Cause The fungus Peronospora digitalidis has been found once by the OSU Plant Clinic and reported once from a commercial nursery sample. The disease is common throughout coastal production in California. It can be found in the PNW nursery industry and in the landscape in Washington. Cool wet weather favors this disease. Conidia of this fungus are dispersed by wind or rain-splashed. Digitalis purpurea cultivars Alba and Apricot, as well as Foxy hybrids, were very susceptible, Digitalis x mertonensis appeared moderately susceptible, and D. grandiflora grown in a heavily diseased area was symptomless.
Symptoms Initial symptoms on leaves consist of light green, rectangular areas that are vein-delimited. These spots later become chlorotic, may coalesce and turn necrotic. The purple-gray sporulation of the pathogen can be seen primarily on lower leaf surfaces. However, in some cases, extensive fungal growth may develop on the upper surface of lower leaves.
Cultural control
- Purchase clean stock.
- Do not water from overhead, or adjust irrigation so plants are not wet for extended periods.
- Space plants for good air circulation.
- Remove and destroy infected plants and plant debris.
Chemical control Rotate fungicides from different groups that have a different mode of action for resistance management.
- Compass 50 WDG at 2 to 4 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Heritage at 2 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Did not do well in one trial in NC. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Mefenoxam 2 AQ at 0.49 to 0.98 fl oz/100 gal water plus another fungicide with a different mode of action. Group 4 fungicide. 48-hr reentry.
- Monterey Garden Phos at 0.5 to 1.1 fl oz/gal water as a foliar spray. Also labeled for soil drench, see label for details. Group P7 fungicide. H
- Segovis at 0.6 to 3.2 fl oz/100 gal water plus another fungicide. Group U15 fungicide. 4-hr reentry.
- Segway SC at 2.1 to 3.5 fl oz/100 gal water. Drench rate is based on pot diameter. Group 21 fungicide. 12-hr reentry.
Reference Wallace, E.C., and Crouch, J.A. 2018. First report of Peronospora digitalidis causing downy mildew disease on foxglove in Oregon. Plant Disease 102:827.