Potato plant with brown leaves from disease

Planting25 May 20268 min

What if potatoes get blight (Phytophthora)?

potato blight phytophthora plant diseases garden problems growing potatoes

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What is potato blight (Phytophthora infestans)?

Potato blight, also called late blight or Phytophthora infestans, is one of the worst diseases for potatoes. It is a fungus-like organism (actually a water mold) that attacks potato and tomato plants. It spreads explosively in wet weather and can completely destroy a healthy plant in two weeks.

The disease first eats the foliage (brown, slimy spots), then moves to the underground tubers. Once in the tuber, the game is over. The tuber rots from inside. You have no harvest.

This is not just any fungus. This is the fungus that caused the Great Famine in Ireland (1845-1849). Billions of potatoes lost. It can do the same to your garden potatoes quickly.

How do you recognize potato blight?

Foliage:

Brown, slimy spots on leaves (especially undersides)
Spots grow fast, leaves turn yellow
Whitish mold powder on underside (spore producer)
In severe cases: entire plant brown and dead within days

Stems:

Brown streaks on stems
Weak plants, drooping

Underground tubers:

Brown, hardened spots under skin
Interior can be gray/brown (rot)
Rot rapidly in storage

Timing: Typically July/August, sometimes June. Warm and wet weather (15-25°C, high humidity).

How does potato blight spread?

Spores: The fungus produces millions of spores. These fly through the air and land on other foliage. Rain/water-spray accelerates the process.

Infected potatoes: Too many gardeners use infected seed potatoes (stored from last year). This is a very direct route to disease.

Water spray: Garden hoses, rain showers, mist. Water is the medium on which spores travel and grow.

Storage: Infected potatoes in your shed infect next season's seed potatoes.

How do you prevent potato blight?

Choose seed potatoes carefully:

Buy certified, healthy seed potatoes from recognized nurseries
NEVER use last season's potatoes unless you are certain they are healthy
Small, healthy tubers work better than large old ones

Choose resistant varieties:

Some varieties are more resistant: Sarpo Mira, Sarpo Axona, Setanta, Wilja
Susceptible varieties: Bintje, Desiree (extra risk in wet years)
Check resistance ratings online before buying

Good spacing and air:

Do not plant potatoes too close (30 cm minimum between rows)
Ensure air circulation - wet foliage is ideal for fungus
Cut lower leaves off if they touch ground

Water from bottom:

NEVER spray overhead with hose (water on foliage = perfect conditions for spores)
Water plant feet, not foliage
Drip irrigation is ideal

Raised bed cultivation:

Plant in raised beds or mounds (better drainage)
Wet, flat ground is deadly

Early harvesting:

Do not wait until September/October
Harvest in August if weather turns wet
Blight grows in wet August weather - better prevent than cure

Kill foliage before harvest:

1-2 weeks before harvest: cut or poison all top growth above ground
This prevents spores moving down to tubers
Your tubers grow safely

Step-by-step

Step 1: Start with healthy seed potatoes

Buy from recognized nurseries. NEVER recycle last year's potatoes without certainty.

Step 2: Plant resistant varieties

Choose Sarpo Mira, Sarpo Axona or Setanta. These have built-in resistance.

Step 3: Ensure good drainage and air

Plant in raised beds. At least 30 cm between rows. Cut lower foliage.

Step 4: Water from bottom

Never spray from above. Drip irrigation or foot watering.

Step 5: Monitor foliage daily

Start July. Look for brown spots. At first sign: remove entire leaf and take it away.

Step 6: Harvest early

Do not wait. Harvest August if rain comes. Waiting until October = risk.

Step 7: Kill foliage before harvest

1-2 weeks before harvest: remove all top growth. This stops spore spread.

Frequently asked questions

Can you prevent potato blight with fungicide spray?

Yes, but only preventively. Chlorothalonil, mancozeb, or copper-based fungicides work. Spray weekly from July through August. But this works only if spores have not yet invaded the plant. Once inside, no salvation.

What do you do if you discover disease?

Remove completely infected plants (roots and all) and burn them. Not on compost! This spreads spores. Other plants: remove foliage, water from bottom, spray fungicide, harvest early.

Can you eat infected potatoes?

Small surface infections: cut them off, eat rest. Deep internal spots: discard. Rotted potatoes: do not eat.

How long do spores live in soil?

In dry conditions: a few months. In wet, heavy clay: until next season. Therefore: rotation! Do not plant potatoes two years in same spot.

Are tomatoes also at risk?

Yes! Same fungus (Phytophthora infestans) attacks tomatoes. Same prevention: good air, water from bottom, spray fungicide, harvest early before disease.

Planting potatoes twice per season works?

Yes, for some. Plant early (March) for summer harvest (May/June). Plant later (May) for fall harvest (September). Caveats: second crop sees more summer disease. First crop: better chance of success.