What can be done to avoid tomato plants from getting blight ?

It is called Tomato blight. Make sure you rotate your tomato crops, or try planting them in containers.

In my master gardener class, we were told to avoid planting tomatoes, potatoes, eggplant in the same place, also to avoid composting the above if any sign of blight. Basically rotating your "crops" on a 3 year cycle will help avoid blight.

There is a new type of blight that was discussed in one of our MG classes. The extension office suggests pulling up the whole plant and destroying it. Do not plant tomatoes, potatoes or eggplant in that area again for at least 3 years. To get more healthy, productive plants, space your tomatoes about 3 feet apart, use organic materials i.e. compost around the base and do not water from the top. That at crop rotation seems to be the best way to avoid blight and powdery mildew.