Potato Blight

Most vegetable gardens will be affected by potato blight at some stage. Fortunately, there are measures you can take to treat and prevent it.

What is potato blight?

Potato blight (Phytophthorci Infestans) or ‘late blight’ is a devastating fungus disease which spreads rapidly in wet weather. Like a mildew disease, to which it is related, it attacks the foliage but can get washed down to damage the tubers. Blight also attacks tomatoes, and has been recorded on aubergines. Related plants like peppers seem immune.

How do I recognise potato blight?

The first signs are small, dark spots, often on the edges of the leaves. If the weather is wet, a white mould surrounds these spots, usually beneath the Leaf. This shows that spores are being produced. The spots grow, covering the whole leaf. They also spread on to stems. Infected tubers have brown and purple skin blotches that go into the flesh of the tuber. The flesh is brown/red and granular. Infected tubers may shrivel and dry, but are often infected by rotting organisms and liquefy in storage, smelling strongly and contaminating other tubers.

Could I mistake it for anything else?

‘Early blight’ also blackens leaves, but with concentric rings of darkened leaf. It usually attacks older leaves and does little damage. Viral diseases can also speckle leaves with black spots and stunt growth. Tubers can be infected with scab, spots, rots and gangrene. The spots and rots can be difficult to tell apart from blight as the end result, a liquefied potato, is the same. Fortunately, the same countermeasures apply.

When should I expect an attack?

Blight can develop if the temperature is 10°C plus for two days with high humidity. In these conditions, the disease spreads rapidly. In hot, dry weather it temporarily ‘dries up’, but breaks out again if conditions become favourable. This typically happens during early summer in the west and late summer in the east. At these times, be ready to apply preventative sprays.

How can I avoid it?

Dig out as many tubers as you can when you gather the potatoes. At the of the season, remove all potato tubers even tiny ones and destroy them. Digging over the plot so frost can get at and kill any tubers left behind will help. Throw away, burn or bury deeply any leftover or rotting tubers from storage, so they can’t grow the following year. Protect new crops by earthing up well. A layer of soil will protect tubers from spores falling from infected foliage.

Can potato blight be sprayed?

Spraying every 10-14 days using protective fungicides based on mancozeb or copper will prevent the disease from spreading. Since these sprays prevent but do not cure blight, you need to cover all the foliage, every time you spray. As blight is a problem most years, it is worth starting to spray regularly from early July. In bad years you can only expect spraying to prevent blight partially, but aim to keep the foliage going long enough for tubers to swell.

What should organic gardeners use?

Copper fungicide in moderation is considered to be acceptable for organic growing. Use it occasionally in conjunction with resistant varieties and early harvesting and you should find this gives adequate protection.

Can I rescue infected plants?

Cutting off infected foliage can prevent spores from reaching the tubers. Leave at Least two weeks between removing the foliage and lifting the tubers, so viable spores lurking on the soil surface don’t contaminate the tubers as you lift them. Store the tubers in dry, cool conditions to reduce the disease’s activity and subsequent rotting. Check the potatoes every month, to get rid of rotting ones.

Can potato blight be prevented?

Using certified disease-free seed potatoes and good garden hygiene will help to prevent blight, but the spores of the disease are carried on the breeze, so few gardens will escape. But you could try
covering the crop with an open-ended polythene tent to keep the foliage dry. Avoid high-nitrogen fertilizer, as this stimulates growth of soft, blight-susceptible foliage. To avoid the worst of blight
outbreaks you could also grow early or second-early varieties and lift before August.

Are there any resistant varieties?

Maincrop varieties with some resistance include ‘Cara’, ‘Cosmos’, ‘Lady Balfour’, ‘Pomeroy’,
‘Remarka’, ‘Romano’, ’Sante’, ‘Stirling’, ‘Valor’ and ‘Verity’

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