Whether or not you spray there are a few simple things you can do to avoid or reduce infection with blight.
As with all pests and diseases it pays to know your enemy.
Potato blight is a fungus that is spread by spores. It first appears as small dark brown irregular spots on the leaves which get bigger. Turn over the leaf and you will usually see a grey mould which is the spores. (put potato blight into google images) These spores float in the air and if they land on a leaf cause further infection. This may be inches away on the same plant or at a considerable distance.
Two things to remember which help to combat the disease. First, it can only live on live material. Second, it needs warm weather and high humidity to develop. ( Farmers are warned when there is a "Smith Period". That is two consecutative days when the temperature stays above 10 degrees C and on each days there are 11 hours when the relative humidity is above 90%.) When a Smith Period has occurred it is likely that blight will develop within 15 to 20 days. Even when the weather seems OK it may still be humid down in the furrows, especially if watering has been generous.
Infection starts on the leaves but then spreads to the stems and down to the tubers. Spores on the ground can also be washed down to infect the tubers.
Do not put potatoes on your compost heap because they may be a source of infection, especially if they are blighted. Look at your neighbours plots and see if there are potatoes growing out of their heaps. If there are either quietly pull them out or ask them to do so. Seal them in a plastic bag so that they rot and die.
Look for left over potatoes growing among your other crops. These are not a bonus crop but a source of blight. Root out and destroy.
Look at your neighbours potato crop. If there is blight tell them. Expect blight in your crop soon. This is the time to start spraying if you want to so do.
Now that we have been told not to spray copper in bordeau mixture the only real option is dithane. This does not kill blight but protects the leaf. Although it sticks on the leaf well it does not protect any new growth. Spray weekly when growth is rapid and stretch this to fortnightly in dry weather.
Whether you spray or not eventually blight will begin to get a hold. Cut off the tops at ground level and destroy them. Either burn, seal in plastic bags or transport to the Council green collection. There will still be spores on the soil which can infect the tubers. They will not survive for long once the tops are gone and general advice is to leave for 3 weks before lifting.
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May 01, 2009, 22:53