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Late Blight

Phytophthora infestans

Late Blight, Tomato Blight, Potato Blight

Late Blight is one of the two fungal potato blight diseases that caused the Irish potato famine in the 1840s. The other, Early Blight, is much less severe. The disease will almost exclusively attack potatoes and tomatoes, rendering whole crops inedible. It spreads incredibly fast in humid and wet weather which is why there are weather warnings sent out to potato growers when there is going to be a stretch of warm, humid weather so that they can preemptively spray a fungicide to protect their crops. This disease is incredibly hard to get rid of once it has established itself as it can survive in rotting potatoes that are not removed from the soil.
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Phytophthora infestans can infect both potatoes and tomatoes and presents itself in different ways. Potatoes: The leaves develop dark brown/black blotches and sometimes have pale margins. These usually appear at the leaf tips or edges. If the conditions are damp, white mould can appear on the underside of the blotches. This causes the blotches to grow in size as well as sometimes transmitting to the stems. If humidity is high, the whole foliage can collapse. This often causes the rapid spreading of the disease. The tubers develop brown lesions on the surface that spread and envelop the entire tuber causing dry rot. The internal tissue of the tubers has a brownish-red speckled appearance. In storage, the fungus does not spread from tuber to tuber. Tomatoes: The foliage suffers similar symptoms but to a lesser degree. Dark streaks can appear on the stems and fruits and cause mature fruit to decay rapidly. This decay is usually shown as a dry rot but can remain symptomless for days after infection, meaning that the fruit can begin to rot after being picked. Tomato infections are more common when plants are grown under glass.

Growth factors

Optimum conditions for the spread of the disease are a temperature above 10 degrees and a humidity above 75% over a period of 48 hours. Though damp conditions, in general, are also favourable to the disease.


Dark blotches appear near leaf margins. Can have white mould on underside of blotches
Leaf margins are pale.
Complete death of foliage.
Tubers covered in brownish lesions.
Tuber flesh has reddish brown speckles.
Tuber consumed by a dry rot.
Black stripes on green fruit and stems.
Premature rotting of ripe fruit.

Chemical treatment

Some copper-containing fungicides can help prevent and treat a small outbreak, but these need to be applied at the correct time and are only worth doing if there is a large harvest of potatoes at risk.


The pathogen can survive in previously infected potatoes that have been left in the ground over winter which infect new plants through the roots. Alternatively, spores can be blown on to healthy plants or splashed into the soil and infect a crop.


There are some resistant cultivars of potato, however, they do not possess the same cooking qualities that many of the commercial cultivars do. Don't use land to grow potatoes that has previously had the disease as it can survive in the soil in pieces of plant that have been left behind.

Affected plants

A close up of a white and yellow Solanum tuberosum flower


Solanum tuberosum

Some red Solanum lycopersicum tomatoes in a garden


Solanum lycopersicum

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