Along California's central coast, delphinium (Delphinium species and cultivars) is grown as a cut flower and potted plant. In the spring of 2003, severe downy mildew was observed on various delphinium crops in Santa Cruz County. The disease was detected on cut flower Delphinium ‘Volkerfrieden’ in a greenhouse and adjacent field production areas. At a different nursery, outdoor, containerized D. grandiflorum ‘Blue Butterfly’ and Delphinium elatum (Pacific Hybrids) were also infected. Initial symptoms on leaves consisted of light green patches. These areas quickly turned dark green to blackish green and often were delimited by the central vein of the leaf. Purple-gray sporulation of the pathogen could be seen primarily on the abaxial leaf surfaces. Symptoms were most severe on older foliage. As disease progressed, the lower leaves withered and dried up, giving the plant a blighted appearance. Disease was most severe on D. grandiflorum ‘Blue Butterfly’, in which the disease affected the entire planting and blighted all foliage with the exception of the youngest leaves on the plant's central leader. For this cultivar, the diseased nursery stock had to be discarded, resulting in significant economic loss. Sporangiophores branched dichotomously, with branches ending in slender, slightly curved tips. Sporangia were slightly purplish, ellipsoid to ovoid, and measured 27 to 36 × 19 to 22 μm. Inside the tissue of older symptomatic leaves, extensive numbers of oogonia and oospores were observed. Oospores were globoid, amber to light brown, verruculose, and measured 25 to 27 μm in diameter. The pathogen was identified as Peronospora ficariae (1). Pathogenicity was demonstrated by spraying sporangial suspensions (1.0 × 104 sporangia per ml) onto the leaves of healthy, potted Delphinium elatum (Pacific Hybrids), incubating the plants in a dew chamber at 18°C for 48 h, and then maintaining them in a greenhouse (22 to 24°C). After 10 to 12 days, inoculated plants developed downy mildew symptoms, and the same pathogen sporulated sparsely on the host. If these plants were again placed in the dew chamber for an additional 24 h, leaf lesions sporulated profusely in the following 48 h. Control plants treated with water did not develop any symptoms or signs of downy mildew. To our knowledge, this is the first report of downy mildew caused by P. ficariae on cultivars of delphinium in California.
Reference: (1) G. Hall. Mycopathologia 126:51, 1994.
