Potato and tomato blight

If you grow potatoes or outdoor tomatoes, sooner or later you'll meet blight. You can't always stop it, but you can spot it early, limit the damage and stop it carrying over to next year.

What blight is

Blight is caused by Phytophthora infestans, a fungus-like organism. The RHS calls it the most serious and damaging disease of potatoes, and the same organism attacks tomatoes.

It's most common in warm, wet weather. Infection needs the leaves to stay wet for several hours.

How it spreads

Spores break off infected leaves and can blow a long way on the wind. Most garden infections come from spores blown in from other gardens, allotments and farm crops. Rain can also wash spores into the soil, where they reach the tubers.

On an allotment site, that means your blight probably came from someone else's plot, and yours can go on to theirs. Tell your neighbours if you spot it.

Spotting it

On potato leaves: a fast-spreading, watery rot. Leaves soon collapse, shrivel and turn brown. The edges of patches can look pale green, and you may see white fungal growth underneath.

On stems: brown patches.

On tubers: reddish-brown rot under the skin, firm at first but often turning soft. Many infected tubers rot in store.

On tomatoes: the same organism infects tomato plants. VERIFY: add tomato leaf and fruit symptoms from the RHS tomato blight page before expanding this.

Forecasts

The RHS points growers to AHDB's BlightSpy service for forecasts and confirmed outbreaks.

What to do when it arrives

• Picking off a few leaves may slow it slightly, but it won't get rid of it.
• If it's heavy on potatoes, cut the stems off close to soil level.
• Don't put infected material on your compost heap. The resting spores may survive home composting. Use council green-waste collection or burn it, if your site allows bonfires.
• Don't store blighted potatoes. They won't keep.

There are currently no fungicides for gardeners to use against blight.

Preventing it next year

• Clear up leftovers. Dig out potatoes left in the soil and get rid of storage waste before the following spring.
• Rotate. The RHS suggests rotating crops for at least four years. Potatoes and tomatoes share a rotation group, so move them on together.
• Grow resistant varieties. Some show good resistance, though none are immune. The RHS names Athlete, Alouette, Carolus and the Sarpo varieties. Older varieties such as King Edward are very susceptible.

FAQs

Can I eat potatoes from blighted plants?

VERIFY: find an RHS or other official answer on eating tubers from blighted plants before answering this. What the RHS does say is that blighted potatoes won't store.

Can I compost blighted leaves?

No. The RHS says to use council green waste or burn them, because the spores may survive home composting.

Is there a spray for blight?

There are currently no fungicides available to gardeners for blight.

Your next step

Choose a resistant potato variety for next year and plan where potatoes and tomatoes go in your rotation. The plot planner moves them on each year.