Epidemic outbreaks of downy mildew caused by Plasmopara halstedii on sunflower in Thrace, part of the Marmara region of Turkey

Abstract

Downy mildew of sunflower (Helianthus annuus) caused by the pathogen Plasmopara halstedii is a potentially devastating disease in Europe and the USA (Miller & Gulya, 1987). During the spring of 2007 and 2008, a serious outbreak of downy mildew caused by P. halstedii at the V4 (four true leaves) stage of development was observed on sunflower plants grown in commercial fields in Thrace, part of Marmara, the main producing area in Turkey. It is thought that low temperature and extensive spring rains encouraged the disease, resulting in approximately 85% yield loss and lower quality of sunflower production in some areas. Plants carrying systemic downy mildew infection were stunted and leaves showed characteristic green and chlorotic mottling along the main veins and over the lamella. Under moist conditions, a white downy growth composed of sporangiospores and sporangia of the fungus appeared on the leaf surface corresponding to chlorotic areas above. Microscopic observations revealed the monopodial branching of sporangiospores at nearly right angles, with five or six terminal branches 6–10 µm long and bearing single elliptic, papillate zoosporangia, 15–25 × 12–16 µm. Sporangiophores were 270–450 µm. These morphological characteristics were typical of P. halstedii, as described by Hall (1989). Plasmopara halstedii is listed as an A1 quarantine pest by IAPSC, but not by any other regional plant protection organization. However, Australia treats it as a major quarantine pest. EPPO is currently examining its quarantine status. In the EPPO region, it is potentially dangerous everywhere that sunflower is grown, except where high soil temperature (above 25°C) and/or drought are limiting factors. If control measures are lacking and conditions are favourable, downy mildew can be devastating to sunflower. Plasmopara halstedii was recorded by Karel (1958) in a single crop near Adapazarı, but the events of 2007 and 2008 signal epidemic spread of downy mildew and economic losses to sunflower crops.