The biggest problem in controlling Late Blight with anything once it gets started is early recognition. If you wait til all your plants are infected and it has gone inside the plants no control will work. It doesn't seem to take long for LB to become systemic once the spores get on the plants. In my experience the only time I have had any good results controlling it were when I saw it at it's earliest stages on just a couple of plants and then treated my whole garden very aggressively with the bleach spray. If you wait til all or most all of your plants are showing LB symptoms then it is probably going to be a losing proposition though you may save a few of the least infected plants. The first time I tried the bleach spray I didn't start until at least half my plants were visibly infected. I lost all or most all of those plants and a few others. I sprayed relentlessly and upped the dosage of bleach to no avail on the sicker plants and ended up losing over 2/3rds of my plants. The next time I started much sooner and had far better results. I made sure to spray everything in the garden including the mulch under my plants and all nearby plants. I also sprayed with a strong permethrin spray to knock out any insects that would help spread the spores. The last two times I saw it even earlier and sprayed very aggressively and removed the few plants that I was sure had it after the first bleach spray and they were the only ones I lost. Of course this last time I saw it was this past summer and I spent weeks worrying that it would show up again but it never did. I also used copper and Daconil as a preventative after using the bleach spray for a week or so.
The first time I had LB I lost every single plant though I used every fungicide in the book. I think what has helped me spot it so early is my constant battle with Gray Mold which shows up every summer and can be deceptively similar in appearance to one who hasn't experienced the horror that is full blown Late Blight. I have found that LB seems to almost always show up first on the upper third of the plant whereas Gray Mold almost always starts lower on the plant and deeper in the foliage. I have found that GM if left untreated and the conditions are right will also become systemic and all treatment is futile after that point but it isn't nearly as aggressive nor does it spread like LB.
I don't know if any fungicide will prevent it but I keep using them but I quickly jump to the bleach at the first sign of either LB or GM. The biggest difference in treatment is that I will go to the stronger solution immediately if I suspect LB because I would rather have a few slightly burned leaves than lose my whole crop of tomatoes.
Bill
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