Fighting potato blight
Although trace element deficiencies are more common than many gardeners think, our crops do suffer from their share of genuine, and one of the worst is potato blight which can only be prevented, not cured.
The best organic answer to this problem is to grow one of the many blight-resistant varieties, of which Maris Piper is the best.
If yourhave rounded, brown-black patches that start at the leaf edges, they have potato blight, and your best policy is to cut them down and put the haulms in the middle of the heap, for the blight spores are killed at 50°C (120°F), and a good compost heap should heat up to at least that temperature at the centre. A better alternative, which ensures that you do not spread the spores over the garden (or your neighbour’s), is to spray with Bordeaux mixture.
This is made by tying 270 g (9 oz) of copper sulphate in the toe of an old pair of tights and hanging it overnight in a bucket or watering can containing 18 L (4 gal) of cold water until it is completely dissolved. Stir 180 g (6 oz) of quicklime into a bucket which contains 4.5 L(l gal) of cold water, mix the two solutions and spray the potato haulms. Bordeaux mixture will kill the blight spores very effectively.
By reducing the amount of copper sulphate from 270 g (9 oz) to 180 g (6 oz) and proceeding with mixture as above, the resulting moderate strength Bordeaux mixture can be used to combat apple and pear scab. Ready-made Bordeaux mixture can also be bought either as powder or paste from chemists and garden centres.