Wednesday, September 17, 2008

Greenhouse and Nursery - Crysanthemum White Rust Found Again in Delaware

Chrysanthemum white rust has recently be identified again in Delaware. The following is more information on this regulated disease.

White rust of chrysanthemum was confirmed today, September 16, 2008, on potted mums from a retail location in Dover. Inspections by Delaware Department of Agriculture personnel found the infections at a low incidence (see the previous post for pictures and description of the disease). The following are I made during a webinar hosted by Yoder Brothers on white rust. In it there is a link to a fact bulletin with further information and pictures.

Chrysanthemum White Rust Webinar Notes August 5, 2008

A webinar presented on a live web site training session by Yoder Brothers on August 5, 2008. Information may be found on under the grower section under technical guides. A bulletin pdf link is found at:
The following notes are condensed from information in the webinar presentation:

Chrysanthemum white rust is caused by the obligate parasite Puccinia horiana. Symptoms include a 3 to 4 mm dimpled pale yellow spot on the upper leaf surface. Pustules are found on the lower leaf surface and are white to buff colored with a waxy appearance and feel. Pustules may occasionally develop on flowers. White rust infects twelve species of chrysanthemum in four genera within the Asteracae. Teliospores of the fungus are produced in the pustules that may survive up to 8 weeks in dried tissue. Spores survive only one week if buried in soil. Small basidiospores are produced from the germinating teliospores after 3 hours of moisture at an optimum temperature of 63 F (63-75 degree range). Basidiospores are easily transported by wind and water, but are sensitive to dessication. Germination occurs in a film of water and the fungus infects plant tissue, with symptoms developing after 5 to 14 days. High humidity results in faster disease development.

Imported flowers are a common source of new infections of Puccinia horiana, and should never be handled near mum-growing facilities. Flower crops should never be used to start new cuttings. Scout regularly, keep foliage dry, keep humidity low, and use preventative fungicides.
Effective preventative fungicides include chlorothalonil and mancozeb. See the Yoder bulletin for recommendations and use according to labels. Myclobutanil may be effective after infection has been observed; however, it may not be used after color break.

If chrysanthemum white rust is found, it must be reported to state agricultural officials, as there is a national protocol for eradication and treatment. The best approach is to exclude the pathogen by means of dealing with reputable suppliers, spraying to prevent infection, and eradicating existing infections.

Nancy F. Gregory, Plant Diagnostician, Department of Plant & Soil Sciences, University of Delaware.

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