It isn’t uncommon for the first leaves on a tomato to turn yellow and fall off as the upper plants takes over and starts to shade the base of the plant. If the new growth looks OK without spotting and such then all is probably well. The best way to avoid fungal diseases on your tomatoes is to keeps them as dry and warm as possible. This means watering only when necessary and especially keeping the water off the leaves as much as possible. NEVER SPRAY YOUR PLANTS WITH WATER! It is bad enough that the rain wets the leaves but you shouldn’t add to the problem by spray watering. Get a watering wand so you can direct the water into the soil at the base of the plant without getting the leaves and flowers wet