Myclobutanil as a Curative Agent for Chrysanthemum White Rust. M. R. Bonde, Research Plant Pathologist; Department of Agriculture (USDA), Agricultural Research Service (ARS), Foreign Disease-Weed Science Research, Ft. Detrick, Frederick, MD 21702. G. L. Peterson, Biologist, U.S. Department of Agriculture (USDA), Agricultural Research Service (ARS), Foreign Disease-Weed Science Research, Ft. Detrick, Frederick, MD 21702; S. A. Rizvi, Plant Protection and Quarantine Officer, USDA, Animal and Plant Health Inspection Service (APHIS), PPQ, U.S. Customs House, Baltimore, MD 21202; and J. L. Smilanick, Research Plant Pathologist, USDA-ARS, Horticultural Crops Research Laboratory, Fresno, CA 93727. Plant Dis. 79:500-505. Accepted for publication 3 February 1995. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/PD-79-0500.

In response to a recent outbreak of chrysanthemum white rust, caused by Puccinia horiana, in California, research was conducted to test the fungicide myclobutanil for its effectiveness as a foliar chemical control for the disease. Myclobutanil applied 5 days after exposure of susceptible plants to sporidial inoculum had extremely strong curative properties and usually prevented disease development in either inoculated plants or cuttings obtained from presymptomatic infected mother plants. Even though lesions with pustules developed on inoculated plants when they were sprayed with the fungicide at 10, 15, or 20 days after sporidial inoculation, the pathogen produced few sporidia in an otherwise conducive environment. Myclobutanil at 100 mg a.i./liter, however, was not highly effective for preventing infection when sprayed onto plants 5 days before inoculation, although it did reduce disease incidence. The data indicate that myclobutanil (100 mg a.i./liter) used as a dip treatment for chrysanthemum cuttings prior to planting is suitable as a regulatory treatment for exclusion and eradication of chrysanthemum white rust.