Puccinia horiana (P. Henn.) - Chrysanthemum White Rust

BACKGROUND

Chrysanthemum White Rust (Puccinia horiana Henn.) is a serious fungal disease of chrysanthemum. It was first reported from China and Japan in 1895. It spread to all far east countries in the early 1900's, from there it spread to South Africa and Europe in the 1960's. It has since spread throughout Europe and has reached South America, Australia, New Zealand and Mexico. Localized introductions have occurred in United States and Canada.

The introduction and spread of chrysanthemum white rust could lead to serious losses. The disease can spread rapidly throughout a greenhouse causing complete crop failure. Intensive management and continuous control measures are required where the disease is present. There is evidence that chrysanthemum white rust develops tolerance to certain pesticides used to control the disease. The introduction of the disease could also result in the loss of export markets for chrysanthemums.

HOSTS

Chrysanthemum, Chrysanthemum and Dendranthema species and Leucanthemella serotina (= Chrysanthemum uliginosum).

DISTRIBUTION

• Asia: Brunei Darussalam,China, Hong Kong, Japan, North Korea, South Korea, Malaysia, Russia (Far East), Taiwan, Thailand.
• Africa: South Africa and Tunisia.
• Pacific: Australia, New Zealand
• Europe: Most countries
• North America: Mexico, USA (only found in : California- under eradication since 1994, Oregon- under eradication since 1995)
• South America: Argentina, Brazil, Chile, Columbia, Uruguay, Venezuela

BIOLOGY

The entire life cycle of Puccinia horiana is completed on the same host. Two types of spores are produced, bicellular teliospores and unicellular basidiospores. The mature teliospores germinate while still attached to the sori that produced them. Each cell of the teliospore produces germ-tubes containing four round to kidney-shaped basidiospores, which are discharged from the germ-tubes and dispersed by air currents, splashing water, clothing, pruning tools, stakes, pots or string. These spores are generally short- lived but the teliospores in sori on detached leaves can remain viable for up to two months. High humidity, and a film of moisture, are necessary for the germination of both types of spores. The basidiospores germinate on living leaf blades producing a pad of mycelium under the lower epidermis of the leaf. It is here that the leaf spots and eventually the teliospores are produced. At first the infected area of the leaf will appear as light coloured circular spots on the upper surface, varying in size from a pin head to 5 millimetres across. As the infection advances the spots become sunken and a whitish pustule appears on the under surface of the leaf (figure 1).

DETECTION & IDENTIFICATION

Detection:

This fungus attacks primarily the leaves of chrysanthemums causing spotting and in severe cases, twisting and finally dropping of the leaves (figure 2). Spore forming pustules appear on the lower surface of the leaves and are beige to pink in colour. As these pustules mature, they turn white (figure 3). Under severe infection the flowers and stem can also be infected. The plants may lack vigor and be reduced in size.

Identification:

On leaves, the disease first appears as waxy-white spots. The upper surface of the leaf over these spots becomes sunken and the lower surface raised. At maturity, the lesions may increase in size up to 2 centimetres in diameter become cinnamon gold to light brown dusted with a dirty, white bloom of spores. With age the lesions turn brown and become necrotic at the centre. Occasionally, necrotic flecking occurs on the flowers. Under high levels of infection sori can be seen on bracts and the stem.

As there are other rusts that occur on chrysanthemum, the fungus is identified based on the morphological features of the teliospores. Suspect material should be sent to a suitable diagnostic laboratory for microscopic examination and fungal identification.

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Text: Plant Pest Surveillance Survey Unit
Photos: Agriculture and Agri-food Canada, Research Branch, Ottawa