Downy mildew (Peronospora sp.) attacks a wide range of greenhouse crops such as impatients, coleus, roses, and basil. Luckily, most downy mildew species are plant-specific. For example, what this means is coleus downy mildew will not infect snapdragons. Peronospora antirrhini is the downy mildew species that attacks snapdragons. Disease occurrence is common with cool temperatures (41 to 70F) coupled with high humidity and wet weather that favors prolonged leaf wetness.
During the spring and fall, when dew often forms on leaves, is the prime season for downy mildew. Spores can be introduced into your container production setting through a variety of ways, including spreading by splashing water, air currents, or wind. For cut snapdragons grown in beds, leaf litter and prior crop plant debris that may contain thick-walled resting spores (oospores) can contribute to disease outbreaks. While another common mildew in the greenhouse is powdery mildew, the key difference is that powdery mildew will generally occur on the top of the leaf while downy mildew will be observed on the bottom.