The Disease Cycle and Control of Geranium Rust

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Abstract
Germination of urediniospores of P. pelargonii-zonalis required liquid water and occurred within 3 h at the optimal temperature of 16.degree. C. The pathogen usually formed appressoria and penetrated through the stomata 5-6 h after inoculation. The hyphae were intercellular with intracellular haustoria. Symptom development was most rapid and extensive at 21.degree. C. Of 35 Pelargonium spp. inoculated, only 7 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.

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