Most of the RHS Gardening Advice enquiries regarding rust occur on the following hosts: allium species, antirrhinum, bluebell, box, chrysanthemum, fuchsia, hollyhock, hypericum, mahonia, pear, pelargonium, poplar, rose and vinca.
Rust fungi have very limited host ranges. For example, the rust that attacks antirrhinum is a different species from that affecting hollyhock. By late summer there may be many different rust species present on various plants in the garden.
Infection is favoured by prolonged leaf wetness, so rusts are usually diseases of wet summers.
Some rusts spend their entire life on one plant and produce just one or two types of spore. Others need two, often completely unrelated plants in order to complete their life-cycle, and can produce up to five different spore types. European pear rust, for example, spends part of its life on juniper.
The colour of the rust pustules varies according to the rust species and the type of spore that it is producing. Rose rust for example, produces orange pustules for much of the summer, but in late summer and autumn these are replaced by black pustules containing overwintering spores.