Potato blight is a major cause of trouble for all potato growers whether amateur or professional. It is caused by a fungus, which invades the plant and which causes the leaves, stems, and sometimes the tubers to rot.
It is surprising how few can recognise the symptoms. There are three stages:
1-- the blight spores germinate and start to grow on the leaves, waiting for optimum conditions - high humidity and warmth. This usually occurs in late June or early July. Faint brown freckles appear on some of the higher leaves; in some varieties the tips of some of the leaves go a bit darker.
2-- small brown blotches appear. By this time the disease has got a hold, and without determined action the plants will die.
3-- big brown blotches, shrivelled leaves, yellowing, new leaves furled upwards in some varieties, looking as if they are short of water. By this stage some of the stems will be slimy, and the smell of rot will be apparent.
As a keen amateur grower I've found that blight can be contained but not completely stopped. It all depends on spotting the blight at stage 1. When you've found it, you spray the plants every 2-3 days with Bordeaux mixture - also pull off any infected leaves and dispose of them. The spray will prevent NEW leaves becoming infected and will slow down the spread of infection on older leaves.
There are systemic fungicides around but you need a bank loan to buy them. Bordeaux mixture, if used regularly, is cheap and effective and you dissolve the compound in water, so it goes a long way. If you can't buy Bordeaux, mix copper sulphate crystals with about twice the amount of lime and dissolve in water. About a teaspoonful of solid per litre. I use a small hand sprayer containing about 1 litre and it works fine. Shake it up in a larger bottle and let it settle or you'll block the spray nozzle.
If you grow intensively as I do, another danger is that when the plants start to sprawl, a rainstorm can cause the whole lot to collapse. To avoid this I trim the haulms regularly and stop them getting more than about 15" high, so they remain upright even in heavy rain. This increases rather than decreases the yield. Once the haulms have collapsed, they'll rot if blight is around because there's no way for air to circulate around the stems. It's also worth mentioning that from a blight point of view, irrigating from underneath is better than watering from above.
Nigel Deacon / Diversity website
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