Downy mildew (DM) attacks onions in many parts of the world. It can become severe on leaves of commercially grown onion plants and on leaves and seed stalks of onions grown for seed, especially when relatively cool, moist weather prevails. Other reported hosts are shallot, leek, garlic, and chive. In muckland onion growing areas in the Northeast, DM generally is not detected until mid-to late July. DM rarely, if ever, occurs in growing areas with a warmer climate presumably because of relatively warmer night temperatures during May and June.