Puccinia horiana is a fungal disease that targets Chrysanthemum species, it spreads through airborne spores and thrives in moist, cool conditions. It appears as raised, light-coloured, warty bumps around 5mm in diameter, these are across the underside of the leaves. On the upper surface, these pustules correspond with depressed yellowing of the leaf tissue. Over time these warts turn brown as the disease further infects the plant, leaf health will decline and leaves are likely to distort and die back.
Commonly known as Chrysanthemum White Rust, this fungal pathogen originates from Japan, it has now spread throughout Asia, Europe and South America. Heavy infestations can stunt plant growth and reduce vigour, eventually leading to plant death. This problem is most often identified from late summer-autumn, however it is generally active year-round. Chrysanthemum cultivars differ in their susceptibility to this problem.
Thought to have appeared in the UK in 1963, spread by infected cuttings.