Exclude, Prevent, Eradicate
Chrysanthemum white rust is caused by the fungus Puccinia horiana. Chrysanthemums are the only type of plant infected by this rust fungus, but the disease can spread rapidly in a greenhouse or nursery, resulting in severe product losses.
CWR is endemic to Asia and is now widespread in parts of Europe and South America, but it currently is not established in the United States. Any introduction of this pathogen from offshore is a significant threat to the U.S. floriculture industry.
In the United States and Canada, CWR is a disease of quarantine significance, leading to federal and state regulatory action. It has been reported in 12 U.S. states, as well as British Columbia and Ontario, Canada (see map on page TK). Two additional states have had infected material identified as “trace forwards,” meaning that infected product was shipped to retail establishments in those states, but CWR was not found in commercial operations. The number of discoveries each year has increased since the early 1990s. In 2008, CWR was found in Connecticut, Massachusetts, Pennsylvania and Michigan.
While the hope is that the 2009 chrysanthemum growing season will yield limited finds, all growers who produce chrysanthemums should know the symptoms of CWR and how to prevent it. White rust prevention and control depends on effective plant quarantine laws, healthy planting material, management of humidity and irrigation, and proper selection and use of fungicides.
Sources of Infection
Probable sources of CWR infection in the United States and Canada are imported infected chrysanthemum cut flowers and smuggled infected chrysanthemum material. We know that CWR can move on cut flowers because of the interceptions made at the ports of entry into the United States. Likewise, CWR has been found on material that was originally smuggled into the United States. It is beneficial to keep CWR out of the United States and Canada in the first place, rather than trying to eradicate it after the fact. In the United States, there is a white rust prevention system required by the USDA for countries exporting cut flowers to the United States. Chrysanthemum cut flowers are inspected at the U.S. ports of entry, but there is no inspection in Canada. The other measure to exclude CWR is a quarantine of imported propagation material (cuttings) into the United States.
How to Identify CWR
The first symptoms of CWR are yellow spots (Figure 1) on the upper leaf surfaces up to 4 millimeters in diameter. Prominent pustules subsequently develop on the lower surface of the leaves. The pustules begin as pinkish buff and become a waxy white (Figure 2). If you do find CWR in your crop, please report it to the USDA, CFIA, state or county regulatory officials. First, it is the law, and second, authorities need information about the finds to try to eradicate it.
White Rust Prevention
Here’s a list of white rust prevention measures:
- Use only plant cuttings from a reputable commercial source.
- Never handle imported flowers in or near mum-growing facilities; they can be infected even if they aren’t showing symptoms.
- Scout crops regularly from stick to sale.
- Maintain low humidity and dry foliage. Use fans, vents, plant spacing and heat to move air and reduce humidity. Use drip rather than overhead irrigation to minimize free water on the foliage. Beware of impermeable barriers that retain humidity if using black cloth to control day length.
- Schedule regular applications of preventive fungicides if you are in an area where CWR has been previously reported. Commonly used fungicides known to be effective protectants are Daconil Ultrex and fungicides containing mancozeb (including Dithane 75 DF and Fore).
Other fungicides useful in preventing CWR include:
- Heritage (azoxystrobin)
- Cygnus (kresoxim-methyl)
- Banner Maxx (propiconazole)
- Strike (triadimefon)
- Terraguard (triflumizole)
- Eagle and Hoist (myclobutanil) are eradicants and should be used only in the event that CWR is actually diagnosed.
Federal quarantines require six-month post-entry quarantine of imported cuttings; prohibition of cutting imports from infested countries; inspection of imported cut chrysanthemums at port of entry; certification programs for offshore cut flower producers wishing to export to the United States; and eradication when CWR is found.
Regulatory officials will supervise the eradication and treatment program. An infected nursery will receive an Emergency Action Notice that prevents shipments until declared free. Then, they will be required to destroy symptomatic plants and everything in the surrounding one-meter radius. Three treatments at five- to seven-day intervals with an eradicant fungicide such as Eagle or Hoist (myclobutanil) will be required, followed by a final inspection five to seven days after the third treatment. If no CWR is found, plants are released for sale.