Potato Blight


Potato and Tomato Blight - phytophthora infestans is a disease that has been around for a long time, the Irish potato famine of the 1840's is testament to that!

Since then the disease has adapted to produce more aggressive strains thus making it more difficult to control.

It is quite a serious disease, affecting outdoor tomatoes as well as potatoes!

Tomatoes grown under cover tend to be less susceptible as the cover denies access to the air-borne spores.

As a consequence potatoes are usually infected before tomatoes.

The fungus is spread by windborne spores.

These spores penetrate and infect the host tissue, which subsequently dies.

The disease is more prevalent during wet and or warm humid weather.

Spores can over winter in dead plant tissue or infected potato tubers left in the ground and initiate infections the next season.


The symptoms are seen as brown, watery lesions which rapidly spread on to the leaves and stems.

Affected foliage can rapidly becomes a foul smelling slimy mess, its prompt removal can often save the crop.

If this foliage is allowed to remain in place, the spores may be washed down into the soil during rainy spells and infect the tubers.

Leaf symptoms

Infected tubers will have a brown or purple discolouration on the surface, and reddish-brown spotted markings in the flesh, making them inedible.

Destroy the infected foliage, on no account compost it.


Spray the haulms with a suitable copper based fungicide before symptoms occur and every ten days thereafter.

Spraying will not prevent infection, but may slow it down and save the crop.

Clear away and destroy all foliage whether affected or not as prevention is better than cure.

Similarly, the following year, remove any plants that have formed from tubers that were missed at the previous years harvest in case they are fungus carriers.

Blight affected haulms
Haulms removed and destroyed

The fungus can be seed borne, so do not save seed from infected fruit and destroy all infected plants and do not compost.

Maintaining a good crop rotation plan can avoid the possibility of coming in contact with foliage residue.

Good soil preparation can prevent the need to water plants during the growing season thus reducing humidity around the plants.

If watering is deemed necessary water the base of the plants, do not water over the plants as this can create the ideal conditions for spore adhesion.

Purchasing blight resistant varieties can also help.

This website can be used to track periods when blight is most likely to strike.

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