Tomato leaves turn yellow, brown and die
Tomatoes - leaves turn yellow, then brown and finally die. Starts with the lowest leaves and works it way upward. Does not seem to affect the fruit.
There are many possibilities ranging from insect pests, environmental conditions or disease.
It could be 'bacterial canker'. When tomatoes have bacterial canker, their leaves will wilt, turn yellow then brown and die from the bottom up.
It could be a wilt disease, such as fusarium or verticillium, for which there is no chemical treatment. Solarization of the soil is typically recommended. The YouTube video below will help you identify if you have a wilt disease or not.
Over/under-watering will also cause leaves to turn yellow and wilt. Examine the soil. If it is dry, dusty and cracked, then your plants are under-watered. If the soil seems swampy, the plants are most likely over-watered.
For more information on all of the above, I recommend you review the following article: