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Downy mildew is a disease of the foliage caused by a fungus-like (Oomycete) organism. It is spread from plant to plant by airborne spores. It is a disease of wet weather as infection is favored by prolonged leaf wetness. A range of common edible and ornamental plants can be affected including brassicas, carrots, grapevines, lettuce, onions, peas, roses and spinach.
Unlike with powdery mildew, plants affected by downy mildew are not so easy to recognize. Look out for the following:
- Discolored blotches on the upper leaf surface. These may be pale green, yellow, purple or brown depending on the plant affected. The blotches sometimes have straight edges (e.g. on lettuce) if they are bordered by the leaf veins.
- A mould-like growth on the underside of the leaf, corresponding to the blotch on the upper surface. This growth may be white, grey or purple depending on the species of downy mildew.
- On some plants (e.g. Peas) the growth is easy to see with the naked eye. On others (e.g. Roses) it can be difficult to see, even with a hand lens.
- Severely affected leaves may shrivel and turn brown or turn yellow and fall prematurely.
- Occasionally other plant parts can be affected e.g. Cauliflower curds, Pea pods etc.
- Severely affected plants are often stunted and lack vigor. In some cases the plant may die.
- Pick off and dispose of by burying or burning affected leaves as soon as symptoms are seen.
- Remove and destroy severely affected plants.
- Avoid dense planting and control weeds so that there is good air circulation around the plants.
- In glasshouses, try to avoid prolonged leaf wetness or periods of high humidity. Avoid overhead watering where possible. Open the doors and vents when conditions allow encouraging air movement.
- Avoid watering plants in the evening as this can lead to high humidity or leaf wetness that persists throughout the night. Water early in the morning so that leaf surfaces dry out rapidly.
- To avoid infection from soil-borne resting spores, practice crop rotation for vegetables and avoid re-planting with the same host for at least a year where an ornamental plant has been affected.
Resistance: Cultivars with resistance to downy mildew are available for some vegetable crops. However, the disease resistance can break down if the downy mildew mutates to produce a genetically different ‘race’ so growing a resistant cultivar does not always guarantee freedom from infection.
Fungicides applied specifically for downy mildew control may be unnecessary. The following broad spectrum protectant fungicides may be considered as they are at least somewhat effective in protecting against downy mildew infection:
Copper oxychloride 85wp, Dithane M45/Mancozeb, Copper hydroxide 77, Bravo/Chlorothalonil, and Metalaxyl + mancozeb/Chemalaxyl.
Always consult Agricura Agronomists before using any of the stated products as there are a whole lot of unique technical aspects involved of which failure to adhere to these instructions may result in permanent crop or tree damage and significant reduction in yield and quality!