Late Blight is a fungal plant disease that primarily affects potatoes and tomatoes. It can also affect crops in the same nightshade family like tomatillos. The disease primarily spreads through spores that travel through the air and land on infected plants. It can also travel through the soil into the potato tubers. Late blight is particularly potent during hot and humid summers, and under wet conditions where it can spread throughout neighbourhoods and even destroy commercial crops.

A form of late blight was a factor in the Irish potato famine in the 1850’s, as the disease can survive inside potatoes and spread when planted out the following year.

Symptoms:
On potato tubers, symptoms of late blight include shallow brownish or purplish lesions on the surface. A dry brown rot can extend from these lesions into the flesh as well.

On plant matter, symptoms can be seem on both stems and leaves. Typically black lesions will appear and turn brown, particularly on the underside of the leaf. As the disease progresses, white spore masses will be visible when the lesions dry up on both the stem and leaves. Tomatoes will turn brown and continue to spread the pathogen if left untreated.

Preventing Late Blight
Spores from late blight do not survive above ground in Northern/cold climates. It is important to carefully plant disease free potatoes either certified from a seed supplier or from your own stock. Some potatoes and tomato varieties are resistant to the disease (though not immune).

In Northern Climates like we have here in Ontario, late blight typically doesn’t show up until late in the season after it migrates up from the deep south. However, late blight can be introduced from infected greenhouse plants

As the season progresses, check plants regularly (we recommend twice per week) for symptoms and remove, discard and/or bury affected plants to prevent continued spread of the blight. Be especially vigilant if warm humid conditions are present. For potatoes, hill up potatoes to increase the distance spores must travel to reach the potatoes and cut potato vines or wait 2-3 weeks after vines die back before harvesting potatoes.

Cover Image by Scot Nelson, used under its Creative Commons license.

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