Potatoes: Destroy plants with blackleg ASAP and rotate your crop

Martyn Davey, Head of Horticulture and Design, Easton College
Thursday, June 21, 2012
9:21 AM

Question: Some of our potato plants have unexpectedly wilted – and when we dug then up we found the seed potato completely rotted and the thing enclosed eating away at the main stem of the potato plant. The long yellow or brown creatures were also found in the soil. The potato tubers unfortunately had also been attacked having holes in them. We do suffer damage by small black slugs if we fail to get the crop lifted early enough, but these holes did not resemble slug damage. The potato variety is Kestrel. We have been gardening here for the past 41 years and not had a problem before. (J and W Knights, Toftwood)

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The creatures you have enclosed are centipedes. They have segmented bodies with one pair of legs in each segment. When a centipede hatches it only has seven pairs of legs but as it grows, commonly reaching 2-3cm in length, its skin splits and a new skin appears, along with an extra set of legs. This continues until the centipede is fully grown. They do not have 100 legs as the name suggests but most have many fewer, with the common centipede having 30 legs. However, some species have in excess of 300 legs. During the day they remain in damp places.

There are 55 UK species. Centipedes are nocturnal creatures which hunt invertebrates on the surface of the soil and are active all year. Centipedes are carnivores, using their poison claws to eat their prey, including woodlice, spiders, worms and slugs and also the eggs of slugs and snails. Its main enemies are birds, ground beetles and other centipedes. These are incidental to the potatoes rotting and are probably attracted by the slugs that are feeding on the rioting potatoes.

Potatoes require an open, frost-free site with deep, fertile, moisture-retentive, friable soil, for high quality and heavy yields. Improve soils by adding organic matter, such as well-rotted manure, in the autumn. Before planting, supplement with a general fertiliser, such as Growmore or blood, fish and bone, applied to the soil surface or spread along the sides of the drill during sowing, at the rate of 1kg per 10m (2.2lb per 33ft) row. Half of this amount will be enough if the garden is known to be fertile.

Ketsrel is a good variety for dry sandy soil so the wet weather we have been having since the hosepipe ban will not be helping them. You may want to try some of the slug resistant varieties next year these include Charlotte, Estima, Golden Wonder, Pentland Dell, Pentland Ivory, Pentland Squire, Stemster, Sante and Wilja.

Potatoes are well-known for suffering from numerous problems. Thankfully, many of these can be controlled or managed with a little help. Cutworms ,slugs, wireworms and potato cyst eelworm may all cause damage to potato crops. Precautions against slug damage are particularly necessary when planting through polythene. Grow cultivars resistant to slugs (see Cultivar Selection above).

Potatoes can also be affected by potato blackleg, potato scab and various potato tuber rots.

The disease affecting your crop, I think, is potato blackleg, a common bacterial disease which causes black rotting at the stem base. Initial infections cause stunted growth and yellowing stems. It is one of the earliest potato diseases to appear, and you may see symptoms as early as June, especially in wet years. The stems of potato plants will appear stunted and pale green or yellow. Leaves at the top of affected stems may be small, stiff and have margins curled inwards. At ground level, these affected stems appear black and rotted. If tubers form, the flesh may be grey or brown and rotten. At ground level, affected stems appear black and rotted.

There are a number of control measures that do not require chemicals. The most important is prompt removal and destruction of infected plants as soon as symptoms are noticed. Remember that commercially-produced seed has very low (but not zero) levels of infection. If you must keep your own seed potatoes, then dry storage of seed or ware potatoes will minimise the risk of spread in storage

Rotation is important, since the bacteria may survive over winter in ground where potatoes were previously grown, albeit at a low level.

The British Potato Variety Database lists resistant varieties including Charlotte, Pixie, Saxon and Vales Sovereign.

There are no chemical controls available to gardeners for blackleg.

Blackleg is one of the few important plant diseases caused by bacteria in the UK. This disease normally comes into gardens (or allotments) via infected seed potatoes. Infected plants are removed from commercial seed crops, but a small number of infected tubers escape detection. If the infected plants that grow from these are not removed from the crop promptly, the disease will spread through the soil.