The rust fungi are described as biotrophs; that is, they grow within the living tissues of the plant and extract nutrients from the cells. Although they do not kill tissues rapidly, heavy attacks by rusts can cause tissues to collapse and die prematurely and this is the case with heuchera rust.
The orange to brown pustules contain numerous spores called teliospores that remain embedded within the leaf. Under humid conditions the teliospores germinate to produce a second spore type called a basidiospore. It is the production of the minute basidiospores that causes the pustules to turn a greyish-white colour, and it is these spores that are carried in air currents to create new infections when they land on other heuchera leaves.
Infection is favoured by wet or humid conditions. The disease is therefore most problematic during wet summers. The fungus overwinters on any affected leaves remaining on the plants. New leaves emerging in spring soon produce rust pustules.
Like many rust diseases, heuchera rust has a ‘latent period’, when infection of the plant has occurred but symptoms are not yet visible. Depending on environmental conditions this latent period could range from a few days to several weeks. It is therefore possible that bought-in plants could be harbouring the disease.
In the UK the disease has so far been confined to Heuchera, but in the USA other members of the Saxifragaceae can be affected, including Lithophragma, Mitella, Saxifraga, Tellima, Tiarella and Tolmiea. There is considerable variation in susceptibility between different cultivars of Heuchera.
Unlike some other rust diseases no alternate host is involved in its lifecycle.