In the summer of eighteen forty-five, potato fields across Europe turned black and rotted in a matter of days.
In Ireland, where millions depended on the potato,
the result was the Great Famine.
The cause was late blight.
More than a century and a half later, it is still one of the most damaging diseases of potato in the world, and it attacks tomato too.
In this video: what late blight really is, how to recognise it and tell it from early blight, the weather that drives it, where it comes from each season, how to prevent it, how to use fungicides by FRAC group, and how to protect the tubers at harvest.
The advice is general and global.
Your local extension service and product labels always come first.
Late blight is caused by Phytophthora infestans. Although it is often called a fungus, it is a water mould, an oomycete, more closely related to brown algae than to true fungi.
That difference matters, because many fungicides that control true fungi do little against it.
It spreads by sporangia, tiny spore cases that form on infected leaves in humid weather.
Wind and rain carry them from plant to plant and from field to field, sometimes for many kilometres. In cool, wet weather a new generation of spores can be produced in about five days, so a few infected plants can become a ruined field within two weeks.
On leaves, late blight starts as pale green or greyish, water-soaked spots, often at the tips and edges of the leaflets.
The spots grow quickly into large brown to almost black patches, often with a pale green border. The single best sign is on the underside of the leaf. In humid weather, especially in the early morning, a fine white, downy growth appears at the edge of the lesion, where it meets green tissue. That white ring is the pathogen producing its spores.
On stems, look for dark brown lesions that are often greasy and can girdle the stem so that it collapses.
In a badly affected field, the canopy looks scorched, and there can be a distinctive smell of rotting foliage.
On tubers, late blight causes a reddish-brown to purplish discolouration of the skin.
Cut the tuber and you see a granular, rusty-brown rot spreading inward from the surface.
Other rots often follow, turning infected tubers soft in storage.
On tomato, fruits develop firm, greasy, brown patches, often on the upper side.
The disease most often confused with it is early blight,
caused by the true fungus Alternaria solani. Early blight spots are smaller, dry and brown, with dark concentric rings like a target, often with a yellow halo, and they start on the older, lower leaves.
Late blight lesions are larger, water-soaked at the edge, grow fast in wet weather, and carry that white downy ring underneath. Early blight never shows the white growth.
Getting this right matters, because some fungicides that work well against early blight have little effect on late blight, and the other way round.
Late blight loves cool, humid weather.
The most dangerous conditions are mild temperatures,
roughly ten to twenty-five degrees Celsius, with long periods of high humidity or wet leaves:
foggy mornings, drizzle, heavy dew, or evening irrigation that keeps the canopy wet overnight.
Many countries run forecasting systems based on these conditions.
In Britain, for example, a Hutton Criteria period is two consecutive days with a minimum temperature of at least ten degrees and at least six hours of relative humidity of ninety percent or more. Other regions use other models and decision-support tools. Find the one for your area, and treat a warning as a reminder to protect the crop before the infection, not after.
Late blight does not appear from nowhere.
Each season it usually starts from one of a few sources.
Infected seed tubers, which can carry the pathogen hidden inside.
Cull piles, the heaps of discarded potatoes at the edge of fields and stores, which sprout and become infected early.
Volunteer potatoes growing from tubers left in the soil after last year's crop. And nearby infected tomatoes or potatoes in gardens.
Where both mating types of the pathogen are present, it can also form thick-walled oospores that survive in soil for years.
These populations also mix their genes more, so new strains appear faster.
Remove the sources, and you delay the first infection, sometimes by weeks.
Prevention starts before planting.
Plant certified, healthy seed.
Get rid of cull piles completely, by burying them deeply, feeding them to livestock where that is allowed, or covering them so that nothing sprouts.
Destroy volunteer potatoes in other crops.
Choose a more resistant variety where you can. Some modern varieties carry several resistance genes and hold out much longer, although the pathogen can adapt, so resistance works best combined with the other steps.
In the field, hill the rows well, so that the tubers sit under a thick layer of soil.
Spores washed down by rain then have further to travel to reach them. Give the canopy room to dry.
If you irrigate, water in the morning, not the evening.
And scout often, especially after blight weather, starting at wet, sheltered corners and near any source.
Fungicides against late blight are mainly protectants. They must be on the leaf before the spores arrive, because no fungicide cures an established lesion. In high-risk weather, sprays are typically repeated every five to seven days, so that new growth is protected.
Interval and dose come from the label.
Choose products by FRAC group, the mode-of-action code on the label.
Multi-site protectants, such as the copper compounds, group M one, carry a low risk of resistance,
and copper is the main option in organic systems, though it is less effective.
Several single-site groups are active against oomycetes, for example group twenty-one, group forty, group forty-three, group forty-nine, and fluazinam in group twenty-nine. Some, like group twenty-seven, have a short curative action of a day or two, and are used in mixtures.
The phenylamides, group four, were once the backbone of blight control.
Resistant strains spread widely, so in many regions group four is no longer reliable on its own. In recent years, strains with reduced sensitivity to some newer single-site groups have also appeared in parts of Europe. So use single-site products in mixtures with a partner from another group, rotate groups between sprays, never exceed the number of applications on the label, and follow your local resistance advice.
Registrations differ from country to country, and some products are restricted in some regions.
The last weeks matter as much as the first.
If blight is in the field, the foliage can drop spores onto the soil and infect tubers at harvest.
Kill the haulm, the green tops, and wait about two to three weeks before lifting. This lets the skins set and gives any spores on the soil surface time to die.
Lift in dry conditions, handle the tubers gently, and do not store tubers with blight symptoms.
In storage, check regularly, keep the store cool,
dry and well ventilated, and remove rotting tubers at once.
Finally, when the season ends, clear volunteers and culls, so that this year's blight does not become next year's.
To recap.
Late blight is a water mould, not a true fungus.
The best sign is the white downy ring at the edge of a lesion, under the leaf.
Early blight has target-like rings and no white growth.
Watch for cool, humid blight weather and use your local forecast.
Remove the sources: infected seed, cull piles and volunteers.
Plant resistant varieties, hill well, and water in the morning.
Spray protectants before infection, choose by FRAC group, mix and rotate.
And kill the haulm two to three weeks before a dry, gentle harvest.
For more on late blight, and on every other crop disease, visit laisha.ai. Thank you for watching.
