You need to ask Carolyn about prevention of Late Blight since it is a disease which we only see every few years if that often although I did have it two years in a row once. As to prevention of Gray Mold I have had the best luck with a copper spray. I know that Daconil is supposed to prevent it but I start out every season spraying weekly with Daconil because it works best preventing Early Blight; but I switch to copper when the first Gray Mold makes an appearance. Before switching to copper I do spray any infected plant and those next to it with a diluted bleach spray which really does a number on Gray Mold. I follow up two days later by removing all the shriveled leaves and then spray with copper. If the Gray Mold returns I repeat the process and can keep my plant alive indefinitely unless we enter a long rainy spell and then it is almost impossible to keep it from spreading; but even then by spraying the bleach spray between rains every day or two you can slow it down quite a bit.
Keeping your plants pruned so that air and sunlight can enter the plant will greatly reduce the chance of Gray Mold getting a good foothold and increase the effectiveness of your copper spray. One of the earliest and easiest signs of GM starting is a stem or a few leaves near the bottom of the plant and in shade wilting and looking dark and wet. It almost looks like damping off hitting a seedling. Another symptom is those light gray or grayish tan dusty looking spots on the tops of some leaves. If you see it early you can't wait too long to treat it as it has actually spread further up the plant than the visible symptoms. If left untreated for too long it will get into the stems and fruit and at that point the plant is probably not salvageable and even if it is there will be little foliage left after treating it.
Before I started growing black varieties I had never experienced Gray Mold but almost every year it will hit some of my black tomato varieties and some of the GWR varieties. I have only had it spread to red or pink tomatoes one time and that was because I didn't treat the nearby plants or spray them with any kind of fungicide until after it had spread. Late in the fall if we have any damp cool weather the GM will hit all my black varieties but if it is relatively dry with low humidity it will not spread that fast. It does seem to spread the fastest when the humidity is high and the plants are loaded with fruit. I have found that quick treatment, preventive copper sprays and pruning have made GM a controllable although bothersome nuisance. Late Blight is another kettle of fish as it strikes so fast and so hard.
I assumed Late Blight because I thought your plants were dead in two weeks and I have never had Gray Mold kill quite that fast but if you were saying that all your plants were infected in those two weeks then it could certainly be very advanced Gray Mold infection. If left untreated Gray Mold will slowly turn your plants into a mess similar to what Late Blight will turn them into only far quicker. I have only had plants get that wrapped up with GM three times. Once was at the end of the season when health problems prevented me from treating the problem but even then only a couple of the plants actually died from it before freezing weather finally ended them. Another time was a rainy summer and one Black from Tula plant that was in a very shady location and I just could never get the Gray Mold under control on that one plant because I waited too long to treat it initially. The other time was the first summer I grew some black tomato varieties and had no idea what I was seeing until it was far too late.
Good luck this year and I hope you don't have that problem again. I plan on setting out a fair number of black varieties and one GWR variety so I will probably have Gray Mold to some degree or another unless this low humidity we are experiencing now continues through the summer.
Bill
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