In response to a recent outbreak of chrysanthemum white rust, caused b
y Puccinia horiana in California, research was conducted to test the f
ungicide myclobutanil for its effectiveness as a foliar chemical contr
ol for the disease. Myclobutanil applied 5 days after exposure of susc
eptible plants to sporidial inoculum had extremely strong curative pro
perties and usually prevented disease development in either inoculated
plants or cuttings obtained from presymptomatic infected mother plant
s. Even though lesions with pustules developed on inoculated plants wh
en they were sprayed with the fungicide at 10, 15, or 20 days after sp
oridial inoculation, the pathogen produced few sporidia in an otherwis
e conducive environment. Myclobutanil at 100 mg a.i./liter, however, w
as not highly effective for preventing infection when sprayed onto pla
nts 5 days before inoculation, although it did reduce disease incidenc
e. The data indicate that myclobutanil (100 mg a.i./liter) used as a d
ip treatment for chrysanthemum cuttings prior to planting is suitable
as a regulatory treatment for exclusion and eradication of chrysanthem
um white rust.