Bordeaux Mixture, a mix of Copper Sulphate, Hydrated Lime and Water, can be used to prevent potato blight. Using Bordeaux Mixture after the blight is apparent is ineffective. Potato blight is most likely when the weather is warm, wet and windy in June and July, so spraying potatoes with Bordeaux mixture around this time of the season can be most effective in ensuring your crop isn’t ruined.
The mixture is approved for organic use and thorough coverage of the plants is necessary using a spray or watering can. A 1% Bordeaux Mixture can be prepared using the formula 1:1:100, where the 1’s are the weight in Kilograms of Copper Sulphate and Hydrated Lime and the 100 represents the water in lites (1kg Copper Sulphate, 1kg Hydrated Lime, 100 litres of water). The Copper Sulphate and Hydrated Lime are dissolved SEPARATELY in a small amount of water using two buckets, one for each ingredient. The mix of Copper Sulphate is then mixed into the bulk water container and then the mixed Hydrated Lime is added and the whole mixed together. The resultant mixture “Bordeaux Mixture” can then be sprayed or the plants can be watered with this using a watering can.
Bordeaux Mixture must be treated with care as it can be toxic if swallowed, it can also kill earthworms if the mixture builds up in the ground. Using Bordeaux Mixture on a few potato plants on the allotment should carry little risk to you or your soil, but keep applications to a minimum to be on the safe side.