Q What causes brassica leaves to go mouldy?
A Yellow spots on leaves and corresponding white, fluffy patches on the leaf undersides, are signs of downy mildew (Peronospora parasitica).
You don't usually see downy mildew outdoors, except in warm, humid areas near the sea, or in very sheltered, low-lying gardens. Mild, wet autumn weather is the most likely period for attacks. Look out for yellow-brown patches between the veins on the upper leaf surfaces, with white, downy patches beneath the leaves showing up only in wet weather. By then, the plants are big enough not to be seriously affected, although leaves may be lost and sometimes edible parts like cauliflower curds are damaged. Yield loss can also be caused by black spots and streaks inside veg, although other diseases may also cause this. Downy mildew damage can open the way for other diseases.
Plants in cloches, frames and greenhouses are much more likely to be infected. In spring and autumn, seedlings are the usual victims, when the weather is mild and humid and the plants are close together. Wallflower plants that are raised indoors are especially at risk in early autumn; they can be stunted or even killed by these attacks.
Caption: Yellow-brown patches appear between the veins when downy mildew occurs on brassicas
Q What can I do about downy mildew on brassicas?
A There's little you can do to protect plants as there are no chemical controls available to the gardener. Keeping down weeds and avoiding overcrowding will help prevent attacks. In cloches, frames and greenhouses, thin out the plants and ventilate as much as you can.
Q Are there any ways to avoid downy mildew?
A Downy mildew is soil-borne, so growing brassicas in a different spot each year will help prevent the disease. It gets into the plant through the roots and spreads to the leaves, where the downy patches form. Then it spreads from plant to plant on the breeze. Prevent spread, including to the soil, by pulling up and binning or burning affected plants.
Ensuring the soil has sufficient potassium and magnesium is said to help plants resist these attacks. Some varieties, such as the calabrese 'Marathon' are said to be tolerant of downy mildew.