Judging by number of queries and enthusiastic feedback relating to potatoes we receive here at GrowVeg, you're clearly mad for these totally tasty tubers! And I don't blame you – they're easy to grow and there's nothing more satisfying than unearthing those nuggets of goodness from the soil at harvesting time. Power to the potato I say!
The trouble is potatoes are notorious for one particularly devastating disease: potato blight. Unfortunately for us, like all good diseases blight has a habit of evolving as quickly as the breeders' efforts to outwit it, always remaining just one step ahead of the game.
Symptoms and spread
Blight in potatoes is caused by a fungus that goes by the Latin name Phytophthora infestans. Its symptoms are unmistakable: small brown-black spots appear on the leaves, often surrounded by a pale halo, while the underside of the leaves may take on a white, downy appearance in wet weather – these are the hyphae by which the fungus colonises. Blight can spread with impressive speed, causing the complete collapse of foliage within a few short days. In severe cases the tubers underground can become infected, giving rise to sunken patches and a brown rot. This usually leads to secondary infection by other bacteria and fungi to give a most unpleasant stink as your prized potatoes turn into a mushy mess.
Originating from seaweed, blight loves nothing more than a spell of warm, humid weather (harking back to its maritime roots perhaps?). Infection reaches a peak during 'Smith Periods' – defined as two consecutive days when temperatures fail to dip below 10°C (50°F) and humidity remains above 90 per cent for the majority of the day. Spores can blow in from literally miles away, so trying to stop the spores from reaching your plants is futile.
Infection in ideal conditions is almost guaranteed if the recipient leaves are moist, which allows for efficient transfer of the troublesome spores into the plant's vascular system. If it happens to be raining then the spores from blight-bashed foliage will drip down onto the soil to wheedle their way towards those precious tubers.
What to do
There's a lot you can do to avoid blight in the first place, but if it suddenly strikes the first task is to remain calm – don't panic just yet! Cut out the infection as it's possible you may have caught it just in the nick of time. Wipe the blades of your pruners with detergent between cuts to avoid inadvertently spreading the disease from plant to plant. Then pray for dry weather.
If more than 10 per cent of the foliage is infected I'm afraid you'll have to cut away the foliage completely as this is the point of no return. This will clearly stop the tubers underground from growing any further but will equally stop the disease in its tracks (left alone it will have inevitably destroyed your entire crop). Once the foliage is removed, leave well alone for three weeks before excavating your tubers. By this time they will have developed a thicker skin and will be ready for storage while any lurking blight spores should have perished. Thoroughly inspect the tubers for any signs of blight, discarding any that have succumbed. Dry the sound tubers (never, ever wash them clean!) before storing in a cool, dark and dry place.
Infected foliage is fine for composting in a thriving compost heap. Bury the foliage among other material and keep the composting process progressing smoothly by turning the material to keep it hot. The spores won't remain active for long, so the chances of the finished compost infecting a new generation of spuds is exceedingly remote. The spores can, however, overwinter on potatoes, so keep blighted tubers well clear of your compost heap.
As my granny always used to tell me, prevention's better than cure, so the ideal solution is to sidestep blight altogether. There's a three-pronged plan for this: growing outside of the blight danger period, choosing blight-resistant varieties, and meticulous cultivation and hygiene.
Growing outside the danger period: Potatoes are grouped according to planting date and the time they take to reach harvest time. The earliest to latest spuds are in this order: first earlies, second earlies and maincrops. When started off early enough in the growing season, most early varieties and some of the earliest-to-mature maincrops will be harvested well before high summer when the risk of blight increases. One idea to guarantee a potato haul ahead of the blight period is to plant potato growing bags of spuds under cover in early spring, taking care to protect against frost, before moving them outside by late spring. This will almost certainly give a crop by early summer. See my blog entry on growing potatoes in containers for more on this.
Blight-resistant varieties: Some varieties display a reasonable amount of resistance to blight. However, this is a constantly changing picture as varieties once resistant fall susceptible to the ever-adapting fungus. Pick a variety that is known to have both resistant foliage and tubers. Suitable varieties include those in the 'Sarpo' range – such as 'Sarpo Mira' or 'Sarpo Axona'. North American gardeners should also keep an eye out for 'Defender'.
Cultivation techniques and hygiene: Only ever plant fresh, certified disease-free seed potatoes. Saving your own tubers is a false economy and serves to dramatically improve the prospects for blight and other diseases.
If watering during a dry spell, apply the water to the base of plants to prevent wetting the leaves. Do this in the morning so that any moisture left on the foliage evaporates during the warm of the day. Take the time to properly earth up and mulch with organic matter to lock in the moisture at ground level. Extra-thick mulches, for example with straw, will give any spores dropping to the soil surface much further to travel, insulating the developing tubers against infection.
Never allow potato volunteers – plants that spring up from old tubers – to remain. Grub them out so you don't inadvertently carry over problems from one year to the next. In the same vein, ensure you harvest every last potato at harvest time. This will give blight nowhere to hide during the winter or inactive periods.
All of the above may feel like a military operation and in most years you probably wouldn't get blight at all. But forewarned is forearmed with a little knowledge and careful cultivation there is no reason why potato blight should ever dampen your efforts.
By Benedict Vanheems.
Photographs courtesy of: Thompson & Morgan, Kai Hendry & Scot Nelson