Hi- I may be having the same problem - mainly my lowers leaves are turning yellow and have a few icky brown spots -
at first I thought it might be fusarium disease from looking up pictures - but I went to a gardening center today and they said one of the main reasons tomatoes start turning yellow is because of too much water - and the problem is that when leaves start turning yellow, often times the gardener will think the opposite and start watering them even more - thereby making the problem worse AND beginning to encourage fungus/mold (spots) -

I have no definitive advice for you, except to tell you what I'm going to try - let them dry out a bit and get some daconil or organicide to spray my leaves to prevent further molding.

sadly, the next three days are expected to be endless thunderstorms - of course.

If you suspect that blight is the problem, all infected leaves should be removed as soon as they show signs of disease. Then, if you havent already, you should start spraying your plants weekly and after heavy rains with a fungicide like daconil (ortho garden disease control).

Continue to remove the affected leaves and optimize air circulation around the base of the plant by removing lower limbs, weeds etc in addition...

I thought the rain was great till, my zucchini and tomatoes started an early mold..... yellowing of leaves and dying.. But I found a dirty little secret no one wants us to find out about.. Someone in South America discovered spraying milk on your zucchini, cucumbers, melons and tomatoes prevented, mold, milky spore and tomato blight. The study I read suggested a 10-30% mixture of milk to water was optimal... 1 part milk to 9 parts water in a spray bottle works fine. With all the rain we have been having you may have to reapply but is sure is alot safer and cheaper than fungicides. It seems the natural bacteria in milk somehow combats the mold and blight in addition to building the plants immunity by absorption through the leaves.
According to the study I read, this milk solution was 90% effective and a whole lot cheaper than traditional copper, fungicide etc. According to a New Zealand study on the same solution they found that skim milk was just as effective as whole milk and as a result of the lower fat content also reduced the possibility of an odor problem.

It seems to be working well so far for me... let me know if anyone else has any luck with this...

Milk can work on powdery mildew -- but PM is a rare problem for tomatoes. Much more common is the Early Blight that I suspect the top two posters are having issues with.

Do pull off the worst leaves, and spray with Ortho Garden Disease Control (you'll see some of us call it Daconil). The other thing you can do is mulch the ground around the plants, because the spores are in the soil, and can get splashed up onto the leaves by rain. In future years, mulch as soon as you put the plants out (that day) and start spraying regularly with Daconil a week after transplant. It helps tremendously.

--Alison