To get rid of tomato blight, you strip off the sick leaves, clean up the air around the plant, and protect the healthy growth that is left. Be honest with yourself first though. There is no instant cure once blight has a real grip. You are working to save the rest of the plant, not to reverse the damage you already see.
This is why so many gardeners feel their efforts fail. They spray a plant that is already covered in brown, spreading spots and wait for it to bounce back. It rarely does. The leaf damage you see is dead tissue, and no spray brings dead tissue back to life. The plant looks worse a week later and the treatment gets the blame.
The real reason comes down to how these sprays work. Almost every fungicide you can buy is preventive, not curative. It coats clean tissue and stops new spores from taking hold. It does not heal a leaf that is already infected. So a spray works wonders on a plant you protect early and does almost nothing on a plant you treat late. Think of it as a raincoat, not a bandage. It keeps the next storm of spores off the dry leaves, but it cannot patch the holes already torn in the wet ones.
Knowing that, your job splits into two parts. You remove infected leaves to slow the spread, then you guard the clean growth with a spray. Do both on the same dry afternoon for the best result. Here is the order that works.
Wet leaves spread spores on your hands and tools. Wait for a dry afternoon so the plant is not damp when you start.
Pinch off the spotted, yellowing leaves and seal them in a bag for the trash. Never toss them on your compost pile.
Take no more than a third of the plant in one go. Strip more than that and you starve the plant of the leaves it needs to feed itself.
Trim crowded lower stems and stake the plant upright. Dry leaves catch far less blight than damp, packed ones.
Coat the clean leaves with a labeled fungicide, top and underside. Repeat every 7 to 10 days through the wet stretch.
For the spray, copper fungicide for blight is the classic organic choice and it covers both early and late blight. Mix it to the label rate and reapply every 7 to 10 days, plus after any heavy rain that washes it off. Hit the undersides of the leaves, since that is where spores love to settle. Copper builds up in soil over time, so follow the label and do not double the dose hoping for a faster fix.
Before any of this, name the disease. Early blight shows dark spots with rings like a target, usually starting low on older leaves. You can treat it calmly and most plants still give you a harvest. Late blight is the scary one. It spreads as greasy, gray-green patches that turn whole stems black in days, and it jumps from garden to garden on the wind.
If you spot late blight, do not try to nurse the plant. Pull it, bag it, and get it out the same day. Then warn your neighbors and call your local extension office, because late blight can wipe out a whole street of tomatoes and nearby potatoes. Acting fast protects far more than your own row. That single phone call often saves a season for everyone close by.
Read the full article: Tomato Blight: Identify, Treat, and Prevent It
