The following is a good article on Chrysanthemum white rust from the New England Greenhouse Update.
Growers of garden mums are reminded to monitor for and take precautions against Chrysanthemum white rust (Puccinia horiana), especially during cool, wet weather. Train staff to recognize symptoms and monitor plants throughout the growing season since infections can appear at any stage in crop production. Symptoms are distinctive. The first symptoms of white rust are yellow spots on upper leaf surfaces. These may become sunken and necrotic. Anyone not familiar with this disease might confuse these early symptoms with spray or insect damage. Any sign of yellow spotting should be closely examined for further evidence of this disease. Diagnostic symptoms appear as raised pink or white waxy pustules on the undersides of the leaf. Severely infected leaves dry up and persist on the stems. Infected plants may remain symptomless for weeks, especially during hot and dry conditions. Symptoms usually appear during cooler, wet weather.
Fungicide applications may suppress disease development. Protectant fungicides include: Azoxystrobin (Heritage), Chlorothalonil (Chlorothalonil, Daconil, Echo, Spectro 90 WDG with thiophanate-methyl), Kresoxim-methyl (Cygnus), Mancozeb (Fore, Dithane75DF, Protect DF, Pentathlon), Myclobutanil (Eagle 20 EW), Triadimefon (Strike), Triflumizole (Terraguard). Always read and follow label directions. The label is the law.
Keep susceptible varieties away from others. Varieties that have tested positive for chrysanthemum white rust (CWR) in the past few years include several ‘Gretchen’™ cultivars (’Bold’, ‘Bright’, ‘Flashy’, and ‘Sunny’), as well as the cultivars ‘Dark Veria’™ ‘Debonair’,™ ‘Ashley’,™ ‘Vicki’,™ ‘Okra’,™ ‘Cesaro’,™ and ‘Flamingo Pink’™ , ‘Gold Finch Yellow’ , Pink Symphony’, ‘Bold Melissa’, ‘Canelli’, ‘Heidi’, ‘Beth’, ‘Helen’, ‘Canelli’, and ‘Galatino’ that are not in the ‘Gretchen’™ series. In addition, cultivars not specified tested positive in a wide range of colors and types (some of which were grass-mum combination pots). Chrysanthemum white rust infects 12 species of chrysanthemum, including garden mums, pot mums, and Nippon daisies.
CWR is still, (at this date) a regulated pest of quarantine significance. If CWR is diagnosed, state and federal regulators remove and destroy infected plants from sites in compliance with the CWR Eradication Protocol established by the USDA APHIS-PPQ.
What to do if you see this disease:
Call the Plant Industries Section of the Delaware Department of Agriculture (302) 698-4500. Regulatory personnel will supervise the eradication and treatment procedures.
Chrysanthemum White Rust.
Information reprinted from the current edition of the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/index.php