Potato and Tomato blight is caused by the same fungal organism (Phytophthora infestans)
This is the scourge of commercial potato growers and can devastate a hole seasons crop for the allotment gardener.
The disease over winters in infected tubers that have not been adequately disposed of, and as the spores are distributed by the wind, infections on allotments and in vegetable gardens can originate many miles away.
Blight disease spreads during periods of warm and humid weather with rain, any time from June onwards but most prolific in July and August.
Symptoms of Potato and Tomato Blight:
The initial symptoms of blight are quiet difficult to spot so regular inspection of vulnerable crops is always a wise move.
The first sign of blight are dark or blackish spots on the leaves or stems, these rapidly increase in size if the conditions are damp and humid. White spores will become visible around the dark patch as the disease progresses.
If the conditions change and the weather turns very dry the spread of the fungus may look to have stopped, but once conditions become favourable again the spread will restart.
For the amateur grower there is no effective cure for blight on Potatoes or Tomatoes so the only cause of action is protection.
Good crop rotation and the removal of any infected crop debris from the previous season is essential.
Chemical control of blight for the allotment gardener is becoming increasingly difficult because of the constant regulations restricting their use.
Buyer Garden, Fruit and Vegetable Disease Control is one of the few chemicals that helps protect Potatoes and Tomatoes from blight and is available to the vegetable growing public.
This is a copper biased fungicide that needs to be applied before the onset of the disease.
Crops are at their most vulnerable when the weather is consistently warm, wet and humid ( Minimum temperature 10°C with 11 hours of 90% humidity each day) known as a Smith Period.
A lot of information about Smith Periods, Blight and potato growing can be obtained from the Potato Council where you can sign up to Blight Watch. They will then send you alerts when the weather has met the criteria of a Smith Period, which is a good time to reinforce the protection you’ve given the plants.
Blight On Potatoes:
The potato tuber is infected with blight by the spores of the disease being washed down into the soil by the rain. If the disease attacks late enough for the potato tubers to have grown to a usable size all is not lost.
Remove the foliage of the plant completely and harvest the crop a few weeks later. This wait is to allow any spores on top of the soil to die, thus you wont infect the crop as you harvest. The potatoes will not be suitable for storage so will need to be used.
Early varieties of potatoes are less vulnerable, as these are lifted before the blight becomes widespread. There are also varieties that claim to be blight resistant these are Sárpo Mira and Sárpo Axona.
Blight On Tomatoes:
Outdoor tomatoes are the most vulnerable but greenhouse crops are not immune and if the disease penetrates the protection provided by the glasshouse the spread is very rapid due to the warm, humid conditions inside.
Once the Tomato plant becomes infected with blight there is very little that can be done by the amateur gardener to rescue the crop.
Removing the infected leaves and regular spraying with a copper biased fungicide may reduce the affects but it is very unlikely that the crop will yield any reasonable fruit.
Other sources of information on blight: