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Cytology and Histology

The Disease Cycle and Control of Geranium Rust. C. A. Harwood, Graduate student, Department of Plant Pathology, University of California, Berkeley, 94720; R. D. Raabe, professor, Department of Plant Pathology, University of California, Berkeley, 94720. Phytopathology 69:923-927. Accepted for publication 17 December 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-923.

Germination of urediniospores of Puccinia pelargonii-zonalis required liquid water and occurred within 3 hr at the optimal temperature of 16 C. The pathogen usually formed appressoria and penetrated through the stomata 5–6 hr after inoculation. The hyphae were intercellular with intracellular haustoria. Symptom development was most rapid and extensive at 21 C. Of 35 Pelargonium spp. inoculated, only seven were susceptible. Cultivars of P. hortorum varied in susceptibility and isolates of the fungus varied in virulence. Viability of urediniospores from air-dried leaves dropped from 100 to 0.8% in 12 wk. Four fungicides (triadimefon, mancozeb, oxycarboxin, and triforine) effectively controlled P. pelargonii-zonalis on geraniums in greenhouse and field experiments.