Late blight, also called potato blight, is caused by a microorganism called Phytophthora infestans. This is not a fungal, bacterial or viral disease - it is caused by a type of protist (eukaryote) called an oomycete, which is fungus-like. Late blight is the disease that was responsible for the Irish potato famine in the mid 1800s. As discovered by the Irish at that time, late blight can multiply rapidly, decimating a whole crop in just a few days.
How it Spreads
Late blight spreads primarily through water and by wind. It is most likely to cause a lot of damage to plants when conditions are cool and wet. The microorganisms can't survive long in soil or water but they can survive for many years in host plant material - eg. decomposing tomato plants or potato tubers left in the ground (with the aim of storing them until they're to be eaten or because they were overlooked during harvest).
Late blight mostly infects potatoes (as its other name suggests) and tomatoes but is known to affect other members of the Solanaceae family.
Symptoms include dark blotches on the tips of plant leaves and on plant stems. If conditions are quite humid, you might also see a white mould like substance on the underside of leaves. Plants can die quite quickly if infected. Potato tubers can become infected if spores are blown or washed off leaves and into the soil. If infected, the usually have dark patches on the skin and the flesh beneath appears reddish brown in colour. Such tubers usually rot in the ground but if the infection is mild enough that symptoms aren't visible, the harvested potatoes won't last well in storage.
If you detect an infection withing 24-48 hours, commercially available, systemic pesticides can control the disease. After this time, there is a small possibility of success but the chance is slim. It is much better to prevent late blight than to try and treat it.
As the microorganisms that cause late blight can live within plant residues and potato tubers that remain in the soil, in order to prevent the disease spreading to crops in the next season you must ensure you remove all plant residues, fallen fruit and all potato tubers at the end of the current season if any of your plants show symptoms of infection. Similarly, if your plants have an infection, don't save seeds or tubers for planting the next season.
Preventative techniques can include: selecting resistant varieties; only using certified 'seed potatoes' and tomato seedlings etc.; watering the soil and not the foliage; using commercial fungicides registered for prevention of late blight; using companion plants and green manures that fumigate the soil; using companion plants and green manures that increase soil copper levels.
While some plants can withstand mild attacks of late blight, it is best to remove and destroy any plant known to be infected with the disease in order to prevent it's spread.