Tomato and Potato Blight: How to Spot, Stop and Prevent It

Late blight strikes fear into UK gardeners' hearts, turning lush tomato and potato crops into slimy ruin within days. This relentless pathogen, Phytophthora infestans, thrives in our damp, cool…

Late blight strikes fear into UK gardeners' hearts, turning lush tomato and potato crops into slimy ruin within days. This relentless pathogen, Phytophthora infestans, thrives in our damp, cool springs and autumns, making it a constant threat. But with early detection and smart action, you can save your harvest and protect next year’s crop.

Spotting Blight Before It Spreads

Don’t wait for the whole plant to collapse. Early symptoms are subtle but critical to catch. Look for dark green, water-soaked spots on the lower leaves, especially near the leaf tips or edges where dew collects – often appearing by late June or early July when temperatures hover around 12-18°C (54-64°F) and humidity is high. Under humid conditions (over 75% humidity for two days), a faint white, fuzzy mold may develop on the undersides of these leaves. As the disease progresses, spots turn brown then black, leaves curl and die, and stems develop dark, sunken lesions. Crucially, infected tubers show grey or dark patches beneath the skin, turning reddish-brown and rapidly decaying into foul-smelling mush – a sign you might have missed it earlier.

Key UK spotting checklist:

• Timing: Check daily during cool, wet spells (typically April–August, peaking July–August).
• Location: Focus on lower leaves first, near the soil line where moisture lingers.
• Conditions: Prioritise inspection after rain or heavy dew, especially if temperatures are 15–20°C (59–68°F).
• Varieties: Susceptible UK varieties include 'Desiree' potatoes and 'Ailsa Craig' tomatoes; resistant options like 'Sarpo Mira' potatoes or 'Defiant' tomatoes show fewer symptoms.

Immediate Action for Active Infections

If you spot blight, act within 24 hours – it spreads rapidly. Remove all infected leaves, stems, and fruit immediately. Do not compost these; bag them securely and dispose of them in the household bin (not the green bin) to prevent spores from spreading. For tomatoes, prune infected stems back to healthy growth, cutting at least 10cm below visible damage. For potatoes, carefully dig up and discard infected tubers, but leave healthy ones in the ground for a few more days to allow the soil to dry. Avoid overhead watering for at least a week – water at soil level using a soaker hose instead. The RHS advises that no fungicides are currently available to UK home gardeners for tomato or potato blight. Focus on non-chemical measures: inspect crops during warm, wet spells, remove and dispose of infected material appropriately, avoid wetting foliage when watering, and consider resistant varieties.

Critical UK-specific steps:

• Timing: Inspect outdoor tomatoes and potatoes frequently after prolonged warm, wet weather for rapidly spreading lesions or rotting stems.
• Removal: Remove affected material and dispose of it according to local green-waste guidance; do not place blighted foliage in ordinary home compost.
• Prevention: Choose resistant cultivars where possible, improve airflow, water at soil level, and avoid long periods of leaf wetness.
• Avoid: Using nitrogen-heavy fertilisers during an outbreak – this encourages soft, succulent growth that blight loves.

Preventing Future Outbreaks: Crop Rotation & Soil Management

Potato and tomato late blight is caused by Phytophthora infestans, a fungus-like organism. In UK gardens a key source of recurring infection is affected tubers and volunteer potatoes, alongside airborne spores during suitable weather. Remove volunteer potatoes and do not leave infected tubers in the ground. Rotation is helpful for overall crop health, but a fixed three-year rotation alone does not prevent airborne late blight.

UK soil management tips:

• Soil Type: Heavy clay soils (common in UK gardens) retain moisture longer, increasing blight risk. Improve drainage with grit or well-rotted compost before planting.
• Timing: Plant potatoes in early spring (March–April) when soil warms to 7–10°C (45–50°F), avoiding cold, wet periods.
• Tuber Storage: Store harvested potatoes in a cool, dark, dry place (not a damp cellar) at 4–7°C (40–45°F) to prevent rotting and secondary infection.

Choosing Resistant Varieties for UK Gardens

Resistant varieties are your best defence. For potatoes, choose 'Sarpo Mira' (highly resistant, great for chipping) or 'Sarpo Axona' (resistant, good for baking). Avoid susceptible UK classics like 'King Edward' or 'Russet Burbank' if blight is a recurring issue. For tomatoes, opt for 'Defiant PHR' (resistant to blight and other diseases) or 'Plum Regal' (disease-resistant, good for sauces). These varieties have been bred to resist Phytophthora infestans without needing heavy fungicide use.

UK planting advice:

• Tomatoes: Start seeds indoors in February/March, transplant outdoors after the last frost (late May/early June) to avoid early-season cool, wet conditions.
• Potatoes: Plant 'Sarpo' varieties in March for a July harvest, ensuring they’re in a sunny, well-drained spot.
• Spacing: Space plants 45cm (18in) apart to improve air circulation – dense planting traps moisture, encouraging blight.

Handling Infected Tubers & Storage

Potatoes from plants affected by blight require particularly careful handling. Check tubers for brown or soft rotting, discard damaged ones and avoid storing suspect tubers near healthy potatoes. Sound tubers may be kept separately with regular checks if conditions are suitable, but seek crop-specific RHS guidance and do not promise that a blighted crop will store safely.

Storage checklist:

• Temperature: 4–7°C (40–45°F) – never above 10°C (50°F).
• Humidity: 85–90% (use a damp cloth in a storage box if too dry).
• Inspection: Check weekly for soft spots; remove any affected tubers immediately.
• Avoid: Storing potatoes near apples or onions – they release ethylene gas, which speeds up rotting.

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Tomato Potato