The USDA has recently confirmed that Chrysanthemum White Rust (CWR) or Puccinia horiana has been confirmed in three states on mums. In September, CWR was confirmed in a Pennsylvania nursery. More recently the infection has spread to Montgomery county Maryland and New Castle county in Delaware, according to Nolan Lemon, from the office of public affairs at APHIS. At this time Lemon could only confirm that the plants in Delaware, where four private properties and five retail nurseries tested positive for the disease, have been destroyed and was not sure if plants in Pennsylvania or Maryland have been destroyed yet.
As of right now the USDA is continuing to survey the area for further infection, but as of press time there are only the three states that have tested positive for CWR. Lemon said, “A suspected nursery in Pennsylvania is where it [CWR] originated.”
Lemon also said, “It is not considered to be as serious of a risk because most people aren’t using the mums for propagation, usually these days people dig up the mums after the season anyway.”
Within the last 25 years the pathogen, caused by the fungal pathogen Puccinia horiana, has come into the United States from infected plant material. The disease causes conspicuous and debilitating lesions on all of the green above-ground parts of florist chrysanthemum and some close relatives. The symptoms appear mostly on the leaves as light yellow chlorotic spots on the upper leaf surface, while corresponding buff-white raised pustules appear eventually on the lower leaf surface. There are only two spore stages in this rust's life cycle.
The telial stage spores are present in the large unsightly protuberant lesions on the leaf underside. These teliospores germinate in place under very high humidity to produce the other airborne or water-splashed spore stage, basidiospores. These basidiospores die when they dry out, so very moist conditions cause Chrysanthemum White Rust to spread.
Basidiospore production from teliospores can occur in as little as three hours after wetness, and two hours of leaf wetness is all that is required for successful infection by dispersed basidiospores. New telial pustules appear in about 2-4 weeks after infection.
The last reported cases of CWR in the United States were in early January 2004 in New York, New Jersey and California. Due to the recent amount of outbreaks, industry officials are wondering if it should be taken off of the quarantine list in the realization that it is here to stay.
For a copy of the CWR Management Plan from the UDSA, visit
http://www.aphis.usda.gov/ppq/ispm/cwr/cwrplan.pdf  for more information.