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Cause Peronospora grisea, a disease that typically occurs on plants crowded together in moist, shaded environments. In nursery production, plants that are overwintering under plastic are susceptible, particularly if they are placed too close together and if the young plants become rootbound. Overhead irrigation that keeps leaves wet can also favor the disease. The disease is rare in the landscape in the Pacific Northwest but plants can be infected that are subject to heavy shade. The disease has also been observed on potted plants grown in coastal areas that are subject to summer fog.
Symptoms The disease is usually noticed first by a yellowish discoloration of the leaves, often on the lower leaves of the plants. This discoloration can affect large portions of individual leaves. Leaves become curled and distorted. A fuzzy fungal growth may develop on the underside of the leaves. Eventually the affected parts of leaves turn brown and shrivel. Plants can be entirely defoliated and killed.
Cultural control
- In the landscape, ensure that plants are well-spaced and in full sun.
- Avoid overhead irrigation unless the foliage can dry quickly and thoroughly.
- Removal of individually affected leaves or stem sections can assist in plant recovery.
- In nursery production, avoid crowding plants together while overwintering in plastic hoop-houses.
- Scout plants carefully for symptoms of the disease especially the undersides of leaves and any stock plants or plants held over from the previous year.
- Maintain adequate fertility.
Chemical control There are no chemicals specifically labeled for this crop; however, many can be used if not phytotoxic. Tests in England have shown that the following chemicals are effective at 7- to 14-day intervals. Rotate or tank-mix materials from different groups with different modes of action to prevent buildup of resistant fungi.
- Aliette at 2 lb/100 gal water. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Heritage at 2 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Segovis at 0.6 to 3.2 fl oz/100 gal water plus another fungicide. Group 49 fungicide. 4-hr reentry.
Reference O'Neill, T. M. and Bobbin, P. 2000. Evaluation of fungicide programs for control of downy mildew (Peronospora grisea) in protected Hebe. Tests of Agrichemicals and Cultivars 21:5-6.