Downy mildew causes disease on a large number of common ornamental nursery plants including Cissus, lisianthus, poppy, ranunculus, rose, snapdragon, stock, pansy, alyssum, impatiens, sweet pea, cucurbits, helichrysum basil and brassica seedlings. Each species of downy mildew only infects a small number of closely related plants.
Common leaf symptoms include white to grey or lavender fluffy growth on the underside of leaves. Pale yellow or necrotic spots appear on the upper surface that can be angular and vein delimited. As disease becomes more severe the entire leaf can become infected and may drop off. Downy mildew can also become systemic, particularly in small plants causing wilt and death of the entire plant. Infection of growing tips can cause leaf distortion, stunting and stem cracking.
Management includes correct identification (not to be mistaken for powdery mildew). It is best to use cultural practices, including reducing leaf wetness and increasing ventilation, removal of unsaleable infected plants, disinfesting the growing area, resistant cultivars and use of appropriate fungicides.
For more information refer to the downy mildew factsheet and the appropriate section of the leaf spot management plan (which has a more up to date list of fungicides). All production nurseries receive 6 free samples through the diagnostic service, Grow Help Australia, who can assist you with correct identification of this disease.
Figure caption: Downy mildew on Hellebore.