The fungus causing pelargonium rust is, like all rusts, a biotroph: it infects the host tissues for extended periods, feeding on the living cells. Unlike many rust diseases no alternate host is required to complete its life cycle.
Puccinia pelargonii-zonalis produces only two types of spore and only one of these, the rusty brown urediniospores, appear to be involved in the disease cycle. The other spore type is rare and has not been observed to germinate. The urediniospores are capable of surviving for several weeks in leaf debris.
The rusty brown spores are spread by wind and water splash and germinate on the leaves, immediately penetrating into the leaf tissues. The fungus forms feeding structures called haustoria, which extract nutrients from the leaf cells. Later the fungus forms fresh spores in pustules which burst out of the leaf surface.
The spores require moist conditions to germinate and infect, and the disease is often more severe during cooler months on overwintering plants under glass, but can also be very damaging outdoors during wet summers. Infection can also be spread via cuttings. Like many rust diseases, pelargonium rust can have a lengthy latent period (when the plant tissue is infected but no symptoms are yet visible). The latent period can frequently be as long as two weeks (sometimes much longer), and in this time cuttings from an infected batch may have been transported to many destinations.
This rust originates in southern Africa, but has now spread around most of the world. It was first detected in the UK in 1965.