Had the same problem one year with mine, solved the problem the next years by adding a small amount of lime to the soil.You have to be careful not to have standing water around your plants, if you have to, make a small trench for drainage.Go to the store and buy something suitable for plant fungus or if it is like it is here in the South the season is almost over and your plants are about to give up bearing fruit so they will turn brown.Pull the plants up and till the soil and add the lime in the ground for next year.Tomatoes like a acid soil, that's where the lime comes in.Use the green tomatoes on the vines for Tomato Relish, it's great. Hope this helped.