Contrary to what you might read elsewhere there isn’t really much you can do to eradicate tomato blight once you see your plants have it. Yes, once you notice those brown patches of rot on your fruit it’s too late and all you can do it try to save any plants that are as yet unaffected. You may be lucky, you may not.
I won’t go into the various strains of this infection since they are largely geographical and the symptoms manifest themselves in the same way anyway; suffice to say tomato blight is not something to be neglectful of because, if you do not act appropriately, it could affect your crop year after year.
The problem is that tomato blight is caused by a fungus and the spores are spread by water and by wind and there isn’t much you can do about the elements. If you also grow potatoes you should also watch them carefully as they usually show signs of blight first, then it spreads to your tomatoes.
You can spray your plants with a protective fungicide but these days so many of us are loathe to use such treatments; if you want to you need to use something containing mancozeb or copper. You need to use it early, once the signs appear it is too late.
If your plants do get it you need to prune the affected branches straightaway and get rid of them – the spores could still be there and be transferred to your healthy plants with just one gust of wind. Don’t save seeds from infected fruit either – this is just asking to be re-infected.
Defence is the best way to eradicate tomato blight and there are several easy steps to remember. Put down a good layer of mulch which will prevent water splashing back when you water. Don’t water your plants directly from above because this makes splashing more likely. Leave plenty of space between plants – give them a sporting chance of avoiding infection. If you have blight this season, do NOT plant tomatoes in the same place for next season; similarly do not grow tomatoes anywhere you had potato blight last season since spores can rest in dead plant tissue and come back to haunt you the following season.
Finally, look out for blight-resistant varieties; there are several that may suit your needs, do your research to find which best give the sort of crop you want.