Potato cyst nematodes are microscopic soil worms that attack potato roots, stunting growth and reducing yields. The two main species form tough, egg-filled cysts that survive in soil for years, making crop rotation and resistant varieties essential for control. Symptoms include yellowing leaves and poor plant vigour, though the nematodes themselves remain invisible underground.
Potato cyst nematodes, often called PCN or eelworms, are tiny soil-dwelling worms that attack potato roots, stunting growth and slashing yields. Common in UK gardens and allotments where potatoes or tomatoes have grown before, the two main species—golden cyst (G. rostochiensis) and white cyst (G. pallida)—form tough cysts packed with eggs that linger in soil for years. You won't see them easily above ground, but they sap your plants' vigour. Rotation and resistant varieties are your best defences for long-term control.
Identification
Look for pinhead-sized cysts (0.5 mm) on potato roots—white or creamy at first, turning yellow for G. rostochiensis or brown for G. pallida. Use a hand lens to spot them near harvest. Above ground, plants show non-specific signs like patches of stunted growth, yellowing or wilting leaves that mimic drought or nutrient lack. Dig up suspect plants carefully; roots will be stubby, branched and discoloured. Send soil samples to a lab for confirmation if unsure—kits are available from RHS or advisory services.
Damage & symptoms
Affected potatoes grow poorly from the start, with weak emergence and stunted shoots. Leaves yellow from the base up, wilt in patches, and flowering delays. Tubers are small, fewer and prone to rot. Roots look stubby, knotted and brownish, unable to take up water or nutrients properly. In heavy attacks, whole beds fail; lighter ones just cut yields by 20-50%. Symptoms worsen in dry spells, mimicking other stresses, so always check roots for cysts.
Lifecycle
Second-stage juveniles hatch from soil cysts in spring, triggered by potato root exudates (March-May). They invade roots, feed and moult into adults over 3-6 weeks. Females swell into visible cysts on roots, each holding 200-600 eggs. Cysts drop into soil at harvest, hardening and surviving 10+ years without hosts. G. pallida hatches later and slower than G. rostochiensis. Spread via infested soil on tools, boots or tubers.
Prevention — Rotate crops rigorously
Never grow potatoes, tomatoes or aubergines in the same spot for 6-8 years. Plant brassicas, cereals or tagetes instead to starve nematodes. Track your plot history to avoid repeats—essential in small gardens.
Prevention — Use certified seed tubers
Buy only UK-certified, PCN-free seed potatoes. Avoid unknown sources that might carry cysts in soil or eyes. Check supplier guarantees for testing.
Prevention — Clean equipment thoroughly
Wash spades, boots and machinery after digging infested plots to stop soil spread. Dry thoroughly. Quarantine new plots if buying in plants.
Prevention — Test your soil
Send samples to SASA or labs for PCN counts before planting. Know your infestation level to plan rotations or varieties.
Organic control — Plant resistant varieties
Choose UK-approved resistant potatoes like 'Harmony', 'Sarpo Mira' or 'Red Duke of York' (check AHDB lists for current ratings). They limit cyst buildup, though not fully immune. Rotate with them in cycles.
Organic control — Encourage natural enemies
Boost soil health with compost to support predatory fungi and bacteria that attack cysts. Avoid over-tilling, which exposes cysts. Plant green manures like mustard to trap and decline populations.
Organic control — Try biofumigants
Grow mustard or tagetes (French marigolds) as pre-crops; their roots release compounds toxic to nematodes. Mow and incorporate before planting potatoes.
Organic control — Fallow if feasible
Leave ground uncropped or under grass for years if infestation is high. Monitor with trap crops like susceptible potatoes to assess decline.
Frequently asked questions
How long do cysts survive in soil?
Cysts remain viable for 10-20 years without host plants, but populations decline slowly. Rotation is key—after 8 years, numbers often drop enough for safe planting if monitored.
Can I still grow potatoes in infested soil?
Yes, with resistant varieties and strict rotation. Test soil first; if over 10 cysts/g, delay planting. Combine with organic traps for best results.
Are tomatoes safe near potatoes?
No—tomatoes host PCN too. Keep all nightshades out of infested rotations. Grow them in separate beds with their own 4-year break.
What's the difference between the two species?
G. rostochiensis (golden) cysts stay yellow longer and hatch earlier; G. pallida (white) turns brown fast and persists in cooler soils. Labs distinguish them for variety choice.
Will compost kill nematodes?
Hot composting (over 55°C) can reduce cysts, but soil populations need rotation. Focus on non-host crops rather than relying on compost alone.