Potato blight

What is blight? And how can you beat it?

Blight (Phytopthora infestans) is a virulent fungal pathogen that is capable of wiping out potato crops very quickly. Most of us who have grown potatoes have seen it in our crops. The best organic method of managing this disease is to take a range of preventative measures which reduce its chances of breaking out.

First we need to be able to recognise blight and how it behaves. It can strike from any time after June, and the incidence increases through the growing season. As it is airborne, it will infect the leaves first. You will notice it as distinct brown to grey lesions, about the size of a penny, close to the edges of leaves. These blotches often have a yellowish halo - not to be confused with smaller brown dots which are more likely to be nutrient potassium or magnesium deficiency. The leaves quickly become weak and watery and rot.

The disease is far more likely to take hold if the weather is damp and still, as this is ideal for allowing the fungal spores to settle and germinate on the surface of the leaf. Under these weather conditions it is a good idea to look for signs of infection. If you notice just a few infected leaves, remove them and dispose of them in your household rubbish. If dry conditions follow, you may be lucky and the crop can be salvaged.

If you are unfortunate, and the disease becomes widespread throughout the leaves, it might be time to cut your losses. It is best to remove all the tops at this stage as they will shed spores that wash down and infect the tubers, causing them to rot. Dispose of the leaves in your rubbish - do not put them on the compost heap as most domestic heaps are not warm enough to kill off infected material. You can still harvest your tubers, but leave them in the ground for 2 weeks, as this will allow the skins to set, sealing them up from disease.

Steps to avoid getting blight

  • Harvest early

Blight pressure increases through the season, so the earlier you can harvest your potatoes, the less likely they are to succumb to the disease. The best way to ensure earlier harvests is to select earlier maturing varieties, so go for first earlies, and early maincrops such as Maris Bard and Orla. Chitting your seed (letting it sprout first) before planting will also help your crop advance more quickly, but choice of variety will have, by far, the biggest effect.

  • Grow resistant varieties

Select varieties which have a better resistance to blight. Many lists showing which varieties have good resistance are out of date, as new strains of the fungus evolve quite regularly. However, Sarpo varieties, originating from Hungary show remarkable resistance to the disease (see below).

  • Tidy up tubers

Blight will not survive in the soil on its own, but it will remain on diseased tubers left in the ground. These are the main source of infection for next year’s crops, as are dumped tubers in piles or on compost heaps.

Sarpo varieties

These were developed at the Savari Trust (www.sarpo.co.uk) in order to minimise the chemical inputs that went into growing potatoes. The varieties show good blight resistance, compete vigorously against weeds, and can be stored for a long time without sprouting.

Sarpo Axona This red-skinned maincrop variety is suitable for baking, roasting and mashing. Both the tubers and the leaves show very good resistance to blight.

Blue Danube An early maincrop variety that has striking purple tubers and is very good for roasting and mashing. Unlike most Sarpo varieties, the leaves do not show good resistance to blight, but the tubers are very resistant so won’t rot if the disease strikes.

Sarpo Kifli Kifli is a waxy salad variety. It is an early maincrop, and both the leaves and the tubers show good resistance to blight.