Chrysanthemum White Rust

Chrysanthemum white rust (Puccinia horiana) is a serious fungal disease of chrysanthemum. This disease was first discovered in Japan in 1895 and was confined to China and Japan until the 1960s. Today it is established in Europe, Africa, Australia, Central America, South America, and the Far East. White rust can spread quickly in greenhouse and nursery environments causing severe crop losses. Eradication of this pathogen is feasible because P. horiana has a limited host range, requires green host tissue, and is a poor disperser.

Chrysanthemum white rust (CWR) is a quarantine significant pest in the United States; therefore, occurrence of this disease leads to state and federal regulatory action. In the last 25 years localized introductions of chrysanthemum white rust have occurred within the United States or Canada and have subsequently been eradicated or are being eradicated. In recent years, CWR has been detected in Massachusetts and Connecticut. The MA Department of Agricultural Resources worked with the grower to eliminate the spread of disease from their facilities and surveyed the surrounding area to insure the rust had not spread. The federal management plan for CWR includes the following measures destroying symptomatic plants, fungicide treatments on remaining asymptomatic plants, surveys of the surrounding area, and trace backs of infected stock. The full management plan can be viewed on the web on the USDA, APHIS website.

Hosts

Twelve species of chrysanthemum are susceptible including:

• pot mums, spray mums, and garden mums ( Dendranthema X grandiflorum = Chrysanthemum morifolium ).
• Other hosts include 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 )

How to spot it:

The symptoms of chrysanthemum white rust are very distinct; light green to yellow spots up to 5mm in diameter appear on the upper surface of the leaf. These spots become brown and necrotic withage. Raised beige to pink pustules form on the underside of leaves beneath the spots. Pustules become white with age. Pustules are most common on young leaves and flower bracts but may form on any green tissue or the petals. Symptoms usually occur during cool, wet weather.

How does it spread:

The disease is brought into the greenhouse on infected plant materials. Infected plants may look normal until correct environmental conditions encourage symptom development. Hot and dry weather may delay onset of symptoms for up to eight weeks. Fungicide applications may suppress disease development. CWR can also be spread to uninfected plants on contaminated soil, litter, dead leaves, gardening equipment, clothes, shoes, and hands. The fungus only grows and reproduces on susceptible host plants.

CWR produces two types of spores. Teliospores are in the pustules on the underside of leaves and remain on the pustule unless they are aggressively brushed off. Teliospores can persist up to 8 weeks on pustules attached to leaves. Teliospores produce basidiospores under cool, moist conditions (96-100% relative humidity for at least three hours). Basidiospores are fragile and short-lived (1 hour or less depending on the humidity) but they are responsible for explosive epidemics under the correct environmental conditions. Infection can occur when temperatures are cool (between 40-73F with an optimal temperature of 63F) and relative humidity is high (96-100%). A film of free water on the leaves is required for infection. Symptom development usually occurs in 5 to 14 days.

What to do:

The first step is to prevent CWR from being introduced into your facilities. Obtain materials from a reputable supplier. Use good cultural techniques to limit spread of disease. If CWR is introduced to Massachusetts, early detection and cooperative actions between growers and state and federal agencies will aid in preventing this pathogen from establishing in Massachusetts. Look for development of disease symptoms during cool, moist periods. Since CWR is a federal quarantine pest, growers are obligated to report it to the Dept. of Agricultural Resources. Please contact the pesticide hotline at 617-626-1779 or through the pest reporting form if you suspect your mums have chrysanthemum white rust.

Fungicides:

Fungicides applied preventatively to asymptomatic plants can prevent CWR from becoming established fand are part of the federal management plan. Remove and destroy affected plants and a border of healthy appearing plants. Fungicides registered for CWR include azoxystrobin (Heritage), chlorothalonil (Daconil, Echo), chlorothalonil plus thiophanate-methyl (Spectro), mancozib (Dithane, Protect T&O), thiophanate-methyl (Cleary's 3336), and myclobutanil (Eagle).

References and Resources:

• Photos of Chrysanthemum White Rust and other diseases
• Chrysanthemum white rust fact sheet from MA Introduced Pests Outreach Project contains photos of the symptoms http://www.massnrc.org/pests/pestFAQsheets/chrysanthemumwhiterust.html
• USDA, APHIS, PPQ Chrysanthemum White Rust home page contains the national management plan for CWR exclusion and eradication.
  http://www.aphis.usda.gov/ppq/ispm/cwr/index.html
• Chrysanthemum White Rust Bulletin from Yodor Bros.
• MacDonald, L. 2001. Chrysanthemum White Rust Floriculture Factsheet. British Columbia Ministry of Agriculture, Food, and Fisheries
  http://www.agf.gov.bc.ca/cropprot/cwrust.htm
• The American Nursery & Landscape Association (ANLA) and the Society of American Florists (SAF) Powerpoint presentation on Chrysanthemum White Rust

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Revised 11/10 by M.B. Dicklow
UMass Extension Plant Diagnostic Laboratory