In California, marguerite daisy (Argyranthemum frutescens [= Chyrsanthemum frutescens]) is an important, commercially grown, perennial flowering plant that is used as a potted plant, cutflower, and landscape plant. For two seasons (2003 and 2004), a downy mildew disease has been affecting marguerite daisy at wholesale container and field cutflower nurseries and retail nurseries in coastal California (Monterey, Santa Cruz, and San Mateo counties). The disease occurred early in the season (January) and continued to infect new foliage throughout the year whenever cool, foggy weather occurred. The disease primarily affected newly expanded young leaves on shoot tips. Such leaves were chlorotic, twisted and bent, and stunted. In some cases, leaflet tips turned black and necrotic. The abaxial sides of affected leaves were heavily colonized by the extensive purplish brown growth of downy mildew. Older, fully expanded foliage was unaffected. Flowers could be infected with the fungus growing on the undersides of petals and resulting in slightly twisted, bent shapes. Symptomatic plants and cutflower stems were unmarketable. Hyaline conidiophores emerged from stomata, branched dichotomously (rarely trichotomously), and had branches ending in slender, curved branchlets that did not have swollen tips. Conidia were slightly brown, ovoid, mostly nonpapillate, and measured 28.5 to 40.0 × 19.0 to 28.0 μm. Oospores were not observed in plant tissue. On the basis of symptoms and morphology of the organism, the pathogen was identified as Peronospora radii (1,2). To prove pathogenicity, plants were spray inoculated with conidial suspensions, incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C).After 18 to 20 days, symptoms and signs of downy mildew developed only on the newest foliage of inoculated plants, and the pathogen morphology matched that of the originally observed pathogen. Untreated control plants did not develop downy mildew. To our knowledge, this is the first report of downy mildew caused by P. radii on marguerite daisy in California and the United States. The pathogen has not been reported on other hosts in California. P. radii is found on marguerite daisy in England, Germany, Israel, Mexico, and the former Yugoslavia (1,2).
References: (1) I. S. Ben-Ze'ev et al. Phytoparasitica 15:51, 1987. (2) O. Constantinescu. Sydowia Ann. Mycol. 41:79, 1989.