Potato blight is a disease that can rapidly destroy an entire crop of formerly healthy potato plants, leaving you with little more than a row of moldy leaves, and a minuscule crop of tiny potatoes that won’t store and therefore, need to be eaten virtually immediately.
Blight is most likely to occur in warm damp weather conditions, and as little as two days of warmth and high humidity, can result in blight warnings being issued. The disease is carried on the wind in the form of spores that can travel many miles, with outbreaks usually occurring between June and September (mainly in wet areas of the country).
The spores land on the damp potato leaves and before long, dark grey/black colored finger nail sized patches appear around the edges. As the disease advances, a white mold can frequently be seen on the undersides of the leaves (although this is harder to see in damp weather).
Stems also begin to display black areas and mold, eventually beginning to rot away. The haulms (foliage) can turn yellow, even before the fungus has turned the entire plant black. As the blight spreads spores are washed into the soil below, where they then affect the potato tubers too.
This is visible on the tubers in the form of white spots of mold on the skin, and rust brown patches that also spread into the flesh of the potato itself (see image below).
Blight has a strong unpleasant smell to it that some gardeners even claim to be able to smell in the air on a warm damp day. In a matter of a few days, your entire crop can be destroyed, and worst of all, there is no cure!
The best you can hope for is to take all reasonable precautions to prevent your potatoes catching the disease in the first place.
Steps You Can Take to Grow Organic Blight Free Potatoes
Grow Blight Resistant Varieties:
Experts have spent years developing varieties of potato that are naturally resistant to blight. The leaders in this field are the Sárvári Research Trust, and many of the Sárpo varieites such as Sárpo Mira and Sárpo Axona are now readily available. Both of these are maincrop varieties that have a very good flavor, and each are extremely blight resistant as are the other Sárvári varieties.
Plant Resistant Early and Second Early Varieties:
Many other potato varieties can also be somewhat blight resistant e.g. Orla, Charlotte, Sante, Romano, Setanta and Casablanca. First and second earlies have the added advantage of frequently producing a good crop before the blight can strike, and of course, these types are at their tastiest when small anyway. If at all possible, try to plant your potatoes as early as possible in order to give them the maximum amount of growing time before blight spores become a danger.
Only Buy Certified Seed Potatoes:
Do not save your own seed potatoes, especially if you had blight the previous year as they may still be carrying spores even if the tubers look perfectly healthy.
Rotate Your Potato Crops:
Leave three years between growing potatoes on the same patch of ground again. This also helps to prevent problems with potato cyst eelworm and the blackleg disease.
Feed Your Crop:
Potatoes are a hungry crop so feed them regularly with seaweed, well rotted manure, compost or organically approved potato fertilizer. Weak plants are far more vulnerable to disease than healthy ones.
Protect Early Potatoes:
Use horticultural fleece, enviromesh netting, straw, grass clippings, cardboard, old bed sheets etc. to protect emerging shoots from any frosts. Remember that the shoots still need light in order to photosynthesize, so any opaque coverings such as the sheets or cardboard will need to be removed during the daylight hours.
Keep Leaves Dry with Covers:
If the weather turns damp and humid and there is therefore a risk of blight, cover your potato crop with a sheet of clear polythene to physically block the blight spores from making contact with the leaves.
This will also keep the leaves dry which means, any spores that get past the barrier have little chance of being successful. As soon as the weather improves, remove the polythene so that air can circulate freely again.
If you can’t keep the leaves of your potatoes dry, then there is a theory that spraying/splashing the leaves with liquid seaweed, manure (or even compost) diluted in water introduces microorganisms that will compete with the blight spores for leaf space.
Earth Up Your Potatoes:
Draw up a good ridge of soil around your potato plants as this too, acts as a physical barrier that should help reduce the amount of blight spores washing down into the soil, where they can infect the tubers.
Destroy Damaged Foliage:
As soon as you see any signs of the disease on the leaves of your plants, make sure you remove the infected foliage and burn it (do not under any circumstances add these infected leaves to your compost heap or bin). If you act promptly, you may even prevent the disease from returning.
If the problem does return, then cut down the foliage to ground level before the problem can spread to the roots. Alternatively, stand astride the plant and pull the stems out leaving the roots below ground.
Firm the soil after the bed is clear of foliage, then wait a few weeks before lifting the crop to make sure none of the blight spores have survived on the surface of the bare soil.
Lift Every Potato:
Make sure you completely dig over your potato bed, and that not even small potatoes remain in the soil. Even just a few missed tubers could carry blight spores through the winter, sprout the following spring and then spread blight throughout your new crop.
Inspect Your Stored Tubers:
Store your potato crop in large flat trays or boxes, so that you can easily check through them every few weeks and remove any tubers that are showing signs of disease or damage. It doesn’t take long for one rotten potato to infect its neighbor and so on. As a further precaution, make sure you also burn any tubers that start to show signs of damage or disease.
Don’t panic about every single brown spot or patch you see on your potato leaves, there are lots of reasons you may see these, including other less destructive diseases (although you should still destroy the foliage at the end of the growing season as a precaution, and avoid saving those potatoes as seed potatoes).
Even watering in the sunshine can cause scorch marks on the leaves, when the droplets of water sitting on the foliage, act as miniature magnifying glasses that cause the sun to burn patches on the surface beneath.
If you are going to manually water your potato crop, then it is best to do it early in the morning, so that the leaves have a chance to dry out before the sun is at its strongest. This also means that unlike if you water in the evenings, the leaves will not spend all night damp which can itself encourage fungal diseases.
Be aware that both potatoes and tomatoes come from the same Solanaceae family (the nightshades) which means, potato blight can also spread to your tomato crop (especially outdoor crops). For this reason, if either your tomatoes or your potatoes become infected with blight, take appropriate precautions with the other crop.
Tomatoes in the greenhouse are not safe either, as blight spores can blow in through vents and be brought in on your clothes. If there is any humidity in the greenhouse, the blight can also destroy your tomato crop very rapidly.