Gardening: potatoes

Potatoes. See PA Feature GARDENING Gardening Column. Picture credit should read: PA Photo/thinkstockphotos. WARNING: This picture must only be used to accompany PA Feature GARDENING Gardening Column.
Potatoes. See PA Feature GARDENING Gardening Column. Picture credit should read: PA Photo/thinkstockphotos. WARNING: This picture must only be used to accompany PA Feature GARDENING Gardening Column.

Potato aficionados will now have their windowsills awash with trays, on which seed potatoes are being ‘chitted’, encouraging the sprouts to appear before planting begins in earnest.

While many of us have visions of digging up delicious spuds and cooking them immediately the soil’s been wiped off them, sometimes in reality we have to tackle a fair few pests and diseases which can leave our potatoes mottled, mouldy or full of holes.

Potato blight is one of the most deadly diseases which can destroy your plants overnight. It’s caused by a Phytophthora fungus, P. infestans, which turns the stems brown and the leaves yellow, causing the plant to collapse. It is always fatal although you may save later crops by removing top growth as soon as the blight is spotted.

Blight generally starts from June onwards especially in warm, wet spells. The first signs are brown patches on the leaves. If you have neighbouring tomatoes, they’re also likely to be infected. There is no treatment once the disease has taken hold.

To minimise the risk, follow a three or four-year crop rotation, never growing potatoes on the same piece of land more than once every four years, and earth up the plants regularly and deeply to protect the tubers.

Use copper-based fungicides during warm, damp spells as a preventative measure. Alternatively, grow early potatoes which can usually be lifted before blight strikes, or grow resistant cultivars such as ‘Sarpo’.

Three of the most common threats to potatoes are pests - potato cyst eelworm, slugs and wireworm. Eelworms are minute worm-like creatures which attack the roots, causing plants to wilt and present reddish-brown cysts on the roots. The lower leaves wither away while upper leaves are pale green and wilt during the day. You’ll need to destroy all traces of the affected crop and avoid growing potatoes in that spot for around eight years.

Slugs and snails can be a major problem on heavy, wet soil. Attacks begin in August. Early lifting of maincrops may help reduce damage and slug pellets may help a little, although some slugs live permanently underground. Collect slugs and snails after rain or in the evening on damp nights and dispose of them.

Wireworm is also a serious pest, especially in wet summers as tubers can become riddled with narrow tunnels made by the 3cm (1in) orange shiny larvae. There is no treatment or prevention, as no insecticide for use below ground is available.

Other problems which may affect your crops include scab, a fungal disease in which scabs - flat areas of rough skin pitted with shallow splits - appear. It is worse in light and limy soils but is only skin deep so won’t affect your potatoes if you peel them.

The best way to deter it is to incorporate plenty of organic matter when planting potatoes but don’t lime your soil. Make sure you water potatoes during dry weather and try resistant types such as ‘Nadine’ and ‘Wilja’.

Wile there’s no treatment for some pests and diseases, there are things you can do to stop other problems. If aphids invade your plants, spray them with insecticidal soap. If there’s a risk of a severe frost, cover the shoots of earlies with newspaper to prevent damage to plants.

If your leaves start to go brown between the veins, it’s likely they are suffering from a magnesium deficiency, so apply a trace element spray. Alternatively, as a preventative measure, feed the plants with fertiliser which contains magnesium as well as nitrogen, phosphates and potash during the growing season.

To boost your chances of success, always buy certified seed potatoes, work in plenty of organic matter on dry soil to retain moisture and help prevent scab and try the wide range of ‘resistant’ varieties on the market. Then hopefully when you harvest your crops you’ll find the spuds you like.