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 319.37-2). Importation of Chrysanthemum species are prohibited from several countries, territories, and possessions due to the potential of this organism to be transported with prohibited articles of Chrysanthemum. When CWR is found in the US, the States and APHIS cooperate to eradicate it.
Chrysanthemum white rust originated in eastern Asia. It is now established in 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. Weekly fungicide sprays of myclobutanil 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.
Twelve species of chrysanthemum are susceptible hosts, including:
•pot mums, spray mums, and garden mums (Dendranthema X grandiflorum = Chrysanthemum morifolium)
•Other hosts are the Nippon daisy (Nipponicanthemum nipponicum = C. nipponicum)
•High daisy, and C. pacificum = Ajania pacifica.
Resistant species include:
•annual chrysanthemum (C. carinatum)
•crown chrysanthemum (C. coronarium)
•pyrethrum (Tanacetum coccineum = C. coccineum)
•marguerite daisy (Argyanthemum frutescens)
•ox-eye daisy (Leucanthemum vulgare)
•shasta daisy (Leucanthemum X superbum = C. maximum)
•corn marigold (C. 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.