Delphinium is a genus comprising three hundred or more distinct species belonging to the family Ranunculaceae (Bassett and Bassett, 2007). True delphiniums are herbaceous perennials. The common name, larkspur, is shared with a separate, closely related genus, Consolida, consisting of annual species. The two genera are distinguished by flower structure. Flowers of the true wild ancestors of species in the genus Consolida have a single petal and a single carpel (seedpod), whereas flowers of species in the genus Delphinium have two or four petals and three or more carpels (Basset and Basset, 2007; Mabberley, 1997). Delphiniums are native in the northern hemisphere, but a few species are found along the mountain ranges of tropical Africa. Delphiniums produce a range of beautiful flower colors, including blue, pink, purple, cream, and white. They contribute significantly to the aesthetic value of homes, gardens, and the landscape, and are grown commercially for cut flowers. In 2006, the wholesale value of larkspur (Delphinium spp. and Consolida spp.) as a cut flower in the United States for operations with sales worth $100,000 or more was $8 million, with California accounting for 80% of those sales (Jerardo, 2007).
Powdery mildew is one of the major diseases affecting field-grown delphiniums. The disease is caused by fungi in the genera Erysiphe, Golovinomyces, and Podosphaera (Horst, 2001). Growth of these fungi results in gray powdery masses of mycelium and conidia on the surface of leaves, stems, and flowers. In severe infections, young leaves and growing tips become curled and stunted, significantly lowering yield and quality. Ascocarps (sexual fruiting bodies) form on infected areas on the plant late in the growing season. The powdery mildews are obligate parasites. They overwinter as ascocarps on plant debris or as mycelia in dormant plant tissue. Conidia, ascocarps, and ascospores released from ascocarps are dispersed from infected plants or plant debris by wind (Jarvis et al., 2002).
Delphinium cultivars vary greatly in their resistance to powdery mildew (Pirone, 1978). Melquist (1941) screened 13 species of delphiniums native to California for resistance to powdery mildew caused by Erysiphe polygoni. The species varied widely in resistance ranging from highly susceptible [swamp larkspur (Delphinium uliginosum)] to highly resistant [sierra larkspur (Delphinium scopulorum var. glaucum)]. Since publication of Melquist's results in 1941, research has not been done in California to identify delphinium cultivars with resistance to powdery mildew. The use of resistant cultivars in combination with other disease management strategies such as cultural practices and fungicide application can reduce losses. The objective of this study was to evaluate delphinium cultivars for resistance to powdery mildew.
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