Potato cyst eelworms?
Eelworms also known as nematodes are a phylum of worm-like animals. There are more than 25, 000 described species although it is thought there could be more than a million. Most are microscopic, less than 1mm in length although some species reach 5cm or more. They are very abundant animals in almost every habitat. Nematodes feed on a range of materials, some are predatory on bacteria whilst some are plant or animal parasitic.
The two species of potato cyst eelworm that commonly occur in Britain – golden or yellow cyst eelworm (Globodera rostochiensis) and white cyst eelworm (G. pallida) – feed in the roots of potatoes and can cause the crop to fail.
They can be distinguished by the colour of the developing cysts which are just under 1mm in diameter. The golden or yellow cyst eelworm passes through a prolonged pale yellow phase, which can be seen if the roots are examined between mid-June and July. White cyst eelworm changes rapidly from creamy-white to brown and intermediate colours are not often seen. Golden cyst eelworm is the more common species in the southern half of Britain while the white cyst eelworm predominates north of Yorkshire and Lancashire, and also in much of Lincolnshire.
Affected potato are stunted and the leaves yellow, turn brown and hang down. These symptoms develop from the ground upwards.
If an affected plant is lifted carefully, it should be possible to see many pin head-sized spherical objects, known as cysts, on the roots. Eelworm cysts are the dead bodies of females which can contain up to 600 eggs each. The cysts may be white, yellow or chestnut brown in colour and can be seen more readily with the aid of a hand lens.
Heavily infested plants die prematurely and yield a poor crop of under-sized tubers. When an area is first infected by potato cyst eelworm only a small part of it may be affected but in successive years the infested area will enlarge until it becomes impossible to grow worthwhile potatoes.
The eggs contained within the cysts can remain viable for many years as they hatch at a slow rate in the absence of host plants. When potatoes or tomatoes are grown in infested soil the eggs are stimulated to hatch in large numbers by chemicals exuding from the roots. The immature eelworms are microscopic worm-like creatures which enter the roots and feed internally on the cell contents. During their development the eelworms disrupt the uptake of water and nutrients, causing stunted growth and poor leaf colour. When almost mature, the female eelworms become globular in shape and burst through the root wall and are then fertilised by males. There is one generation a year on outdoor crops but two generations may be possible on glasshouse tomatoes.
Wild plants of the Solanaceae family are rarely attacked and are of no importance as host plants in the UK although some, such as the weed black nightshade (Solanum nigrum), will induce egg hatching from the cysts.
There are no pesticides available to amateur gardeners which will control potato cyst eelworms, and so cultural methods should be used to avoid spreading the pest and to reduce infestations.
A high population of cysts may soon develop if potatoes or tomatoes are grown frequently in the same piece of soil. This can be avoided by adopting as long a rotation as possible. Self-set or ‘volunteer’ potatoes can maintain an infestation and so weed them out in early summer before eelworms can complete their development. Eelworm eggs can survive for up to ten years in some soils and so very long rotations are needed to starve out the pest completely. This process can be speeded up by sowing a half-hardy annual, Solanum sisymbriifolium. This plant’s roots induce egg hatching but eelworms cannot develop in the roots.
A worthwhile crop of early potatoes can usually be grown in infested ground after a break of five or six years. The increase in cyst numbers can be limited by lifting the crop as soon as the tubers are ready and not allowing the plants to continue growing beyond that point. Once soil becomes infested with cysts it is difficult to prevent them being spread around the garden, but steps should be taken to prevent this occurring on a large scale. Plants grown in infested soil should not be transplanted to cyst-free areas, and the roots, including those of weeds, should not be put on the compost heap.
There are some potato cultivars that have resistance to the golden cyst eelworm. These include ‘Accent’, ‘Lady Christl’, ‘Pentland Javelin’, ‘Premiere’, ‘Rocket’, ‘Swift’, ‘Winston’ (earlies); ‘Blue Danube’, ‘Kestrel’, ‘Nadine’, ‘Saxon’, (second earlies); ‘Amour’, ‘Cara’, ‘Harmony’, ‘Lady Balfour’, ‘Maris Piper’, ‘Maxine’, ‘Nicola’, ‘Picasso’, ‘Sante’, ‘Spey’, ‘Stemster’, ‘Valor’ (maincrop). The root exudates produced by these cultivars still induce egg hatching and the roots are attacked in the usual way. Females, however, are unable to develop inside the roots and only males are produced, thus reducing the number of eggs in the soil. White cyst eelworms can reproduce normally on these varieties. Crop rotation is still important, even in areas where the golden cyst eelworm predominates, as there is a danger that the other species may also be present and will increase in numbers if potatoes are grown too frequently.
Some cultivars also have some tolerance of white cyst eelworm, in addition to resistance to golden cyst eelworm. These include ‘Harmony’, ‘Kestrel’, ‘Lady Balfour’, ‘Maxine’, ‘Sante’, ‘Spey’ and ‘Valor’. The white cyst eelworm is able to complete its normal life cycle and reproduce in the roots of these varieties, but they are nevertheless able to produce a worthwhile crop, provided the soil is not heavily infested.
Soil that has grown potatoes should never be used in the greenhouse for growing tomatoes. If greenhouse beds become infested, the problem can be overcome by complete re-soiling, or by growing tomatoes in growbags or other systems, such as ring culture or straw bales, which reduce root contact with the infested soil.