Plasmopara halstedii. [Descriptions of Fungi and Bacteria].
Abstract A description is provided for Plasmopara halstedii. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Over 80 hosts from a wide range of genera in the Compositae have been reported, including wild and cultivated species of Helianthus. For lists see Leppik (1966) and Novotel'nova (1977). DISEASE: Downy mildew of sunflower (Helianthus annuus var. macrocarpus); the fungus is an obligately biotrophic plant pathogen. Leaves of infected plants develop chlorotic mottling which spreads from the veins near the petiole across the lamina, and increases in area and intensity as leaves age. Plants become stunted, having thin stems, very much smaller capitula without seeds, and smaller and darker roots. The disease is primarily systemic and mycelium can be found throughout the plant from roots to capitulum and achenes, in all except meristematic tissues. Under humid conditions, a white felt of sporangiophores develops on the undersurface of chlorotic areas. Localized secondary infection of the leaves and heads occasionally develops, resulting in spots, delimited by veins. Such secondary infection may also become systemic. Some infected plants show no disease symptoms, but produce lower yields of poorer quality seeds, which lose vitality and have lower germination rates (latent infection). Cotyledons are also infected causing damping-off in seed beds. A basal gall may also be produced. GEOGRAPHICAL DISTRIBUTION: Plasmopara halstedii is a fungus characteristic of the Americas, its putative origin, It has spread throughout Europe to parts of Africa and Asia, and has recently been reported from New Zealand. See CMI Distribution Maps of Plant Diseases 286. TRANSMISSION: Soil-borne oospores and mycelium (in systemically infected roots) overwinter, infecting subsequent crops. Sporangia form on the surface of infected seedling roots, releasing zoospores which encyst and germinate c root hairs of other seedlings, producing a systemic infection. Sporangia are dispersed by rain-splash from leaves, producing a secondary infection in plants up to the six-leaf stage, but infect only the apical growing points of olde plants. Transmission by oospores in seeds has been responsible for the spread of this fungus around the world, especially since these spores can germinate to produce only a latent infection in the host plant (53, 4545).