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Chrysanthemum White Rust by Paul Pilon

June 22, 2010

Chrysanthemum white rust (CWR) caused by the pathogen Puccinia horiana is a federally quarantined disease that is increasing in incidence within the United States in recent years. CWR is a mircocyclic rust which does not require an alternate host to complete its life cycle. Several species are susceptible to CWR including Dendranthema x grandiflora syn. Chrysanthemum morifolium, Nipponicanthumum nipponicum syn. C. nipponicum, Leucanthemella serotina syn. C. serotinum, and Ajania pacifica syn. C. pacificum. Chrysanthemum white rust can be a very serious problem; when it occurs CWR could potentially result in 100% loss of all chrysanthemum plants that are in production. It is classified as a quarantine action pest and requires state and federal regulatory action upon detection.

With confirmed cases of white rust in the United States, growers should scout their chrysanthemum crops regularly to detect its presence as early in the production cycle as possible to prevent significant injury, severe crop losses, and to prevent this pathogen from becoming established in this country.

The first indication of white rust infections on chrysanthemums is the presence of small light green to yellow spots on the upper leaf surfaces. These spots range in size from 1.0 – 4.0 mm across and may appear sunken and necrotic with age. At first glance, the spotting on the upper foliage appears similar to spotting which commonly develops from insect feeding, plant viruses, or chemical spray injury. The occurrence of any yellow leaf spots on mums should lead to further examination of the foliage to determine if white rust is present.

The development of raised pustules on the undersides of the leaves confirms the plants are infected with rust. The pustules of white rust infections in the early stages have a pinkish or beige appearance; the pustules become white overtime. As the disease advances, it is not uncommon for the pustules to also develop on plant stems and flowers. Under severe infections, infected leaves may turn necrotic and abscise from the plant.

White rust infections usually occur during or following periods of high humidity, extended periods with free water on the foliage, and moderate temperatures. Spores landing on plant surfaces can germinate and penetrate wet leaf surfaces in as little as two hours. A film of free water on the leaves is required with temperatures between 40 and 73° F (optimum 63°F) for infections to occur. The pathogen grows within the plant for 5 to 14 days before the yellow spotting becomes visible on the upper leaf surfaces and ultimately pustules develop under the leaves.

The length of time plant leaves remain wet is critical for spore germination and should be reduced to eliminate germination from occurring. Avoid overhead irrigation in the evenings or at night, instead water mums in the mid-morning, allowing the foliage to dry quickly. When irrigating, avoid getting the foliage wet or splashing water from leaf to leaf (drip irrigation is best). Rust infections are also likely to arise in areas where nightly dew formation occurs (warm days and cool nights) and temperatures remain conducive for spore germination. 

CRW is most commonly introduced into production sites on unrooted cuttings or rooted liners. These starting materials may not display symptoms for up to two weeks after infection; do not assume that if the starting materials appear healthy that your crops are not or will not become infected. Due to quarantine issues and severe plant losses that can occur following infection, it is best to obtain clean non-infected starting materials from a reliable source that includes testing for white rust prior to shipping as a standard procedure. Spores can be carried short distances by wind currents, disseminated on infected plant debris, and plants that have been overwintered may also serve as sources for future infections.

It is also recommended to inspect all new plant materials, such as unrooted cuttings or rooted liners, for rust symptoms when these materials are received. If infected plants are detected, remove them from the production site; infected plants and fallen leaves should be removed promptly and destroyed. Also avoid recycling the soil from plants previously infected with chrysanthemum white rust.

Until the threat of CWR disappears, consider a preventative fungicide program to protect mums against infections. Several fungicides including those products containing the active ingredients chlorothalonil, kresoxim-methyl, mancozeb, myclobutanil, propiconazole, pyraclostrobin, pyraclostrobin + boscalid, triadimefon, and triflumizole and are effective at controlling chrysanthemum white rust. These products are most effective when used on a preventative basis or when applied immediately after white rust has been detected. It is beneficial to apply preventative applications just prior to expecting conditions conducive for disease development. 

Paul Pilon
Perennial Solutions Consulting
paul@perennial-solutions.com

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