Downy mildew is a fungal disease of leaves and sometimes of pods and seed. The fungi that cause downy mildews as a group are considered to be obligate parasites, meaning that they are restricted typically to one specific host. The disease is more common on herbaceous plants and is favored by temperatures cooler than 65 degrees and by 100% relative humidity. Symptoms include the yellowing of leaves and the advanced development of white fungal growth on the undersides of older leaves. Because downy mildew symptoms are mild and generally restricted to older growth, they present few problems in the landscape. Downy mildew can, however, become systemic in small seedlings and lead to rapid death, however this is rare and more commonly associated with infected seed that has been planted. The three most important downy mildew diseases are those of impatiens, snapdragon and grape.