Shade the plants with a sheet to keep the fruit from sunburning.

I agree with Amy. I cut off and remove as many leaves as I can, but am careful to leave enough to shade the fruit so that sunscald doesn't ruin the tomatoes before they can ripen.

Yellowing leaves are merely an indication of stress and can indicate virtually anything---plants are too wet, too dry, too hot, too cold, malnourished, etc. Unfortunately, when yellowing leaves begin yellowing from the ground up and the yellowing is progressing upward very quickly, that usually is an indication of a fungal or bacterial disease, especially when accompanied by dark spots or splotches on the leaves.

You can prevent fungal diseases by spraying preventively with chlorothalonil from Day 1 when the plants go into the ground, and by continuing to spray on a regular basis at the intervals specfied on the product label. I generally don't use fungicides preventively as I'd rather eat tomatoes that haven't been sprayed with anything. Serenade is an organic alternative and it worked well for me in the first year I used it which was a year in which we received a little under 19" of rain, but is less effective in rainy years.

Chlorothalonil will not cure a plant of fungal disease once the disease already has shown up on the plant, but often will keep it from progressing upward and killing the plant. I sprayed my plants twice this spring with Daconil (its active ingredient is cholorothalonil) and it probably helped slow down the progression of the fungal diseases for a while, but I haven't sprayed it at 7-10 day intervals in order to keep the diseases under control. Usually, by the time we have yellowing leaves taking over the entire tomato plant, we are getting sick of tomatoes and I just let the disease win. Then, I pull out the plants and plant new ones sometime in July for fall tomatoes.

The best way to stop the progression of fungal and bacterial diseases on tomatoes is to remove each and every infected leaf and to spray regularly. Spraying alone is not very effective if you leave diseased leaves on the plants. You cannot, however, remove too many leaves or you lose the fruit. Shading the plants will help prevent the fruit from getting sunscald, but it doesn't nothing to stop or slow down the progression of the diseases.

My garden has both early blight and septoria leaf spot running rampantly from one tomato plant to another this year, which is not surprising with all the rain that has fallen. If I only had a handful of plants, I might have sprayed them regularly and kept them in much better shape than they're in now, but I have a lot of tomato plants and spraying them all takes hours which is another reason I rarely do it.

The most disease-resistant tomato plant in my garden this year is Marion, and I intend to grow it every year for that reason. Once you get the fungal and bacterial issues showing up on your plants, they tend to be a problem every single year, but rotating the tomato plants next year to a different spot will help keep the plants healthier longer.

Dawn