Diseases of onion and garlic (
Downy mildew
Downy mildew is caused by Peronospora destructor. It can develop on both garlic and onions. Infection sites will look like elongated patches up to 1.25 inch long that are somewhat paler than the rest of the foliage or have turned a light tan to brown in color (photo 1). A furry growth, grayish violet in color, may be visible on the surface of infected leaves or seed stems during moist periods, especially early morning. Downy mildew lesions may be violet to purple and can be confused with initial lesions of purple blotch (photo 2). Infected leaves gradually turn pale green to yellow and diseased parts such as leaf tips fold over and collapse (photo 3). The downy mildew pathogen requires cool temperatures and the presence of free moisture from rainfall or dew to infect onion plants.
Cultural practices that facilitate air movement and drying of leaves will reduce disease severity. Preventive applications of fungicides to the foliage when climatic conditions favor disease development can effectively control downy mildew. Fungicides include: copper hydroxide, chlorothalonil, fosetyl-Al, mancozeb, maneb, and mefenoxem. Two or more of these materials should be rotated throughout the season as a resistance management strategy.
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February 10, 2003