The first symptoms of chrysanthemum white rust (Puccinia horiana) are visible on the upper side of the leaf. These are light green to yellow coloured spots of 2 to 5 mm in size. The centres turn brown and necrotic. The characteristic pustules or teliospores occur on the underside of the leaf. When there is a heavy infection the leaves wilt.
High humidity and a period of wet leaves are conditions during which this fungus is able to spread. Under optimal conditions new infections of white rust are possible within five hours. Climate changes in the greenhouse and watering at an unfavourable time increase the risk of contamination.
The fungus is mainly present on the leaves but stems can also be affected. Spores are spread via water, air, plant material, tools, hands and pets. White rust only occurs on chrysanthemums and has a quarantine status in several countries.