Chrysanthemum White Rust Background
Chrysanthemum White Rust, caused by the fungus Puccinia horiana P. Henn., is a quarantine significant pest in the United States (Title 7, Code of Federal Regulations, Part 319.37 and 319.74). Importation of certain Chrysanthemum (including Dendranthema), Leucanthemella, and Nipponanthemum species are prohibited from several countries, territories, and possessions due to the potential of this organism to be transported with prohibited host articles. When CWR is found in the United States, the States and APHIS cooperate to eradicate the disease.
Chrysanthemum white rust originated in eastern Asia. It is now established in the Far East, Europe, Africa, Australia, Central America and South America. There have been outbreaks in Canada and the United States, but the pest is eradicated when found. Best management practices, including the use of proper cultural techniques, scouting for disease symptoms, sanitation, fungicide applications, and worker training/education are required to manage this disease.
Chrysanthemum white rust may be recognized by the small white to yellow spots, up to 4 mm wide, on the upper surface of the leaf. These slightly dimpled spots become brown over time. Pustules form on the underside of the leaf, beneath the small spots. These are buff to pink-colored but become white as they age. Pustules are most common on young leaves and flower bracts but can be found on any green tissue and flowers. Infected plants do not always express symptoms during hot and dry conditions. Symptoms usually appear during cooler, wet weather.
Various species of chrysanthemum are susceptible hosts, including:
- Arctic chrysanthemum, Arctic daisy (Chrysanthemum articum = Arctanthemum arcticum, Dendranthema articum)
- Chrysanthemum boreale = Chrysanthemum indicum var. boreale, Dendranthema boreale
- Chrysanthemum indicum = Dendranthema indicum
- Nojigiku (Chrysanthemum japonense = Dendranthema japonense, Dendranthema occidentali-japonense)
- Ryuno-giku (Chrysanthemum japonicum = Chrysanthemum makinoi, Dendranthema japonicum)
- Florist's chrysanthemum, Chrysanthemum, Mum (Chrysanthemum ×morifolium = Anthemis grandiflorum, Anthemis stipulacea, Chrysanthemum sinense, Chrysanthemum stipulaceum, Dendranthema × grandiflorum,Dendranthema ×morifolium, Matricaria morifolia )
- Iso-giku (Chrysanthemum pacificum = Ajania pacifica, Dendranthema pacificum)
- Shio-giku (Chrysanthemum shiwogiku = Ajania shiwogiku, Dendranthema shiwogiku)
- Chrysanthemum yoshinaganthum (Dendranthema yoshinaganthum)
- Chrysanthemum zawadskii = Chrysanthemum arcticum subsp. Maekawanum, Chrysanthemum arcticum var. yezoense, Chrysanthemum yezoense, Dendranthema yezoense, Leucanthemum yezoense
- Chrysanthemum zawadskii subsp. Zawadskii = Chrysanthemum sibiricum, Dendranthema zawadskii, Dendranthema zawadskii var. zawadskii
- Leucanthemella serotina = Chrysanthemum serotinum, Chrysanthemum uliginosum, Pyrethrum uliginosum
- Nippon daisy, Nippon-chrysanthemum (Nipponanthemum nipponicum = Chrysanthemum nipponicum, Leucanthemum nipponicum)
Resistant species include:
- Annual chrysanthemum (Chrysanthemum carinatum)
- Crown chrysanthemum (Chrysanthemum coronarium)
- Pyrethrum (Tanacetum coccineum = Chrysanthemum coccineum)
- Marguerite daisy (Argyanthemum frutescens)
- Ox-eye daisy (Leucanthemum vulgare)
- Shasta daisy (Leucanthemum X superbum = Chrysanthemum maximum)
- Corn marigold (Chrysanthemum segetum)
The fungus produces two types of spores:
- Teliospores - produced in pustules and remain in the pustules unless they are aggressively brushed off. Teliospores can survive up to 8 weeks if they remain in pustules on detached leaves at 50% relative humidity or less. They die sooner under moist conditions. Under moist conditions (96% to 100% relative humidity) for at least 3 hours, teliospores produce basidiospores.
- Basidiospores - these can cause an epidemic if conditions are right. They spread from plant to plant by splashing water and they must have a film of water on the plant surface for infection. Infection can occur in as little as 2 hours at the optimal temperature of 17 C. Basidiospores can also travel short distances (700 meters) by wind currents during moist weather. The basidiospores are fragile and generally do not survive longer than about one hour. Infections occur when infected cuttings or viable spores are brought into a greenhouse, thereby exposing the uninfected cuttings to inoculum. Infected cuttings may appear normal even though the fungus is present. Basidiospores survive 5 minutes when the relative humidity is 80% or less, and up to 60 minutes when the relative humidity is 81 to 90%.
Symptoms usually develop within 5 to 14 days after infection. The fungus itself will only grow and reproduce on susceptible plants. It does not develop outside the plant.