Blight arrives on our potatoes around Mid-June onwards and on outdoor tomatoes shortly afterwards. There is a website that will alert you to the likelihood of occurrence if you register (for free) with them.
What is Blight?
Blight is a microscopic, fungus-like organism whose sporangia (spore-bearing structures) easily break away from infected foliage and may be wind-blown for long distances. The actual infective spores are released from the sporangia into water and need to swim in a water film before settling on the plant surface and penetrating into leaf tissues; this is why the disease is so serious in wet summers.
How do I identify it?
The immediate signs are - Brown freckles of patches on the leaves. Some curling at the edges. Yellowish border spreading from the brown patches.
When does it occur?
Usually from mid-June onwards. The ideal conditions for blight to occur are with a 90%+ humidity and minimum temperatures going below 10C for two consecutive days. It is airborne so others around you can pass it on. Register with Blightwatch.
Can I avoid it?
Not easily. If it becomes a frequent annual problem consider growing a more blight-resistant variety. Early (and second early) potato varieties are usually less susceptible than main crop potatoes. Keep rows as open as possible to allow air to pass through rather than be trapped in a high humidity micro-climate.
There are sprays which need to be applied before the signs are seen, but they don't cure only help to prevent it. As Tomatoes are in the same family of plants, flowering plant of the nightshade family (Solanaceae), DO NOT SPRAY as we eat the edible fruits.
Can it be cured? No
What actions should I take if I have it?
Keep a keen watch for early indications on the leaves - see RHS website for pictures. Remove infected leaves as soon as seen and dispose off-site, not in your compost or green waste. This will slow its progress down but not eliminate it. Removing all above ground leaves and stalks is preferable, but it tends to stop further tuber growth.
Prepare to lift your potatoes early to stop tubers being affected.
If diseased foliage is left on the plant the danger is that the tubers become diseased, blackened and have no keeping quality.
Reference - https://www.rhs.org.uk/advice/profile?pid=217