russell N. Spear, Senior research specialist, Department of Plant Pathology, University of Wisconsin-Madison 53706. Phytopathology 79:539-547. Accepted for publication 26 October 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-539.

The white pine blister rust fungus has been recognized as comprising two formae speciales, Cronartium ribicola f. sp. ribicola and C. ribicola f. sp. pedicularis, each with different alternate hosts and different expressions of pathogenicity to some pine species. We observed interactions of a Wisconsin isolate of C. r. ribicola with a conventional alternate host, an alternate host commonly infected by the East Asian form (C. r. pedicularis), and plants considered as nonhosts for our North American form. Leaf samples collected at different times after inoculation with urediniospores were examined by light microscopy for penetration and infection or colonization. In the conventional alternate host, Ribes nigrum, the fungus achieved an established infection through the formation of haustoria, which were first visible 6 days after inoculation. The development of typical symptoms and signs followed within 79 days after inoculation. In Pedicularis resupinata, the host for the East Asian form, penetration and early stages of colonization were similar to those stages in R. nigrum. Infection hyphae branched sparingly in the intercellular spaces and occasionally reached as far as the upper epidermis between 5 and 16 days after inoculation but did not develop further. Colonization in the native P. canadensis and the common bean, Phaseolus vulgaris, was similar to but slightly less extensive than in P. resupinata. Colonization in localized portions of leaves of some plants of Castilleja miniata was much more extensive than in the other nonhost species. In leaf sections taken 63 days after inoculation, intercellular space, in portions up to 2 mm long, was densely packed with profusely branched hyphae. Haustorial mother cells developed in all species, but no haustoria were formed in any species except R. nigrum. The results supported previous indications of a difference in pathogenicity between the two forms of C. ribicola. The surprising amount of colonization in C. miniata led to speculation about the possible presence of the East Asian form of the rust in the Pacific Northwest, or the possibility of adaptation of C. miniata as an alternate host for our North American form.