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Tomato Blight, Identify and Treatment

The first signs of tomato blight will be on the leaves. Brown marks nearby will appear and they will spread rapidly to other leaves and plants. If the tomato plants have fruit on them at the time of infection then the next signs will be browning of the fruit which will begin to rot.


It's caused by the fungus Phytophthora infestans which is also the cause of Potato Blight. It's a wind and rain borne fungus. If you have an allotment the fungus will be there you can be sure of that. It is simply waiting for the correct weather conditions to spread to growing crops.

When the weather becomes warmish and wet the fungus begins to get ready to disperse on the wind and rain. I takes roughly 3 to four days of these conditions for the spores to release into the atmosphere. potatoes normally show the first signs, tomato plants next. The period of warm, wet weather necessary for the spores to become active is know as the "Mills Period".


There are no organic or chemical treatments which will prevent Tomato Blight. There are organic actions you can take which will lessen the risk of Tomato Blight and there are chemical sprays which can slow down its progress significantly but only if the sprays are applied before infection. These are discussed below in what we believe is their order of importance.


1. Destroy infected plants, leaves, fruits, everything. Burning them is the best way. Leave nothing form an infected plant on the ground, it may well be the cause of re-infection next year.

2. The spread of the disease can be slowed significantly if a copper fungicide (not organic) is applied before the plants become affected. Spraying after the plants are affected is a waste of time.
The risk of Tomato Blight begins to increase significantly after 48 hours of wet and warm weather. After about 90 hours of those weather conditions, the spores will certainly begin to be airborne. This period of wet and warm weather is known as the "Mills Period".
Farmers will only spray their crops with a copper fungicide when they believe the weather is warm and wet enough for their crops to be at risk. Spraying needs to be repeated every 10 to 14 days for it to be effective.

3. If tomato plants are not badly affected by Tomato Blight, a period of dry weather may also slow down the progress of the disease. You can help increase your chances of success against this disease if you remove and burn any leaves or fruit which appear to have tomato blight as soon as you see them. Your hope is for the weather to turn dry. Removing affected parts in combination with dry weather may slow down the infection.