Puccinia horiana, also known as chrysanthemum white rust, was detected at six locations in two counties in Connecticut, the Litchfield County Times (Conn.) reported last week. Additionally, the parasite also was found in Michigan, according to a USDA bulletin. According to the USDA, “Chrysanthemum white rust may be recognized by the small white to yellow spots, up to 4 mm wide, on the upper surface of the leaf. These slightly dimpled spots become brown over time.” The disease targets only certain species of chrysanthemums, and has poor dispersal mechanisms, but in a greenhouse environment, it can spread rapidly, resulting in the death of large numbers of plants. According to Oregon State University, “It has been found and eradicated in several nurseries in the greater Portland, Ore. area off and on since 1995 and in British Columbia since 2001. Usually it is found in nurseries that ‘hold-over’ mums from the previous growing season. High humidity, free water, and moderate temperatures favor the fungus.” The USDA recommends certain containment measures (PDF) to halt the spread of the disease.
