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 white rust. Some rusts require two different plant hosts in order to complete their life-cycle, but white rust spends its entire life on chrysanthemum.
The buff-coloured pustules on the leaf undersides 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 white, and it is these spores that are carried in air currents to create new infections when they land on other chrysanthemum leaves.
Infection is favoured by cool, wet or humid conditions. The disease is therefore most problematic in late summer and autumn, when suitable temperatures combine with prolonged leaf wetness caused by rain or morning dews.
The fungus cannot survive for extended periods on leaf debris, but can persist on overwintering plants or stools (rootstocks).
Like many rust diseases, white 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 can range from a few days to several weeks. Transport of symptomless but infected plants or cuttings is the main way in which the disease has been able to spread between continents.
Susceptibility: White rust does not affect any plants other than chrysanthemum. There are considerable differences in susceptibility between different chrysanthemum cultivars.